oapi_codegen/lower/

paths.rs

//! Lowering OpenAPI paths/operations into the server [`crate::ir::Service`].
//!
//! Each operation is lowered into typed inputs (path/query/header parameters and
//! a JSON request body) plus a response enum. The generator only models what it
//! can translate faithfully. Anything else is rejected with an error rather than
//! mis-generated.
//!
//! Supported:
//!
//! - **Path parameters** — inline scalars, or a same-document `$ref` to a scalar.
//! - **Query parameters** — scalars and arrays of scalars, lowered into a
//!   per-operation `Deserialize` struct extracted via
//!   `axum_extra::extract::Query`. Required parameters stay bare. optional ones
//!   become `Option<..>`. Arrays must use the default `form`/`explode: true`
//!   encoding (repeated keys).
//! - **Header parameters** — scalars only, lowered into a per-operation struct
//!   extracted via a generated `FromRequestParts` impl. The reserved
//!   `Accept`/`Content-Type`/`Authorization` headers are ignored.
//! - **Cookie parameters** — scalars only, lowered into a per-operation struct
//!   extracted via a generated `FromRequestParts` impl backed by
//!   `axum_extra`'s `CookieJar`.
//! - **Component `$ref` parameters and request bodies** — `$ref`s to
//!   `#/components/parameters/*` and `#/components/requestBodies/*` are resolved
//!   against the document.
//! - **Responses** — explicit status codes, the `default` catch-all, and ranges
//!   (`5XX`). Component `$ref` responses are resolved against the document.
//! - **Cross-file `$ref` parameters, request bodies, and responses** — the
//!   referenced structural object is read from the sibling file (resolved
//!   relative to the main spec's directory), following chains across files.
//!   Parameter inner schemas must still resolve to scalars. body/response inner
//!   schema `$ref`s route through the `import-mapping` to an external type
//!   (they are never inlined).
//!
//! Rejected: object or other non-scalar parameters, non-default query-array
//! encodings, `content` parameters, array or `byte`/`binary` header parameters,
//! `byte`/`binary` cookie parameters, path-item `$ref`s, cross-file schema-type
//! `$ref`s (in a body or response) that lack an `import-mapping` entry for the
//! referenced file, and an unrecognised response status code.

use std::collections::BTreeMap;

use http::StatusCode as HttpStatus;
use openapiv3::ObjectType;
use openapiv3::Operation as OasOperation;
use openapiv3::Parameter;
use openapiv3::ParameterData;
use openapiv3::ParameterSchemaOrContent;
use openapiv3::PathStyle;
use openapiv3::QueryStyle;
use openapiv3::ReferenceOr;
use openapiv3::RequestBody;
use openapiv3::Response as OasResponse;
use openapiv3::Schema;
use openapiv3::SchemaData;
use openapiv3::SchemaKind;
use openapiv3::StatusCode;
use openapiv3::Type;

use crate::diagnostic::Warning;
use crate::diagnostic::report_warnings;
use crate::error::Error;
use crate::error::Result;
use crate::ir::Body;
use crate::ir::BodyKind;
use crate::ir::BodyVariant;
use crate::ir::CookieParam;
use crate::ir::Cookies;
use crate::ir::Field;
use crate::ir::HeaderParam;
use crate::ir::Headers;
use crate::ir::Multipart;
use crate::ir::MultipartField;
use crate::ir::NegotiatedBody;
use crate::ir::Operation;
use crate::ir::Param;
use crate::ir::RequestPayload;
use crate::ir::ResponseBody;
use crate::ir::ResponseCase;
use crate::ir::ResponseStatus;
use crate::ir::RustType;
use crate::ir::Service;
use crate::ir::Struct;
use crate::loader::Resolved;
use crate::loader::Spec;
use crate::loader::ref_component_name;
use crate::loader::ref_file_part;
use crate::lower::default::lower_default;
use crate::lower::schema::integer_type;
use crate::lower::schema::string_format_type;
use crate::lower::security;
use crate::naming::Case;
use crate::naming::RustIdent;
use crate::naming::X_RUST_NAME;
use crate::naming::operations;
use crate::naming::to_ident;

/// Header parameter names that OpenAPI mandates be ignored when declared with
/// `in: header`, since they are governed by content negotiation / security
/// mechanisms rather than the parameter object (compared case-insensitively).
pub(crate) const IGNORED_HEADER_NAMES: [&str; 3] = ["accept", "content-type", "authorization"];

/// Content-type selection priority for request bodies. `multipart/form-data` is
/// only offered here (not for responses): axum has a multipart *extractor* but
/// no multipart *response* writer.
pub(crate) const REQUEST_BODY_PRIORITY: [BodyKind; 4] =
    [BodyKind::Json, BodyKind::Form, BodyKind::Multipart, BodyKind::Text];

/// Content-type selection priority for response bodies. Multipart is excluded:
/// axum has a multipart *extractor* but no multipart *response* writer. A
/// multipart-only response is therefore rejected, as any response whose every
/// content type is unsupported is.
pub(crate) const RESPONSE_BODY_PRIORITY: [BodyKind; 3] = [BodyKind::Json, BodyKind::Form, BodyKind::Text];

/// A lowered response body before it is named. A single content type yields a
/// [`Body`]. several yield the per-representation variants, which the caller
/// names into a [`NegotiatedBody`] (the name depends on the response variant).
enum LoweredResponseBody {
    Single(Body),
    Negotiated(Vec<BodyVariant>),
}

/// Rust field names the response emitter injects into a header-bearing struct
/// variant (`status` for dynamic responses, `body` when a body is present). A
/// declared response header whose `snake_case` identifier equals one of these
/// will collide, so such headers are rejected during lowering.
const RESERVED_RESPONSE_FIELDS: [&str; 2] = ["status", "body"];

/// Check whether a header name is valid for use with `HeaderName::from_static`.
/// Enforces the HTTP `tchar` token set (RFC 9110 §5.6.2 / RFC 7230): ASCII
/// alphanumerics plus ``!#$%&'*+-.^_`|~``. This prevents a later panic when
/// emitting `HeaderName::from_static`.
fn is_valid_header_name(name: &str) -> bool {
    if name.is_empty() {
        return false;
    }
    for byte in name.as_bytes() {
        // RFC 7230 tchar. `-`..`9` (0x2D..0x39) will wrongly include `/`
        // (0x2F), which is not a valid header-name char, so digits are their
        // own range and `-`/`.` are listed explicitly.
        let valid = matches!(
            byte,
            b'!' | b'#'..=b'\'' | b'*'..=b'+' | b'-' | b'.' | b'0'..=b'9' | b'A'..=b'Z' | b'^'..=b'z' | b'|' | b'~'
        );
        if !valid {
            return false;
        }
    }
    return true;
}

/// Lower every operation in `spec` into the server IR, resolving cross-file
/// schema references through `import_mapping`. `response_type_suffix` is
/// appended to each operation's response-enum name.
pub fn generate_service(
    spec: &Spec,
    import_mapping: &BTreeMap<String, String>,
    response_type_suffix: &str,
) -> Result<Service> {
    let lowerer = Lowerer {
        spec,
        import_mapping,
        response_type_suffix,
    };
    return lowerer.lower();
}

/// Carries the document and its import mapping through operation lowering.
struct Lowerer<'a> {
    spec: &'a Spec,
    import_mapping: &'a BTreeMap<String, String>,
    response_type_suffix: &'a str,
}

impl Lowerer<'_> {
    /// Lower every operation in the document into the server IR.
    fn lower(&self) -> Result<Service> {
        let catalogue = security::scheme_catalogue(self.spec);
        let mut operations = Vec::new();
        let mut used_schemes: Vec<String> = Vec::new();
        // Maps a claimed method name back to the `method path` that claimed it, so
        // a collision error can name the earlier operation and not the identifier
        // alone. Filtering runs before lowering (see `crate::filter`), and nothing
        // prunes an operation afterwards, so every name claimed here reaches the
        // file and every collision found here is real.
        let mut claimed: BTreeMap<String, String> = BTreeMap::new();
        let mut collisions = crate::lower::validate::Diagnostics::new();
        for (path, entry) in self.spec.paths().iter() {
            let item = match entry {
                ReferenceOr::Item(item) => item,
                ReferenceOr::Reference { .. } => {
                    return Err(Error::UnsupportedOperation {
                        method: "*".to_owned(),
                        path: path.clone(),
                        reason: "path-item `$ref`s are not supported".to_owned(),
                    });
                }
            };
            for (method, operation) in item.iter() {
                let mut lowered = self.lower_operation(path, method, operation, &item.parameters)?;
                let route = format!("{method} {path}");
                match claimed.get(lowered.name.logical()) {
                    Some(first) => {
                        collisions.push(Error::OperationNameCollision {
                            ident: lowered.name.logical().to_owned(),
                            first: first.clone(),
                            second: route,
                            hint: operation_collision_hint(operation),
                        });
                        // Do not lower this operation into the service. Its
                        // artifacts all derive from the colliding name, so keeping
                        // it would emit the duplicates this check exists to stop.
                        continue;
                    }
                    None => {
                        claimed.insert(lowered.name.logical().to_owned(), route);
                    }
                }
                lowered.security = self.operation_security(operation);
                for key in &lowered.security {
                    if !used_schemes.iter().any(|existing| return existing == key) {
                        used_schemes.push(key.clone());
                    }
                }
                operations.push(lowered);
            }
        }
        collisions.into_result()?;
        let security_schemes = catalogue
            .into_iter()
            .filter(|scheme| return used_schemes.iter().any(|key| return *key == scheme.key))
            .collect();
        return Ok(Service {
            operations,
            security_schemes,
        });
    }

    /// Resolve an operation's effective security requirement into the ordered
    /// keys of the schemes it applies.
    ///
    /// Keys are kept verbatim, including any not declared in
    /// `components.securitySchemes`: the client emitter rejects an unresolved key
    /// rather than silently dropping it, while the server emitter ignores
    /// security entirely, so an unused-by-server dangling reference never blocks
    /// server generation.
    fn operation_security(&self, operation: &OasOperation) -> Vec<String> {
        let effective = security::effective_requirements(operation.security.as_deref(), self.spec.global_security());
        let Some(requirements) = effective else {
            return Vec::new();
        };
        return security::required_keys(requirements);
    }

    /// Lower a single operation, given its path, method and path-item parameters.
    fn lower_operation(
        &self,
        path: &str,
        method: &str,
        operation: &OasOperation,
        shared_params: &[ReferenceOr<Parameter>],
    ) -> Result<Operation> {
        let name = operation_name(path, method, operation)?;
        let response_enum = operations::response_enum_name(&name, self.response_type_suffix);

        let params = self.resolve_parameters(operation, shared_params)?;
        let path_params = self.lower_path_params(path, method, &params)?;
        let query = self.lower_query_params(path, method, &params, &name)?;
        let headers = self.lower_header_params(path, method, &params, &name)?;
        let cookies = self.lower_cookie_params(path, method, &params, &name)?;
        let request = self.lower_request_body(path, method, &name, operation)?;
        let responses = self.lower_responses(path, method, &name, operation)?;

        return Ok(Operation {
            name,
            response_enum,
            doc: operation_doc(operation),
            method: method.to_owned(),
            path: path.to_owned(),
            path_params,
            query,
            headers,
            cookies,
            request,
            responses,
            security: Vec::new(),
        });
    }

    /// Resolve an operation's parameters (its own, then the path-item's shared
    /// parameters) into concrete `Parameter`s paired with the referenced file
    /// each was resolved from (`None` for inline or same-document entries), so
    /// inner schema `$ref`s can later be interpreted against the right document.
    /// Operation-level entries precede shared ones, preserving override order.
    fn resolve_parameters(
        &self,
        operation: &OasOperation,
        shared_params: &[ReferenceOr<Parameter>],
    ) -> Result<Vec<Resolved<Parameter>>> {
        let mut resolved = Vec::new();
        for parameter in operation.parameters.iter().chain(shared_params) {
            let entry = match parameter {
                ReferenceOr::Item(param) => Resolved {
                    value: param.clone(),
                    origin: None,
                },
                ReferenceOr::Reference { reference } => self.spec.resolve_parameter(reference)?,
            };
            resolved.push(entry);
        }
        return Ok(resolved);
    }

    /// Resolve the typed path parameters in their path-template order, which is
    /// the order axum extracts a `Path<(..)>` tuple in.
    fn lower_path_params(&self, path: &str, method: &str, params: &[Resolved<Parameter>]) -> Result<Vec<Param>> {
        let placeholders = path_param_names(path);
        let mut path_params = Vec::new();
        for name in &placeholders {
            let declared = path_param_schema(name, params);
            let ty = match declared {
                Some((format, origin)) => self.param_type(path, method, name, origin, format)?,
                None => {
                    return Err(Error::UndeclaredPathParameter {
                        method: method.to_owned(),
                        path: path.to_owned(),
                        name: name.clone(),
                    });
                }
            };
            path_params.push(Param {
                name: to_ident(name, Case::Snake),
                ty,
            });
        }
        // A parameter declared `in: path` must have a matching `{placeholder}` in
        // the template. Driving the loop above from the template alone will
        // otherwise silently drop such a parameter from the generated signature,
        // producing a handler that omits a required input.
        let mut seen = Vec::new();
        for parameter in params {
            let Parameter::Path { parameter_data, style } = &parameter.value else {
                continue;
            };
            if !placeholders
                .iter()
                .any(|placeholder| return placeholder == &parameter_data.name)
            {
                return Err(Error::InvalidPathParameter {
                    method: method.to_owned(),
                    path: path.to_owned(),
                    name: parameter_data.name.clone(),
                });
            }
            if seen.contains(&parameter_data.name.as_str()) {
                continue;
            }
            seen.push(parameter_data.name.as_str());
            if !matches!(style, PathStyle::Simple) {
                self.warn(
                    parameter.origin.as_deref(),
                    &format!("{method} {path}"),
                    format!(
                        "path parameter `{}` uses unsupported style `{style:?}`. Generated code uses `simple` encoding",
                        parameter_data.name
                    ),
                );
            }
        }
        return Ok(path_params);
    }

    /// Lower an operation's query parameters into a generated `Deserialize`
    /// struct, returning `None` when the operation declares none. Per OpenAPI's
    /// override rule, an operation-level parameter takes precedence over a
    /// path-item one with the same name, so duplicates are de-duplicated keeping
    /// the first (operation-level) definition.
    fn lower_query_params(
        &self,
        path: &str,
        method: &str,
        params: &[Resolved<Parameter>],
        operation_name: &RustIdent,
    ) -> Result<Option<Struct>> {
        let owner = operations::query_struct_name(operation_name);
        let mut fields = Vec::new();
        let mut seen: Vec<&str> = Vec::new();
        for parameter in params {
            let Parameter::Query {
                parameter_data, style, ..
            } = &parameter.value
            else {
                continue;
            };
            if seen.contains(&parameter_data.name.as_str()) {
                continue;
            }
            seen.push(&parameter_data.name);
            fields.push(self.query_field(path, method, parameter.origin.as_deref(), parameter_data, style, &owner)?);
        }
        if fields.is_empty() {
            return Ok(None);
        }
        let name = owner;
        return Ok(Some(Struct {
            name,
            doc: None,
            deprecated: None,
            fields,
            additional_properties: None,
            // A query struct denies no unknown key. A query string commonly
            // carries a parameter the spec does not declare, such as one a proxy
            // or an analytics tool adds, and rejecting the whole request for it
            // would break a client that the spec permits.
            deny_unknown_fields: false,
        }));
    }

    /// Build a query struct field from a single query parameter's metadata,
    /// wrapping optional parameters in `Option<..>`.
    fn query_field(
        &self,
        path: &str,
        method: &str,
        origin: Option<&str>,
        data: &ParameterData,
        style: &QueryStyle,
        owner: &RustIdent,
    ) -> Result<Field> {
        let schema = self.query_param_schema(path, method, origin, data)?;
        let mut ty = self.query_param_type(path, method, origin, data, style, &schema)?;

        // A required parameter is always present, so its `default` never fires.
        let declared = match data.required {
            true => None,
            false => schema.schema_data.default.clone(),
        };
        if !data.required && declared.is_none() {
            ty = ty.optional();
        }
        // A query parameter never has a named type, so its default can never
        // name an enum variant.
        let default = match &declared {
            Some(json) => Some(lower_default(json, &ty, &|_| return None, owner.logical(), &data.name)?),
            None => None,
        };

        let ident = to_ident(&data.name, Case::Snake);
        let rename = crate::naming::rename_for(&data.name, &ident);
        // A query parameter is the one place a server reads a value it does not
        // own, so a constraint here does the most work.
        let constraints = crate::lower::constraints::constraints_of(&schema);
        let field = Field {
            name: ident,
            rename,
            doc: data.description.as_deref().and_then(trimmed),
            deprecated: None,
            ty,
            required: data.required,
            omit_empty: None,
            serde_skip: false,
            default,
            constraints,
            // A parameter travels in a request only, so a direction mark on its
            // schema states nothing the generator can act on.
            access: crate::ir::Access::ReadWrite,
        };
        crate::lower::constraints::check_constraints(&field)?;
        return Ok(field);
    }

    /// The schema of a query parameter, resolved one time for both the type and
    /// the `default` to read.
    ///
    /// A `content` parameter is rejected here. This generator reads a query
    /// parameter from a schema only.
    fn query_param_schema(
        &self,
        path: &str,
        method: &str,
        origin: Option<&str>,
        data: &ParameterData,
    ) -> Result<Schema> {
        let ParameterSchemaOrContent::Schema(schema) = &data.format else {
            return Err(Error::UnsupportedOperation {
                method: method.to_owned(),
                path: path.to_owned(),
                reason: format!("query parameter `{}` uses `content`, which is not supported", data.name),
            });
        };
        return self.resolve_param_schema(path, method, origin, &data.name, schema);
    }

    /// Map a query parameter's schema to a scalar Rust type, or a `Vec<T>` of
    /// scalars. Cross-file `$ref`s, `content`, and non-scalar shapes (including
    /// arrays of non-scalars) are rejected. Array parameters must use OpenAPI's
    /// default `form`/`explode: true` encoding (repeated keys), since the
    /// generated server reads them through `axum-extra`'s `Query` extractor.
    /// other array encodings are rejected rather than silently mis-parsed.
    fn query_param_type(
        &self,
        path: &str,
        method: &str,
        origin: Option<&str>,
        data: &ParameterData,
        style: &QueryStyle,
        schema: &Schema,
    ) -> Result<RustType> {
        let name = data.name.as_str();
        let explode = data.explode;
        if let SchemaKind::Type(Type::Array(array)) = &schema.schema_kind {
            if !matches!(style, QueryStyle::Form) || explode == Some(false) {
                return Err(Error::UnsupportedOperation {
                    method: method.to_owned(),
                    path: path.to_owned(),
                    reason: format!(
                        "query parameter `{name}` uses a non-default array encoding; only `style: form` with `explode: true` (repeated keys) is supported"
                    ),
                });
            }
            let element = match &array.items {
                Some(ReferenceOr::Item(item)) => {
                    self.warn_scalar_enum(
                        origin,
                        &format!("{method} {path} query parameter `{name}` items"),
                        &item.schema_kind,
                    );
                    scalar_type(&item.schema_kind)
                }
                Some(ReferenceOr::Reference { reference }) if ref_file_part(reference).is_some() => {
                    return Err(Error::UnsupportedOperation {
                        method: method.to_owned(),
                        path: path.to_owned(),
                        reason: format!(
                            "query parameter `{name}` uses array items via a cross-file `$ref`, which is not supported"
                        ),
                    });
                }
                Some(ReferenceOr::Reference { reference }) => {
                    let item = self.spec.resolve_schema(origin, reference)?;
                    self.warn_scalar_enum(
                        origin,
                        &format!("{method} {path} query parameter `{name}` items"),
                        &item.schema_kind,
                    );
                    scalar_type(&item.schema_kind)
                }
                None => {
                    return Err(Error::UnsupportedOperation {
                        method: method.to_owned(),
                        path: path.to_owned(),
                        reason: format!("query parameter `{name}` is an array without `items`"),
                    });
                }
            };
            let element = element.ok_or_else(|| {
                return Error::UnsupportedOperation {
                    method: method.to_owned(),
                    path: path.to_owned(),
                    reason: format!("query parameter `{name}` must be an array of scalars"),
                };
            })?;
            return Ok(RustType::Vec(Box::new(element)));
        }
        let ty = scalar_type(&schema.schema_kind).ok_or_else(|| {
            return Error::UnsupportedOperation {
                method: method.to_owned(),
                path: path.to_owned(),
                reason: format!("query parameter `{name}` must be a scalar or an array of scalars"),
            };
        })?;
        self.warn_scalar_enum(
            origin,
            &format!("{method} {path} query parameter `{name}`"),
            &schema.schema_kind,
        );
        if !matches!(style, QueryStyle::Form) {
            self.warn(
                origin,
                &format!("{method} {path}"),
                format!(
                    "query parameter `{name}` uses unsupported scalar style `{style:?}`. Generated code uses `form` encoding"
                ),
            );
        }
        return Ok(ty);
    }

    /// Resolve a parameter schema reference to an owned concrete schema. A
    /// same-document reference is resolved against the main document, or against
    /// the referenced document the parameter came from (`origin`). a cross-file
    /// inner `$ref` is out of scope and rejected.
    fn resolve_param_schema(
        &self,
        path: &str,
        method: &str,
        origin: Option<&str>,
        name: &str,
        schema: &ReferenceOr<Schema>,
    ) -> Result<Schema> {
        match schema {
            ReferenceOr::Item(schema) => return Ok(schema.clone()),
            ReferenceOr::Reference { reference } if ref_file_part(reference).is_some() => {
                return Err(Error::UnsupportedOperation {
                    method: method.to_owned(),
                    path: path.to_owned(),
                    reason: format!("query parameter `{name}` uses a cross-file `$ref`, which is not supported"),
                });
            }
            ReferenceOr::Reference { reference } => return self.spec.resolve_schema(origin, reference),
        }
    }

    /// Lower an operation's header parameters into a generated [`Headers`]
    /// struct, returning `None` when the operation declares none. Per OpenAPI's
    /// override rule the first (operation-level) definition wins on a
    /// case-insensitive name collision, and the `Accept`/`Content-Type`/
    /// `Authorization` headers the specification reserves are skipped.
    fn lower_header_params(
        &self,
        path: &str,
        method: &str,
        params: &[Resolved<Parameter>],
        operation_name: &RustIdent,
    ) -> Result<Option<Headers>> {
        let mut header_params = Vec::new();
        let mut seen: Vec<&str> = Vec::new();
        for parameter in params {
            let Parameter::Header { parameter_data, .. } = &parameter.value else {
                continue;
            };
            let name = parameter_data.name.as_str();
            if IGNORED_HEADER_NAMES
                .iter()
                .any(|ignored| return ignored.eq_ignore_ascii_case(name))
            {
                continue;
            }
            if seen.iter().any(|other| return other.eq_ignore_ascii_case(name)) {
                continue;
            }
            seen.push(name);
            header_params.push(self.header_param(path, method, parameter.origin.as_deref(), parameter_data)?);
        }
        if header_params.is_empty() {
            return Ok(None);
        }
        let name = operations::headers_struct_name(operation_name);
        return Ok(Some(Headers {
            name,
            params: header_params,
        }));
    }

    /// Build a single header field, resolving its scalar type and recording the
    /// exact header name for the generated case-insensitive lookup.
    fn header_param(
        &self,
        path: &str,
        method: &str,
        origin: Option<&str>,
        data: &ParameterData,
    ) -> Result<HeaderParam> {
        let ty = self.header_param_type(path, method, origin, &data.name, &data.format)?;
        return Ok(HeaderParam {
            name: to_ident(&data.name, Case::Snake),
            header_name: data.name.clone(),
            ty,
            required: data.required,
            doc: data.description.as_deref().and_then(trimmed),
        });
    }

    /// Map a header/response-header schema to a scalar Rust type, applying the
    /// shared rules: reject `content`, non-scalar shapes, and `byte`/`binary`.
    /// resolve a same-document/origin schema `$ref` to a scalar. reject a
    /// cross-file schema `$ref`. `kind_label` is used in error messages (for example
    /// "header parameter" or "response header").
    fn scalar_from_format(
        &self,
        path: &str,
        method: &str,
        origin: Option<&str>,
        kind_label: &str,
        name: &str,
        format: &ParameterSchemaOrContent,
    ) -> Result<RustType> {
        let schema = match format {
            ParameterSchemaOrContent::Schema(schema) => schema,
            ParameterSchemaOrContent::Content(_) => {
                return Err(Error::UnsupportedOperation {
                    method: method.to_owned(),
                    path: path.to_owned(),
                    reason: format!("{kind_label} `{name}` uses `content`, which is not supported"),
                });
            }
        };
        let schema = match schema {
            ReferenceOr::Item(schema) => schema.clone(),
            ReferenceOr::Reference { reference } if ref_file_part(reference).is_some() => {
                return Err(Error::UnsupportedOperation {
                    method: method.to_owned(),
                    path: path.to_owned(),
                    reason: format!("{kind_label} `{name}` uses a cross-file `$ref`, which is not supported"),
                });
            }
            ReferenceOr::Reference { reference } => self.spec.resolve_schema(origin, reference)?,
        };
        let ty = scalar_type(&schema.schema_kind).ok_or_else(|| {
            return Error::UnsupportedOperation {
                method: method.to_owned(),
                path: path.to_owned(),
                reason: format!("{kind_label} `{name}` must be a scalar"),
            };
        })?;
        if matches!(ty, RustType::Bytes) {
            return Err(Error::UnsupportedOperation {
                method: method.to_owned(),
                path: path.to_owned(),
                reason: format!("{kind_label} `{name}` uses a `byte`/`binary` format, which is not supported"),
            });
        }
        self.warn_scalar_enum(
            origin,
            &format!("{method} {path} {kind_label} `{name}`"),
            &schema.schema_kind,
        );
        return Ok(ty);
    }

    /// Map a header parameter's schema to a scalar Rust type. `content`,
    /// cross-file `$ref`s, non-scalar shapes (arrays/objects), and `byte`/
    /// `binary` strings (which have no `FromStr`) are rejected.
    fn header_param_type(
        &self,
        path: &str,
        method: &str,
        origin: Option<&str>,
        name: &str,
        format: &ParameterSchemaOrContent,
    ) -> Result<RustType> {
        return self.scalar_from_format(path, method, origin, "header parameter", name, format);
    }

    /// Lower an operation's cookie parameters into a generated [`Cookies`]
    /// struct, returning `None` when the operation declares none. Per OpenAPI's
    /// override rule the first (operation-level) definition wins on a name
    /// collision. Cookies have no reserved-name analogue, so none are skipped.
    fn lower_cookie_params(
        &self,
        path: &str,
        method: &str,
        params: &[Resolved<Parameter>],
        operation_name: &RustIdent,
    ) -> Result<Option<Cookies>> {
        let mut cookie_params = Vec::new();
        let mut seen: Vec<&str> = Vec::new();
        for parameter in params {
            let Parameter::Cookie { parameter_data, .. } = &parameter.value else {
                continue;
            };
            let name = parameter_data.name.as_str();
            if seen.contains(&name) {
                continue;
            }
            seen.push(name);
            let ty = self.cookie_param_type(
                path,
                method,
                parameter.origin.as_deref(),
                &parameter_data.name,
                &parameter_data.format,
            )?;
            cookie_params.push(CookieParam {
                name: to_ident(&parameter_data.name, Case::Snake),
                cookie_name: parameter_data.name.clone(),
                ty,
                required: parameter_data.required,
                doc: parameter_data.description.as_deref().and_then(trimmed),
            });
        }
        if cookie_params.is_empty() {
            return Ok(None);
        }
        let name = operations::cookies_struct_name(operation_name);
        return Ok(Some(Cookies {
            name,
            params: cookie_params,
        }));
    }

    /// Map a cookie parameter's schema to a scalar Rust type. `content`,
    /// cross-file `$ref`s, non-scalar shapes (arrays/objects), and `byte`/
    /// `binary` strings (no `FromStr`) are rejected.
    fn cookie_param_type(
        &self,
        path: &str,
        method: &str,
        origin: Option<&str>,
        name: &str,
        format: &ParameterSchemaOrContent,
    ) -> Result<RustType> {
        let schema = match format {
            ParameterSchemaOrContent::Schema(schema) => schema,
            ParameterSchemaOrContent::Content(_) => {
                return Err(Error::UnsupportedOperation {
                    method: method.to_owned(),
                    path: path.to_owned(),
                    reason: format!("cookie parameter `{name}` uses `content`, which is not supported"),
                });
            }
        };
        let schema = match schema {
            ReferenceOr::Item(schema) => schema.clone(),
            ReferenceOr::Reference { reference } if ref_file_part(reference).is_some() => {
                return Err(Error::UnsupportedOperation {
                    method: method.to_owned(),
                    path: path.to_owned(),
                    reason: format!("cookie parameter `{name}` uses a cross-file `$ref`, which is not supported"),
                });
            }
            ReferenceOr::Reference { reference } => self.spec.resolve_schema(origin, reference)?,
        };
        let ty = scalar_type(&schema.schema_kind).ok_or_else(|| {
            return Error::UnsupportedOperation {
                method: method.to_owned(),
                path: path.to_owned(),
                reason: format!("cookie parameter `{name}` must be a scalar"),
            };
        })?;
        if matches!(ty, RustType::Bytes) {
            return Err(Error::UnsupportedOperation {
                method: method.to_owned(),
                path: path.to_owned(),
                reason: format!("cookie parameter `{name}` uses a `byte`/`binary` format, which is not supported"),
            });
        }
        self.warn_scalar_enum(
            origin,
            &format!("{method} {path} cookie parameter `{name}`"),
            &schema.schema_kind,
        );
        return Ok(ty);
    }

    fn warn_scalar_enum(&self, origin: Option<&str>, context: &str, kind: &SchemaKind) {
        let has_enum = match kind {
            SchemaKind::Type(Type::String(schema)) => !schema.enumeration.is_empty(),
            SchemaKind::Type(Type::Integer(schema)) => !schema.enumeration.is_empty(),
            _ => false,
        };
        if has_enum {
            self.warn(
                origin,
                context,
                "the declared `enum` is ignored. Generated code uses the base scalar type without enum validation"
                    .to_owned(),
            );
        }
    }

    /// Collect every supported content type a body declares, deduplicated by
    /// [`BodyKind`] and ordered by the caller's `priority` (requests and
    /// responses differ — see [`REQUEST_BODY_PRIORITY`] /
    /// [`RESPONSE_BODY_PRIORITY`]). When several media entries map to the same
    /// kind (for example `application/json` and `application/vnd.api+json`), the first
    /// in document order wins. An empty result means the map declares no content
    /// type the `priority` accepts.
    fn supported_bodies<'m>(
        &self,
        content: &'m indexmap::IndexMap<String, openapiv3::MediaType>,
        priority: &[BodyKind],
        origin: Option<&str>,
        context: &str,
    ) -> Vec<(BodyKind, &'m openapiv3::MediaType)> {
        let mut selected = Vec::new();
        for &wanted in priority {
            let mut first = None;
            for (name, media) in content {
                if media_type_kind(name) == Some(wanted) {
                    match first {
                        Some(first) => self.warn(
                            origin,
                            context,
                            format!("media type `{name}` is ignored because `{first}` is the first representation of the same body kind"),
                        ),
                        None => {
                            selected.push((wanted, media));
                            first = Some(name);
                        }
                    }
                }
            }
        }
        if !selected.is_empty() {
            for name in content.keys() {
                if !media_type_kind(name).is_some_and(|kind| return priority.contains(&kind)) {
                    self.warn(origin, context, format!("unsupported media type `{name}` is ignored"));
                }
            }
        }
        return selected;
    }

    fn warn(&self, origin: Option<&str>, context: &str, message: String) {
        let document = self.spec.source().display().to_string();
        let message = match origin {
            Some(origin) => format!("{message} (resolved from `{origin}`)"),
            None => message,
        };
        report_warnings(&document, &[Warning::new(context, message)]);
    }

    /// Lower a selected body media entry into a typed [`Body`] for the given
    /// content kind. Text bodies must be `string`. form bodies must reference a
    /// named object schema. JSON reuses the existing body-type mapping.
    fn body_from_media(
        &self,
        path: &str,
        method: &str,
        origin: Option<&str>,
        kind: BodyKind,
        media: &openapiv3::MediaType,
    ) -> Result<Option<Body>> {
        let schema = match &media.schema {
            Some(schema) => schema,
            None => return Ok(None),
        };
        let ty = match kind {
            BodyKind::Json => self.body_type(path, method, origin, schema)?,
            BodyKind::Text => {
                let resolved = match schema {
                    ReferenceOr::Item(schema) => schema.clone(),
                    ReferenceOr::Reference { reference } => self.spec.resolve_schema(origin, reference)?,
                };
                if !matches!(resolved.schema_kind, SchemaKind::Type(Type::String(_))) {
                    return Err(Error::UnsupportedOperation {
                        method: method.to_owned(),
                        path: path.to_owned(),
                        reason: "text/plain body must be a `string` schema".to_owned(),
                    });
                }
                RustType::String
            }
            BodyKind::Form => match schema {
                ReferenceOr::Reference { reference } => {
                    // A form body must be a struct of scalar fields for
                    // `serde_urlencoded`. When the schema is resolvable here
                    // (same-document, or an origin-file `$ref` without its own
                    // file part) require it to be an `object`. A cross-file
                    // (`file#/...`) ref is opaque — it maps to an external type
                    // via the `import-mapping`, so its shape cannot be inspected
                    // and is trusted.
                    if ref_file_part(reference).is_none() {
                        let resolved = self.spec.resolve_schema(origin, reference)?;
                        if !matches!(resolved.schema_kind, SchemaKind::Type(Type::Object(_))) {
                            return Err(Error::UnsupportedOperation {
                                method: method.to_owned(),
                                path: path.to_owned(),
                                reason:
                                    "form (`application/x-www-form-urlencoded`) body must reference an `object` schema"
                                        .to_owned(),
                            });
                        }
                    }
                    self.schema_ref_type(path, method, origin, reference)?
                }
                ReferenceOr::Item(_) => {
                    return Err(Error::UnsupportedOperation {
                        method: method.to_owned(),
                        path: path.to_owned(),
                        reason: "form (`application/x-www-form-urlencoded`) body must reference a named object schema"
                            .to_owned(),
                    });
                }
            },
            // Multipart is lowered by `lower_multipart_body`, and never selected
            // for responses (`RESPONSE_BODY_PRIORITY` excludes it), so it does
            // not reach the shared body mapping.
            BodyKind::Multipart => {
                return Err(Error::UnsupportedOperation {
                    method: method.to_owned(),
                    path: path.to_owned(),
                    reason: "multipart/form-data is only supported for request bodies".to_owned(),
                });
            }
        };
        return Ok(Some(Body { ty, kind }));
    }
}

/// Locate the `path` parameter named `name` within a resolved parameter list,
/// returning its schema/content and the referenced file it was resolved from.
fn path_param_schema<'a>(
    name: &str,
    params: &'a [Resolved<Parameter>],
) -> Option<(&'a ParameterSchemaOrContent, Option<&'a str>)> {
    for parameter in params {
        let Parameter::Path { parameter_data, .. } = &parameter.value else {
            continue;
        };
        if parameter_data.name == name {
            return Some((&parameter_data.format, parameter.origin.as_deref()));
        }
    }
    return None;
}

/// The enum-variant identifier for a negotiated body's content kind
/// (`Json`, `Form`, `Text`). `Multipart` never participates in negotiation, so
/// its arm is only for exhaustiveness.
fn body_kind_ident(kind: BodyKind) -> RustIdent {
    let name = match kind {
        BodyKind::Json => "Json",
        BodyKind::Form => "Form",
        BodyKind::Text => "Text",
        BodyKind::Multipart => "Multipart",
    };
    return to_ident(name, Case::Pascal);
}

/// The content types a body declares, in document order, for a message. Both
/// directions report the same way, so one function builds the list.
fn declared_content_types(content: &indexmap::IndexMap<String, openapiv3::MediaType>) -> String {
    return content.keys().cloned().collect::<Vec<_>>().join(", ");
}

/// Classify a media type string into a supported [`BodyKind`], or `None`.
/// Parameters after `;` (for example `; charset=utf-8`) are ignored. JSON matches
/// broadly: `application/json` or any `+json`-suffixed type.
pub(crate) fn media_type_kind(name: &str) -> Option<BodyKind> {
    let base = name.split(';').next().unwrap_or(name).trim().to_ascii_lowercase();
    if base == "application/json" || base.ends_with("+json") {
        return Some(BodyKind::Json);
    }
    if base == "application/x-www-form-urlencoded" {
        return Some(BodyKind::Form);
    }
    if base == "multipart/form-data" {
        return Some(BodyKind::Multipart);
    }
    if base == "text/plain" {
        return Some(BodyKind::Text);
    }
    return None;
}

/// Map an OpenAPI schema kind to its Rust type when it is one of the four
/// supported scalars (`string`, `integer`, `number`, `boolean`), else `None`.
fn scalar_type(kind: &SchemaKind) -> Option<RustType> {
    let ty = match kind {
        SchemaKind::Type(Type::String(st)) => string_format_type(&st.format),
        SchemaKind::Type(Type::Integer(it)) => integer_type(it),
        SchemaKind::Type(Type::Number(_)) => RustType::F64,
        SchemaKind::Type(Type::Boolean(_)) => RustType::Bool,
        _ => return None,
    };
    return Some(ty);
}

impl Lowerer<'_> {
    /// Map a path parameter's schema to a scalar Rust type. Path parameters must
    /// be scalars (they are parsed from URL segments into an axum `Path<..>`
    /// tuple), so a `$ref` is resolved to its concrete schema and the scalar-only
    /// rule is enforced — the same as header and cookie parameters. A
    /// same-document `$ref` is resolved against the main document, or against the
    /// referenced document the parameter came from (`origin`). a cross-file inner
    /// `$ref` is rejected.
    fn param_type(
        &self,
        path: &str,
        method: &str,
        name: &str,
        origin: Option<&str>,
        format: &ParameterSchemaOrContent,
    ) -> Result<RustType> {
        let schema = match format {
            ParameterSchemaOrContent::Schema(schema) => schema,
            ParameterSchemaOrContent::Content(_) => {
                return Err(Error::UnsupportedOperation {
                    method: method.to_owned(),
                    path: path.to_owned(),
                    reason: format!("path parameter `{name}` uses `content`, which is not supported"),
                });
            }
        };
        let schema = match schema {
            ReferenceOr::Item(schema) => schema.clone(),
            ReferenceOr::Reference { reference } if ref_file_part(reference).is_some() => {
                return Err(Error::UnsupportedOperation {
                    method: method.to_owned(),
                    path: path.to_owned(),
                    reason: format!("path parameter `{name}` uses a cross-file `$ref`, which is not supported"),
                });
            }
            ReferenceOr::Reference { reference } => self.spec.resolve_schema(origin, reference)?,
        };
        let ty = scalar_type(&schema.schema_kind).ok_or_else(|| {
            return Error::UnsupportedOperation {
                method: method.to_owned(),
                path: path.to_owned(),
                reason: format!("path parameter `{name}` must be a scalar type"),
            };
        })?;
        self.warn_scalar_enum(
            origin,
            &format!("{method} {path} path parameter `{name}`"),
            &schema.schema_kind,
        );
        return Ok(ty);
    }

    /// Lower an operation's request body, if it declares one. A cross-file
    /// wrapper `$ref` is resolved against the referenced file. Its inner schema
    /// `$ref`s are then interpreted against that file (`origin`). A single
    /// supported content type yields a [`RequestPayload::Single`]. a
    /// `multipart/form-data` body yields a per-operation extractor (`op_name`
    /// seeds its name). several supported content types yield a
    /// [`RequestPayload::Negotiated`] dispatch enum. `multipart/form-data`
    /// cannot be combined with other content types (it needs a bespoke
    /// extractor rather than a `Content-Type` branch).
    fn lower_request_body(
        &self,
        path: &str,
        method: &str,
        op_name: &RustIdent,
        operation: &OasOperation,
    ) -> Result<Option<RequestPayload>> {
        let body = match &operation.request_body {
            Some(body) => body,
            None => return Ok(None),
        };
        let (body, origin): (RequestBody, Option<String>) = match body {
            ReferenceOr::Item(body) => (body.clone(), None),
            ReferenceOr::Reference { reference } => {
                let resolved = self.spec.resolve_request_body(reference)?;
                (resolved.value, resolved.origin)
            }
        };
        let supported = self.supported_bodies(
            &body.content,
            &REQUEST_BODY_PRIORITY,
            origin.as_deref(),
            &format!("{method} {path} request body"),
        );
        if supported.is_empty() {
            if body.content.is_empty() {
                return Ok(None);
            }
            return Err(Error::UnsupportedContentType {
                method: method.to_owned(),
                path: path.to_owned(),
                location: "request body".to_owned(),
                declared: declared_content_types(&body.content),
                hint: "A request body must declare `application/json`, `application/x-www-form-urlencoded`, `multipart/form-data`, or `text/plain`. Add one of them, or remove the `requestBody`.".to_owned(),
            });
        }
        let has_multipart = supported.iter().any(|(kind, _)| return *kind == BodyKind::Multipart);
        if has_multipart {
            if supported.len() > 1 {
                return Err(Error::UnsupportedOperation {
                    method: method.to_owned(),
                    path: path.to_owned(),
                    reason: "multipart/form-data cannot be combined with other request content types".to_owned(),
                });
            }
            let Some(&(_, media)) = supported.first() else {
                unreachable!("multipart body content type count already validated to be exactly one");
            };
            let multipart = self.lower_multipart_body(path, method, op_name, origin.as_deref(), media)?;
            return Ok(Some(RequestPayload::Multipart(multipart)));
        }
        let mut variants = Vec::with_capacity(supported.len());
        for (kind, media) in supported {
            if let Some(body) = self.body_from_media(path, method, origin.as_deref(), kind, media)? {
                variants.push(BodyVariant {
                    variant: body_kind_ident(kind),
                    body,
                });
            }
        }
        if variants.len() == 1 {
            let Some(variant) = variants.pop() else {
                unreachable!("length checked to be 1 above");
            };
            return Ok(Some(RequestPayload::Single(variant.body)));
        }
        if variants.is_empty() {
            return Ok(None);
        }
        return Ok(Some(RequestPayload::Negotiated(NegotiatedBody {
            name: operations::request_body_enum_name(op_name),
            variants,
        })));
    }

    /// Lower a `multipart/form-data` request body into a per-operation extractor
    /// struct (`<Op>Multipart`) that parses it. The schema must be an object,
    /// declared either inline or as a same-document `$ref` (so its fields can be
    /// enumerated). each property must be a scalar or a binary/file string.
    /// Composite, nested-object, array, and cross-file/external bodies are
    /// rejected.
    ///
    /// Unlike JSON/form/text bodies, a multipart body does not reuse a component
    /// model type: axum has no typed multipart extractor, so the generator owns
    /// a dedicated struct plus a hand-written `FromRequest`. Generating a
    /// per-operation struct (rather than reusing the referenced component) keeps
    /// multipart working under any model configuration — including
    /// `models: false` with cross-file `import-mapping` — and lets an inline
    /// object be used without declaring a redundant named component.
    fn lower_multipart_body(
        &self,
        path: &str,
        method: &str,
        op_name: &RustIdent,
        origin: Option<&str>,
        media: &openapiv3::MediaType,
    ) -> Result<Multipart> {
        let unsupported = |reason: String| {
            return Error::UnsupportedOperation {
                method: method.to_owned(),
                path: path.to_owned(),
                reason,
            };
        };
        let schema = media
            .schema
            .as_ref()
            .ok_or_else(|| return unsupported("multipart/form-data body must declare a schema".to_owned()))?;
        let object = self.multipart_object(path, method, origin, schema)?;
        let fields = self.lower_multipart_fields(path, method, &object)?;
        return Ok(Multipart {
            name: operations::multipart_struct_name(op_name),
            fields,
        });
    }

    /// Resolve a multipart body schema to its [`ObjectType`]. An inline object is
    /// taken directly. a `$ref` must be same-document (no cross-file part, and
    /// the body must not itself come from a referenced file) and resolve to an
    /// object. Non-object schemas and cross-file/external references are
    /// rejected — their fields cannot be enumerated into a typed extractor.
    fn multipart_object(
        &self,
        path: &str,
        method: &str,
        origin: Option<&str>,
        schema: &ReferenceOr<Schema>,
    ) -> Result<ObjectType> {
        let reject = |reason: &str| {
            return Error::UnsupportedOperation {
                method: method.to_owned(),
                path: path.to_owned(),
                reason: reason.to_owned(),
            };
        };
        let not_object = "multipart/form-data body schema must be an `object`";
        let cross_file = "multipart/form-data body must be an inline object or a same-document `$ref`; cross-file/external multipart is unsupported";
        match schema {
            ReferenceOr::Item(item) => {
                if origin.is_some() {
                    return Err(reject(cross_file));
                }
                match &item.schema_kind {
                    SchemaKind::Type(Type::Object(object)) => return Ok(object.clone()),
                    _ => return Err(reject(not_object)),
                }
            }
            ReferenceOr::Reference { reference } => {
                if origin.is_some() || ref_file_part(reference).is_some() {
                    return Err(reject(cross_file));
                }
                let resolved = self.spec.resolve_schema(origin, reference)?;
                match &resolved.schema_kind {
                    SchemaKind::Type(Type::Object(object)) => return Ok(object.clone()),
                    _ => return Err(reject(not_object)),
                }
            }
        }
    }

    /// Lower a multipart object's properties into [`MultipartField`]s: a field is
    /// `Option<..>` when it is not `required` or its schema is `nullable`, and
    /// its identifier is the property's `snake_case` name. Each property must
    /// resolve to a scalar (a binary/`byte` string becomes a `Vec<u8>` file
    /// field). a same-document `$ref` is resolved first. Non-scalar properties
    /// (nested objects/arrays) and cross-file `$ref` properties are rejected.
    fn lower_multipart_fields(&self, path: &str, method: &str, object: &ObjectType) -> Result<Vec<MultipartField>> {
        let mut fields = Vec::with_capacity(object.properties.len());
        for (wire_name, property) in &object.properties {
            let required = object.required.iter().any(|name| {
                return name == wire_name;
            });
            // A one-member `allOf` around a `$ref` names the schema the `$ref`
            // names. The wrapper adds a keyword such as `description` or
            // `nullable` and nothing else, so the field reads the target.
            let wrapped = match property {
                ReferenceOr::Item(schema) => crate::loader::single_ref_member(schema).map(|reference| {
                    return ReferenceOr::Reference {
                        reference: reference.to_owned(),
                    };
                }),
                ReferenceOr::Reference { .. } => None,
            };
            // The wrapper's own keywords still apply on top of the target's. A
            // target in another file cannot be read here, so for one the wrapper's
            // mark settles the part alone; a same-document target is read and
            // its marks combine with the wrapper's below, so a conflict between
            // the two is reported as it is on any field.
            let wrapper = match (&wrapped, property) {
                (Some(_), ReferenceOr::Item(schema)) => Some(&schema.schema_data),
                _ => None,
            };
            if let (Some(wrapper), Some(ReferenceOr::Reference { reference })) = (wrapper, &wrapped)
                && ref_file_part(reference).is_some()
                && !multipart_part_is_sent(wrapper, path, method, wire_name)?
            {
                continue;
            }
            let (kind, nullable) = match wrapped.as_ref().unwrap_or(property) {
                ReferenceOr::Item(schema) => {
                    if !multipart_part_is_sent(&schema.schema_data, path, method, wire_name)? {
                        continue;
                    }
                    (schema.schema_kind.clone(), schema.schema_data.nullable)
                }
                ReferenceOr::Reference { reference } => {
                    if ref_file_part(reference).is_some() {
                        return Err(Error::UnsupportedOperation {
                            method: method.to_owned(),
                            path: path.to_owned(),
                            reason: format!(
                                "multipart field `{wire_name}` uses a cross-file `$ref`, which is not supported"
                            ),
                        });
                    }
                    let resolved = self.spec.resolve_schema(None, reference)?;
                    let mut data = resolved.schema_data;
                    if let Some(wrapper) = wrapper {
                        data.nullable |= wrapper.nullable;
                        data.read_only |= wrapper.read_only;
                        data.write_only |= wrapper.write_only;
                    }
                    if !multipart_part_is_sent(&data, path, method, wire_name)? {
                        continue;
                    }
                    (resolved.schema_kind, data.nullable)
                }
            };
            let ty = scalar_type(&kind).ok_or_else(|| {
                return Error::UnsupportedOperation {
                    method: method.to_owned(),
                    path: path.to_owned(),
                    reason: format!(
                        "multipart field `{wire_name}` must be a scalar or binary string; nested objects and arrays are not supported"
                    ),
                };
            })?;
            fields.push(MultipartField {
                wire_name: wire_name.clone(),
                rust_name: to_ident(wire_name, Case::Snake),
                is_file: matches!(ty, RustType::Bytes),
                ty,
                optional: !required || nullable,
            });
        }
        return Ok(fields);
    }

    /// Lower a response's declared headers into scalar-typed [`ResponseHeader`]s.
    /// Inline `Header` objects only. a `Header` that is itself a `$ref` is
    /// rejected. De-duplicated by case-insensitive name, first-seen winning.
    fn lower_response_headers(
        &self,
        path: &str,
        method: &str,
        origin: Option<&str>,
        response: &OasResponse,
    ) -> Result<Vec<crate::ir::ResponseHeader>> {
        let mut headers = Vec::new();
        let mut seen: Vec<String> = Vec::new();
        let mut seen_idents: Vec<String> = Vec::new();
        for (header_name, header_ref) in &response.headers {
            let header = match header_ref {
                ReferenceOr::Item(header) => header,
                ReferenceOr::Reference { .. } => {
                    return Err(Error::UnsupportedOperation {
                        method: method.to_owned(),
                        path: path.to_owned(),
                        reason: format!("response header `{header_name}` uses a `$ref`, which is not supported"),
                    });
                }
            };
            if seen.iter().any(|other| return other.eq_ignore_ascii_case(header_name)) {
                continue;
            }
            if !is_valid_header_name(header_name) {
                return Err(Error::UnsupportedOperation {
                    method: method.to_owned(),
                    path: path.to_owned(),
                    reason: format!("response header `{header_name}` has an invalid header name"),
                });
            }
            let ident = to_ident(header_name, Case::Snake);
            // The response emitter injects `status` (dynamic responses) and
            // `body` (responses with a body) fields into the struct variant. A
            // header whose Rust field name collides with one of those will emit
            // duplicate fields. Reject rather than mis-generate.
            if RESERVED_RESPONSE_FIELDS.contains(&ident.logical()) {
                return Err(Error::UnsupportedOperation {
                    method: method.to_owned(),
                    path: path.to_owned(),
                    reason: format!(
                        "response header `{header_name}` maps to the reserved Rust field name `{}`",
                        ident.logical()
                    ),
                });
            }
            // Distinct header names can collapse to the same Rust field
            // identifier (for example `X-Foo` and `X_Foo` both → `x_foo`), which will
            // emit a struct with duplicate fields. Reject rather than
            // mis-generate.
            if seen_idents.iter().any(|other| return other == ident.logical()) {
                return Err(Error::UnsupportedOperation {
                    method: method.to_owned(),
                    path: path.to_owned(),
                    reason: format!(
                        "response header `{header_name}` maps to the same Rust field name as another header (`{}`)",
                        ident.logical()
                    ),
                });
            }
            seen.push(header_name.clone());
            seen_idents.push(ident.logical().to_owned());
            let ty = self.scalar_from_format(path, method, origin, "response header", header_name, &header.format)?;
            headers.push(crate::ir::ResponseHeader {
                name: ident,
                header_name: header_name.clone(),
                ty,
                required: header.required,
                doc: header.description.as_deref().and_then(trimmed),
            });
        }
        return Ok(headers);
    }

    /// Lower an operation's responses into typed enum variants, resolving
    /// component `$ref` responses against the document. A fixed status code
    /// becomes a reason-named variant with a compile-time status constant. a
    /// range (`5XX` → `Status5xx`) or the `default` response becomes a variant
    /// that carries the `axum::http::StatusCode` the handler supplies at runtime.
    fn lower_responses(
        &self,
        path: &str,
        method: &str,
        operation_name: &RustIdent,
        operation: &OasOperation,
    ) -> Result<Vec<ResponseCase>> {
        let mut cases = Vec::new();
        for (status_code, response) in &operation.responses.responses {
            let (status, variant) = match status_code {
                StatusCode::Code(code) => {
                    let reason = HttpStatus::from_u16(*code).ok().and_then(|status| {
                        return status.canonical_reason();
                    });
                    let reason = reason.ok_or_else(|| {
                        return Error::UnsupportedOperation {
                            method: method.to_owned(),
                            path: path.to_owned(),
                            reason: format!("status code `{code}` is not a recognised HTTP status"),
                        };
                    })?;
                    (ResponseStatus::Fixed(*code), to_ident(reason, Case::Pascal))
                }
                StatusCode::Range(range) => {
                    if !(1..=5).contains(range) {
                        return Err(Error::UnsupportedOperation {
                            method: method.to_owned(),
                            path: path.to_owned(),
                            reason: format!("response range `{range}XX` is not a valid HTTP status class"),
                        });
                    }
                    let variant = to_ident(&format!("status_{range}xx"), Case::Pascal);
                    let Ok(range) = u8::try_from(*range) else {
                        unreachable!("range checked to be within 1..=5 above");
                    };
                    (ResponseStatus::Range(range), variant)
                }
            };
            let response = self.resolve_response_ref(response)?;
            // The status code names the response for a message. Two responses of
            // one operation are otherwise indistinguishable to a reader.
            let location = format!("`{status_code}` response");
            let body = self.response_body(path, method, &location, response.origin.as_deref(), &response.value)?;
            let body = self.name_response_body(operation_name, &variant, body);
            let headers = self.lower_response_headers(path, method, response.origin.as_deref(), &response.value)?;
            cases.push(ResponseCase {
                variant,
                status,
                body,
                headers,
                doc: trimmed(&response.value.description),
            });
        }

        if let Some(default) = &operation.responses.default {
            let response = self.resolve_response_ref(default)?;
            let variant = to_ident("default", Case::Pascal);
            let body = self.response_body(
                path,
                method,
                "`default` response",
                response.origin.as_deref(),
                &response.value,
            )?;
            let body = self.name_response_body(operation_name, &variant, body);
            let headers = self.lower_response_headers(path, method, response.origin.as_deref(), &response.value)?;
            cases.push(ResponseCase {
                variant,
                status: ResponseStatus::Default,
                body,
                headers,
                doc: trimmed(&response.value.description),
            });
        }

        if cases.is_empty() {
            return Err(Error::UnsupportedOperation {
                method: method.to_owned(),
                path: path.to_owned(),
                reason: "operation declares no responses".to_owned(),
            });
        }
        return Ok(cases);
    }

    /// Resolve a possibly-referenced response to an owned [`OasResponse`] plus
    /// the referenced file it came from (`None` for inline/same-document).
    fn resolve_response_ref(&self, response: &ReferenceOr<OasResponse>) -> Result<Resolved<OasResponse>> {
        match response {
            ReferenceOr::Item(response) => {
                return Ok(Resolved {
                    value: response.clone(),
                    origin: None,
                });
            }
            ReferenceOr::Reference { reference } => return self.spec.resolve_response(reference),
        }
    }

    /// Extract a response's body, if it declares supported content. Inner
    /// schema `$ref`s are interpreted against the response's origin file when it
    /// was resolved from a referenced document. A single supported content type
    /// yields [`LoweredResponseBody::Single`]. several yield the
    /// per-representation variants the caller names into a [`NegotiatedBody`].
    ///
    /// A response that declares content, and no content type in
    /// [`RESPONSE_BODY_PRIORITY`], is an error. It is not a bodyless response. A
    /// bodyless response declares no `content:` at all, and `204` is the common
    /// case. The two are different statements, and emitting a bodyless variant
    /// for the first drops the body with no message. The request side already
    /// rejects the same input, so this keeps the two directions in agreement.
    fn response_body(
        &self,
        path: &str,
        method: &str,
        location: &str,
        origin: Option<&str>,
        response: &OasResponse,
    ) -> Result<Option<LoweredResponseBody>> {
        let supported = self.supported_bodies(
            &response.content,
            &RESPONSE_BODY_PRIORITY,
            origin,
            &format!("{method} {path} {location}"),
        );
        if supported.is_empty() && !response.content.is_empty() {
            return Err(Error::UnsupportedContentType {
                method: method.to_owned(),
                path: path.to_owned(),
                location: location.to_owned(),
                declared: declared_content_types(&response.content),
                hint: "A response body must declare `application/json`, `application/x-www-form-urlencoded`, or `text/plain`. Add one of them, or declare no `content:` for a bodyless response.".to_owned(),
            });
        }
        let mut variants = Vec::with_capacity(supported.len());
        for (kind, media) in supported {
            if let Some(body) = self.body_from_media(path, method, origin, kind, media)? {
                variants.push(BodyVariant {
                    variant: body_kind_ident(kind),
                    body,
                });
            }
        }
        if variants.len() == 1 {
            let Some(variant) = variants.pop() else {
                unreachable!("length checked to be 1 above");
            };
            return Ok(Some(LoweredResponseBody::Single(variant.body)));
        }
        if variants.is_empty() {
            return Ok(None);
        }
        return Ok(Some(LoweredResponseBody::Negotiated(variants)));
    }

    /// Name a lowered response body against its response variant: a single
    /// content type stays [`ResponseBody::Single`]. several become a
    /// [`ResponseBody::Negotiated`] enum named `<Op><Variant>Body`.
    fn name_response_body(
        &self,
        operation_name: &RustIdent,
        variant: &RustIdent,
        lowered: Option<LoweredResponseBody>,
    ) -> Option<ResponseBody> {
        return lowered.map(|body| {
            return match body {
                LoweredResponseBody::Single(body) => ResponseBody::Single(body),
                LoweredResponseBody::Negotiated(variants) => ResponseBody::Negotiated(NegotiatedBody {
                    name: operations::response_body_enum_name(operation_name, variant),
                    variants,
                }),
            };
        });
    }

    /// Map a request/response body schema to a Rust type. Composite inline
    /// schemas must be referenced by name (`$ref`) so the models pass owns
    /// their emission. `origin` is the referenced file the enclosing wrapper was
    /// resolved from, so a same-document inner `$ref` lowers to the right module.
    fn body_type(
        &self,
        path: &str,
        method: &str,
        origin: Option<&str>,
        schema: &ReferenceOr<Schema>,
    ) -> Result<RustType> {
        match schema {
            ReferenceOr::Reference { reference } => return self.schema_ref_type(path, method, origin, reference),
            ReferenceOr::Item(schema) => return self.inline_body_type(path, method, origin, schema),
        }
    }

    /// Map an inline (non-`$ref`) body schema to a Rust type.
    fn inline_body_type(&self, path: &str, method: &str, origin: Option<&str>, schema: &Schema) -> Result<RustType> {
        let ty = match &schema.schema_kind {
            SchemaKind::Type(Type::String(st)) => string_format_type(&st.format),
            SchemaKind::Type(Type::Integer(it)) => integer_type(it),
            SchemaKind::Type(Type::Number(_)) => RustType::F64,
            SchemaKind::Type(Type::Boolean(_)) => RustType::Bool,
            SchemaKind::Type(Type::Array(at)) => {
                let element = match &at.items {
                    Some(ReferenceOr::Reference { reference }) => {
                        self.schema_ref_type(path, method, origin, reference)?
                    }
                    Some(ReferenceOr::Item(item)) => self.inline_body_type(path, method, origin, item)?,
                    None => RustType::Value,
                };
                RustType::Vec(Box::new(element))
            }
            SchemaKind::Type(Type::Object(object)) if object.properties.is_empty() => {
                let value = match &object.additional_properties {
                    Some(openapiv3::AdditionalProperties::Schema(value)) => {
                        self.body_type(path, method, origin, value)?
                    }
                    Some(openapiv3::AdditionalProperties::Any(true)) | None => RustType::Value,
                    Some(openapiv3::AdditionalProperties::Any(false)) => {
                        return Err(Error::UnsupportedOperation {
                            method: method.to_owned(),
                            path: path.to_owned(),
                            reason: "closed object bodies must reference a named schema (`$ref`)".to_owned(),
                        });
                    }
                };
                RustType::Map(Box::new(value))
            }
            SchemaKind::Any(_) => RustType::Value,
            SchemaKind::AllOf { all_of } => {
                let [member] = all_of.as_slice() else {
                    return Err(Error::UnsupportedOperation {
                        method: method.to_owned(),
                        path: path.to_owned(),
                        reason: "composite request/response bodies must reference a named schema (`$ref`)".to_owned(),
                    });
                };
                self.body_type(path, method, origin, member)?
            }
            _ => {
                return Err(Error::UnsupportedOperation {
                    method: method.to_owned(),
                    path: path.to_owned(),
                    reason: "composite request/response bodies must reference a named schema (`$ref`)".to_owned(),
                });
            }
        };
        return Ok(if schema.schema_data.nullable {
            crate::lower::schema::nullable_type(self.spec, ty)
        } else {
            ty
        });
    }

    /// Decide the Rust type for a schema `$ref`, given the referenced file the
    /// enclosing structural object was resolved from (`origin`). A cross-file
    /// ref, or a same-document ref whose enclosing object came from a referenced
    /// file, resolves through the `import-mapping` to a [`RustType::External`].
    /// a same-document ref in the main document stays a local [`RustType::Named`].
    fn schema_ref_type(&self, path: &str, method: &str, origin: Option<&str>, reference: &str) -> Result<RustType> {
        let target = ref_component_name(reference, "schemas").ok_or_else(|| {
            return Error::UnsupportedOperation {
                method: method.to_owned(),
                path: path.to_owned(),
                reason: format!("reference `{reference}` must point at a component schema"),
            };
        })?;
        let file = ref_file_part(reference)
            .map(str::to_owned)
            .or_else(|| return origin.map(str::to_owned));
        let Some(file) = file else {
            // A local ref must name a schema this document declares. Without the
            // check the name reaches the output, and the generated file does not
            // compile. A cross-file ref below resolves through the
            // `import-mapping` instead, so the other document owns that name.
            if !self.spec.schemas().contains_key(target) {
                return Err(Error::UnresolvedRef(reference.to_owned()));
            }
            return Ok(RustType::Named(target.to_owned()));
        };
        let module = self.import_mapping.get(&file).ok_or_else(|| {
            return Error::UnsupportedOperation {
                method: method.to_owned(),
                path: path.to_owned(),
                reason: format!("cross-file reference `{reference}` needs an `import-mapping` entry for `{file}`"),
            };
        })?;
        return Ok(RustType::External {
            module: module.clone(),
            name: self.spec.external_schema_name(&file, target, reference)?,
        });
    }
}

/// Whether a `multipart/form-data` part travels in a request.
///
/// A multipart body is a request body only, so the generator leaves a `readOnly`
/// part out of the extractor. `writeOnly` states what a multipart part already
/// does.
fn multipart_part_is_sent(data: &SchemaData, path: &str, method: &str, wire_name: &str) -> Result<bool> {
    let at = format!("{method} {path} multipart field `{wire_name}`");
    let access = crate::lower::schema::access_of(data, &at)?;
    return Ok(access.carried_by(crate::ir::Direction::Request));
}

/// Derive the trait method name: an explicit `x-rust-name`, else the
/// `operationId`, else a name synthesised from the method and path (for example
/// `get /v1/widgets` -> `get_v1_widgets`).
///
/// `x-rust-name` is the escape hatch for two `operationId`s that collapse onto one
/// Rust name. It takes precedence over `operationId`, the same way it does for a
/// schema, so the author names the method and the generator does not.
fn operation_name(path: &str, method: &str, operation: &OasOperation) -> Result<crate::naming::RustIdent> {
    let at = format!("{method} {path}");
    if let Some(name) = crate::lower::extension::str_value(&operation.extensions, X_RUST_NAME, &at)? {
        return Ok(operations::operation_method_name(name));
    }
    if let Some(id) = &operation.operation_id {
        return Ok(operations::operation_method_name(id));
    }
    return Ok(operations::operation_method_name(&at));
}

/// Build the remedy text for an operation-name collision.
///
/// `operation` is the one that collided, and the advice depends on what it already
/// declares. An operation that sets `x-rust-name` needs a different name and not
/// the extension it already uses. An operation with no `operationId` derives its
/// name from the method and path, so adding an `operationId` is the natural fix.
///
/// There is no suffix option for operations, unlike `type-name-suffix` for
/// schemas. A method name appears in the trait a consumer implements, so every
/// name a consumer writes stays the author's choice.
fn operation_collision_hint(operation: &OasOperation) -> String {
    if operation.extensions.contains_key(X_RUST_NAME) {
        return format!(
            "This operation already sets `{X_RUST_NAME}`, and that name collides too. \
             Give it a name that no other operation uses."
        );
    }
    if operation.operation_id.is_some() {
        return format!(
            "Two `operationId`s that differ only in case or in punctuation produce one Rust name. \
             Give one of the two operations a different `operationId`, or set `{X_RUST_NAME}` on it \
             to name the generated method directly."
        );
    }
    return format!(
        "This operation declares no `operationId`, so its name comes from the method and the path. \
         Add an `operationId`, or set `{X_RUST_NAME}` on it to name the generated method directly."
    );
}

/// The operation's doc comment, preferring `summary` over `description`.
fn operation_doc(operation: &OasOperation) -> Option<String> {
    if let Some(summary) = &operation.summary
        && let Some(text) = trimmed(summary)
    {
        return Some(text);
    }
    return operation.description.as_ref().and_then(|text| {
        return trimmed(text);
    });
}

/// Extract `{name}` path-parameter names in their order of appearance.
fn path_param_names(path: &str) -> Vec<String> {
    let mut names = Vec::new();
    let mut rest = path;
    while let Some(open) = rest.find('{') {
        let after_open = &rest[open + 1..];
        let Some(close) = after_open.find('}') else {
            break;
        };
        names.push(after_open[..close].to_owned());
        rest = &after_open[close + 1..];
    }
    return names;
}

/// Trim a string and return `None` when it is empty.
fn trimmed(text: &str) -> Option<String> {
    let trimmed = text.trim();
    if trimmed.is_empty() {
        return None;
    }
    return Some(trimmed.to_owned());
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn all_of_inline_json_body_maps_keep_value_types_and_nullability() {
        let document = serde_json::from_value(serde_json::json!({
            "openapi":"3.0.3", "info":{"title":"maps","version":"1"}, "paths":{}
        }))
        .expect("document");
        let spec = Spec::from_parts(document, "maps.yaml".into());
        let import_mapping = BTreeMap::new();
        let lowerer = Lowerer {
            spec: &spec,
            import_mapping: &import_mapping,
            response_type_suffix: "",
        };
        for (member, expected) in [
            (
                serde_json::json!({"type":"object","additionalProperties":{"type":"string"}}),
                RustType::String,
            ),
            (serde_json::json!({"type":"object"}), RustType::Value),
            (
                serde_json::json!({"type":"object","additionalProperties":true}),
                RustType::Value,
            ),
        ] {
            for nullable in [false, true] {
                let schema = serde_json::from_value(serde_json::json!({"nullable":nullable,"allOf":[member]}))
                    .expect("map schema");
                let map = RustType::Map(Box::new(expected.clone()));
                let expected = if nullable {
                    RustType::Nullable(Box::new(map))
                } else {
                    map
                };
                assert_eq!(
                    lowerer
                        .inline_body_type("/maps", "post", None, &schema)
                        .expect("map body"),
                    expected
                );
            }
        }
        let closed = serde_json::from_value(serde_json::json!({
            "allOf":[{"type":"object","additionalProperties":false}]
        }))
        .expect("closed schema");
        let error = lowerer
            .inline_body_type("/maps", "post", None, &closed)
            .expect_err("closed inline body requires a named schema")
            .to_string();
        assert!(error.contains("closed object bodies"), "{error}");
    }

    #[test]
    fn valid_header_names_accept_tokens_and_reject_separators() {
        // Real header names with `-` and digits and `.` are accepted.
        assert!(is_valid_header_name("X-Request-Id"));
        assert!(is_valid_header_name("X-RateLimit-Remaining"));
        assert!(is_valid_header_name("Sec-CH-UA-Platform-Version"));
        assert!(is_valid_header_name("a.b"));
        // Empty and separator characters (which will panic `from_static`) are
        // rejected — notably `/` (0x2F), which sits between `-` (0x2D) and the
        // digits, and `:`, space, and control-ish punctuation.
        assert!(!is_valid_header_name(""));
        assert!(!is_valid_header_name("X/Y"));
        assert!(!is_valid_header_name("X:Y"));
        assert!(!is_valid_header_name("X Y"));
        assert!(!is_valid_header_name("X(Y)"));
    }

    #[test]
    fn extracts_path_param_names_in_order() {
        assert_eq!(path_param_names("/v1/widgets"), Vec::<String>::new());
        assert_eq!(path_param_names("/pets/{id}"), vec!["id".to_owned()]);
        assert_eq!(
            path_param_names("/orgs/{org}/pets/{petId}"),
            vec!["org".to_owned(), "petId".to_owned()],
        );
    }

    #[test]
    fn response_variants_are_named_after_the_status_reason() {
        let variant = |code| {
            let reason = HttpStatus::from_u16(code)
                .expect("test status code is a valid HTTP status")
                .canonical_reason()
                .expect("status code has a canonical reason phrase");
            return to_ident(reason, Case::Pascal).logical().to_owned();
        };
        assert_eq!(variant(200), "Ok");
        assert_eq!(variant(204), "NoContent");
        assert_eq!(variant(404), "NotFound");
        assert_eq!(variant(500), "InternalServerError");
    }

    #[test]
    fn range_response_variants_are_derived_from_the_range_digit() {
        let variant = |range: u16| {
            return to_ident(&format!("status_{range}xx"), Case::Pascal)
                .logical()
                .to_owned();
        };
        assert_eq!(variant(4), "Status4xx");
        assert_eq!(variant(5), "Status5xx");
    }
}