confroid-derive 0.0.1

//! Derive macro for [`confroid`](https://docs.rs/confroid).
//!
//! `#[derive(Config)]` generates a `confroid::FromEnv` implementation for a
//! named struct. See the `confroid` crate for usage.

use darling::util::Flag;
use darling::{FromDeriveInput, FromField, ast};
use proc_macro::TokenStream;
use quote::quote;
use syn::{DeriveInput, parse_macro_input};

/// An arbitrary expression captured from an attribute value.
///
/// darling's built-in `syn::Expr` handling rejects bare literals such as
/// `default = 8080`, so we capture the expression ourselves — literals wrapped
/// as literal expressions, everything else cloned verbatim.
#[derive(Clone)]
struct MetaExpr(#[allow(dead_code)] syn::Expr);

impl darling::FromMeta for MetaExpr {
    fn from_expr(expr: &syn::Expr) -> darling::Result<Self> {
        Ok(MetaExpr(expr.clone()))
    }

    fn from_value(value: &syn::Lit) -> darling::Result<Self> {
        Ok(MetaExpr(lit_to_expr(value)))
    }
}

/// The `default` attribute: either bare (`default` -> `Default::default()`) or
/// an explicit expression (`default = expr`).
///
/// This is a dedicated type rather than `Override<MetaExpr>` because darling's
/// `Override` rejects non-literal expression values before delegating, which
/// would reject a path default such as `default = MY_CONST`.
#[derive(Clone)]
enum DefaultKind {
    Inherit,
    Explicit(syn::Expr),
}

impl darling::FromMeta for DefaultKind {
    fn from_word() -> darling::Result<Self> {
        Ok(DefaultKind::Inherit)
    }

    fn from_expr(expr: &syn::Expr) -> darling::Result<Self> {
        Ok(DefaultKind::Explicit(expr.clone()))
    }

    fn from_value(value: &syn::Lit) -> darling::Result<Self> {
        Ok(DefaultKind::Explicit(lit_to_expr(value)))
    }
}

fn lit_to_expr(lit: &syn::Lit) -> syn::Expr {
    syn::Expr::Lit(syn::ExprLit {
        attrs: Vec::new(),
        lit: lit.clone(),
    })
}

#[derive(FromField)]
#[darling(attributes(confroid), forward_attrs(doc))]
#[allow(dead_code)]
struct FieldOpts {
    ident: Option<syn::Ident>,
    ty: syn::Type,
    /// Override the derived environment variable name / prefix.
    name: Option<String>,
    /// `default` (bare) -> `Default::default()`, `default = expr` -> `expr`.
    default: Option<DefaultKind>,
    /// Example value, used only for documentation generation.
    example: Option<MetaExpr>,
    /// Parse a single delimited variable into a `Vec` instead of indexed keys.
    auto_vec: Flag,
    /// Delimiter for `auto_vec` (defaults to `,`).
    auto_vec_delimiter: Option<String>,
    /// Forwarded `#[doc]` attributes, used for documentation generation.
    attrs: Vec<syn::Attribute>,
}

#[derive(FromDeriveInput)]
#[darling(attributes(confroid), supports(struct_named))]
struct InputOpts {
    ident: syn::Ident,
    generics: syn::Generics,
    data: ast::Data<darling::util::Ignored, FieldOpts>,
}

/// Derive `confroid::FromEnv` for a named struct.
#[proc_macro_derive(Config, attributes(confroid))]
pub fn derive_config(input: TokenStream) -> TokenStream {
    let di = parse_macro_input!(input as DeriveInput);
    let opts = match InputOpts::from_derive_input(&di) {
        Ok(opts) => opts,
        Err(err) => return err.write_errors().into(),
    };
    match expand(&opts) {
        Ok(tokens) => tokens.into(),
        Err(err) => err.write_errors().into(),
    }
}

fn expand(opts: &InputOpts) -> darling::Result<proc_macro2::TokenStream> {
    let ident = &opts.ident;
    let (impl_generics, ty_generics, where_clause) = opts.generics.split_for_impl();

    let fields = opts
        .data
        .as_ref()
        .take_struct()
        .expect("supports(struct_named) guarantees a struct")
        .fields;

    let mut errors = darling::Error::accumulator();
    let mut field_inits = Vec::with_capacity(fields.len());
    let mut field_docs = Vec::with_capacity(fields.len());

    for field in fields {
        let fname = field
            .ident
            .as_ref()
            .expect("named struct fields always have an ident");
        let fname_str = fname.to_string();
        let var_seg = field
            .name
            .clone()
            .unwrap_or_else(|| screaming_snake(&fname_str));
        let fty = &field.ty;

        // The core read expression evaluates to `confroid::Result<#fty>`.
        let read_expr = if field.auto_vec.is_present() {
            match vec_inner(fty) {
                Some(elem_ty) => {
                    let delim = field
                        .auto_vec_delimiter
                        .clone()
                        .unwrap_or_else(|| ",".to_string());
                    auto_vec_read(&var_seg, &fname_str, elem_ty, &delim)
                }
                None => {
                    errors.push(
                        darling::Error::custom("`auto_vec` requires a `Vec<T>` field")
                            .with_span(fty),
                    );
                    continue;
                }
            }
        } else {
            quote! {
                {
                    let __ctx = ctx.field(#var_seg, #fname_str);
                    <#fty as ::confroid::FromEnv>::from_env(&__ctx, env)
                }
            }
        };

        let init = match &field.default {
            None => quote! { #fname: (#read_expr)? },
            Some(DefaultKind::Inherit) => quote! {
                #fname: match #read_expr {
                    ::core::result::Result::Ok(__v) => __v,
                    ::core::result::Result::Err(::confroid::ConfroidError::EnvVarNotFound { .. }) =>
                        <#fty as ::core::default::Default>::default(),
                    ::core::result::Result::Err(__e) => return ::core::result::Result::Err(__e),
                }
            },
            Some(DefaultKind::Explicit(expr)) => {
                let default_val = default_value(expr);
                quote! {
                    #fname: match #read_expr {
                        ::core::result::Result::Ok(__v) => __v,
                        ::core::result::Result::Err(::confroid::ConfroidError::EnvVarNotFound { .. }) =>
                            #default_val,
                        ::core::result::Result::Err(__e) => return ::core::result::Result::Err(__e),
                    }
                }
            }
        };

        field_inits.push(init);

        // Documentation metadata for this field.
        let doc_tokens = match extract_doc(&field.attrs) {
            Some(doc) => quote! { ::core::option::Option::Some(#doc) },
            None => quote! { ::core::option::Option::None },
        };
        // Default/example annotations are only rendered for displayable scalar
        // leaves; collections (and a bare default on an `Option`) have no
        // single displayable value.
        let is_collection = type_is_named(fty, "Vec") || type_is_named(fty, "HashMap");
        let is_option = type_is_named(fty, "Option");
        let none = quote! { ::core::option::Option::None };
        let default_doc = match &field.default {
            Some(DefaultKind::Inherit) if !is_collection && !is_option => quote! {
                ::core::option::Option::Some(
                    ::std::format!("{}", <#fty as ::core::default::Default>::default())
                )
            },
            Some(DefaultKind::Explicit(expr)) if !is_collection => quote! {
                ::core::option::Option::Some(::std::format!("{}", #expr))
            },
            _ => none.clone(),
        };
        let example_doc = match &field.example {
            Some(MetaExpr(expr)) if !is_collection => quote! {
                ::core::option::Option::Some(::std::format!("{}", #expr))
            },
            _ => none.clone(),
        };
        field_docs.push(quote! {
            ::confroid::FieldDoc {
                var_seg: #var_seg,
                name: #fname_str,
                doc: #doc_tokens,
                default: #default_doc,
                example: #example_doc,
                children: <#fty as ::confroid::Documented>::doc_fields(),
            }
        });
    }

    errors.finish()?;

    let documented_impl = if cfg!(feature = "docs") {
        let ident_str = ident.to_string();
        quote! {
            #[automatically_derived]
            impl #impl_generics ::confroid::Documented for #ident #ty_generics #where_clause {
                fn doc_name() -> &'static str {
                    #ident_str
                }

                fn doc_fields() -> ::core::option::Option<::std::vec::Vec<::confroid::FieldDoc>> {
                    ::core::option::Option::Some(::std::vec![
                        #(#field_docs),*
                    ])
                }
            }
        }
    } else {
        quote! {}
    };

    Ok(quote! {
        #[automatically_derived]
        impl #impl_generics ::confroid::FromEnv for #ident #ty_generics #where_clause {
            fn from_env(
                ctx: &::confroid::Ctx,
                env: &::confroid::Env,
            ) -> ::confroid::Result<Self> {
                ::core::result::Result::Ok(Self {
                    #(#field_inits),*
                })
            }

            fn is_present(ctx: &::confroid::Ctx, env: &::confroid::Env) -> bool {
                !env.immediate_children(&ctx.var).is_empty()
            }
        }

        #documented_impl
    })
}

/// Delimited-value parsing for an `auto_vec` field.
fn auto_vec_read(
    var_seg: &str,
    fname_str: &str,
    elem_ty: &syn::Type,
    delim: &str,
) -> proc_macro2::TokenStream {
    quote! {
        {
            let __ctx = ctx.field(#var_seg, #fname_str);
            match env.get(&__ctx.var) {
                ::core::option::Option::None => ::core::result::Result::Err(
                    ::confroid::ConfroidError::EnvVarNotFound {
                        var_name: __ctx.var.clone(),
                        field: __ctx.path.clone(),
                    }
                ),
                ::core::option::Option::Some(__raw) if __raw.is_empty() =>
                    ::core::result::Result::Ok(::std::vec::Vec::new()),
                ::core::option::Option::Some(__raw) => {
                    let mut __out: ::std::vec::Vec<#elem_ty> = ::std::vec::Vec::new();
                    let mut __result = ::core::result::Result::Ok(());
                    for __part in __raw.split(#delim) {
                        match __part.parse::<#elem_ty>() {
                            ::core::result::Result::Ok(__v) => __out.push(__v),
                            ::core::result::Result::Err(__e) => {
                                __result = ::core::result::Result::Err(
                                    ::confroid::ConfroidError::EnvVarInvalid {
                                        var_name: __ctx.var.clone(),
                                        field: __ctx.path.clone(),
                                        value: __part.to_string(),
                                        parser_error: __e.to_string(),
                                    }
                                );
                                break;
                            }
                        }
                    }
                    __result.map(|()| __out)
                }
            }
        }
    }
}

/// Tokens for an explicit `default = expr`.
///
/// Numeric literals are emitted verbatim so integer/float literal inference
/// coerces them to the field type. Everything else is wrapped in `Into::into`
/// so that, for example, a `&str` default lands in a `String` field.
fn default_value(expr: &syn::Expr) -> proc_macro2::TokenStream {
    if is_numeric_literal(expr) {
        quote! { #expr }
    } else {
        quote! { ::core::convert::Into::into(#expr) }
    }
}

fn is_numeric_literal(expr: &syn::Expr) -> bool {
    match expr {
        syn::Expr::Lit(syn::ExprLit { lit, .. }) => {
            matches!(lit, syn::Lit::Int(_) | syn::Lit::Float(_))
        }
        syn::Expr::Unary(syn::ExprUnary {
            op: syn::UnOp::Neg(_),
            expr,
            ..
        }) => is_numeric_literal(expr),
        syn::Expr::Group(g) => is_numeric_literal(&g.expr),
        syn::Expr::Paren(p) => is_numeric_literal(&p.expr),
        _ => false,
    }
}

/// Whether `ty`'s last path segment is the given identifier (e.g. `Vec`).
fn type_is_named(ty: &syn::Type, name: &str) -> bool {
    matches!(ty, syn::Type::Path(tp) if tp.path.segments.last().is_some_and(|s| s.ident == name))
}

/// The element type `T` of a `Vec<T>`, if `ty` is a vector.
fn vec_inner(ty: &syn::Type) -> Option<&syn::Type> {
    let syn::Type::Path(tp) = ty else {
        return None;
    };
    let seg = tp.path.segments.last()?;
    if seg.ident != "Vec" {
        return None;
    }
    let syn::PathArguments::AngleBracketed(args) = &seg.arguments else {
        return None;
    };
    args.args.iter().find_map(|arg| match arg {
        syn::GenericArgument::Type(inner) => Some(inner),
        _ => None,
    })
}

/// Collect and clean the text of `#[doc]` attributes into a single string.
fn extract_doc(attrs: &[syn::Attribute]) -> Option<String> {
    let mut lines = Vec::new();
    for attr in attrs {
        if !attr.path().is_ident("doc") {
            continue;
        }
        if let syn::Meta::NameValue(nv) = &attr.meta
            && let syn::Expr::Lit(syn::ExprLit {
                lit: syn::Lit::Str(s),
                ..
            }) = &nv.value
        {
            lines.push(s.value().trim().to_string());
        }
    }
    if lines.is_empty() {
        None
    } else {
        Some(lines.join(" "))
    }
}

/// Convert an identifier to `SCREAMING_SNAKE_CASE`.
fn screaming_snake(s: &str) -> String {
    let mut out = String::new();
    let mut prev_is_word = false;
    for ch in s.chars() {
        if ch == '_' {
            out.push('_');
            prev_is_word = false;
            continue;
        }
        if ch.is_uppercase() && prev_is_word {
            out.push('_');
        }
        out.extend(ch.to_uppercase());
        prev_is_word = ch.is_lowercase() || ch.is_ascii_digit();
    }
    out
}