oxapi_impl/

method.rs

//! Method transformation for trait methods.

use heck::ToUpperCamelCase;
use proc_macro2::TokenStream;
use quote::{format_ident, quote};
use syn::{FnArg, GenericArgument, PathArguments, Type};

use crate::Generator;
use crate::openapi::Operation;
use crate::types::TypeGenerator;

/// Transforms user-defined trait methods based on OpenAPI operations.
pub struct MethodTransformer<'a> {
    generator: &'a Generator,
    types_mod: &'a syn::Ident,
}

impl<'a> MethodTransformer<'a> {
    pub fn new(generator: &'a Generator, types_mod: &'a syn::Ident) -> Self {
        Self {
            generator,
            types_mod,
        }
    }

    /// Transform a trait method based on its operation.
    pub fn transform(&self, method: &syn::TraitItemFn, op: &Operation) -> syn::Result<TokenStream> {
        let op_name = op
            .operation_id
            .as_deref()
            .unwrap_or(&op.path)
            .to_upper_camel_case();

        let types_mod = self.types_mod;
        let ok_type = format_ident!("{}Ok", op_name);
        let err_type = format_ident!("{}Err", op_name);

        // Transform each parameter, filling in type elisions
        let transformed_params = self.transform_params(method, op)?;

        // Determine if async and build return type
        let is_async = method.sig.asyncness.is_some();

        let return_type = quote! {
            ::core::result::Result<#types_mod::#ok_type, #types_mod::#err_type>
        };

        let method_name = &method.sig.ident;

        // Build the signature
        if is_async {
            // Rewrite async fn to fn -> impl Send + Future<Output = ...>
            Ok(quote! {
                fn #method_name(#transformed_params) -> impl ::core::marker::Send + ::core::future::Future<Output = #return_type>;
            })
        } else {
            // Sync function
            Ok(quote! {
                fn #method_name(#transformed_params) -> #return_type;
            })
        }
    }

    /// Transform the parameters, filling in type elisions.
    fn transform_params(
        &self,
        method: &syn::TraitItemFn,
        op: &Operation,
    ) -> syn::Result<TokenStream> {
        let type_gen = self.generator.type_generator();

        let mut transformed = Vec::new();

        for arg in &method.sig.inputs {
            match arg {
                FnArg::Receiver(_) => {
                    return Err(syn::Error::new_spanned(
                        arg,
                        "oxapi trait methods must be static (no self)",
                    ));
                }
                FnArg::Typed(pat_type) => {
                    let pat = &pat_type.pat;
                    let ty = &pat_type.ty;

                    // Check if this is an extractor with type elision
                    let transformed_ty = self.transform_type(ty, op, type_gen)?;

                    transformed.push(quote! { #pat: #transformed_ty });
                }
            }
        }

        Ok(quote! { #(#transformed),* })
    }

    /// Transform a type, filling in elisions like `Path<_>`.
    fn transform_type(
        &self,
        ty: &Type,
        op: &Operation,
        type_gen: &TypeGenerator,
    ) -> syn::Result<TokenStream> {
        let types_mod = self.types_mod;

        match ty {
            Type::Path(type_path) => {
                let last_segment = type_path
                    .path
                    .segments
                    .last()
                    .ok_or_else(|| syn::Error::new_spanned(ty, "empty type path"))?;

                let type_name = last_segment.ident.to_string();

                match type_name.as_str() {
                    "Path" => {
                        // Fill in path parameter type
                        let inner = self.get_or_infer_inner(&last_segment.arguments, || {
                            type_gen.generate_path_type(op)
                        })?;
                        Ok(quote! { ::axum::extract::Path<#inner> })
                    }
                    "Query" => {
                        // Fill in query parameter type
                        let inner = self.get_or_infer_inner(&last_segment.arguments, || {
                            // Generate a query struct if needed
                            if let Some((name, _)) = type_gen.generate_query_struct(op) {
                                quote! { #types_mod::#name }
                            } else {
                                quote! { () }
                            }
                        })?;
                        Ok(quote! { ::axum::extract::Query<#inner> })
                    }
                    "Json" => {
                        // Fill in request body type
                        let inner = self.get_or_infer_inner(&last_segment.arguments, || {
                            if let Some(body) = &op.request_body {
                                let op_name = op.operation_id.as_deref().unwrap_or(&op.path);
                                type_gen.request_body_type(body, op_name)
                            } else {
                                quote! { serde_json::Value }
                            }
                        })?;
                        Ok(quote! { ::axum::extract::Json<#inner> })
                    }
                    "State" => {
                        // State type is user-provided, don't modify
                        Ok(quote! { #ty })
                    }
                    _ => {
                        // Other types pass through unchanged
                        Ok(quote! { #ty })
                    }
                }
            }
            _ => {
                // Non-path types pass through unchanged
                Ok(quote! { #ty })
            }
        }
    }

    /// Get the inner type from generics, or infer it if it's `_`.
    fn get_or_infer_inner<F>(&self, args: &PathArguments, infer: F) -> syn::Result<TokenStream>
    where
        F: FnOnce() -> TokenStream,
    {
        match args {
            PathArguments::None => {
                // No generics, infer the type
                Ok(infer())
            }
            PathArguments::AngleBracketed(args) => {
                if let Some(GenericArgument::Type(Type::Infer(_))) = args.args.first() {
                    // Type is `_`, infer it
                    Ok(infer())
                } else if let Some(GenericArgument::Type(ty)) = args.args.first() {
                    // Type is explicit, use it
                    Ok(quote! { #ty })
                } else {
                    Err(syn::Error::new_spanned(args, "expected type argument"))
                }
            }
            PathArguments::Parenthesized(_) => Err(syn::Error::new_spanned(
                args,
                "unexpected parenthesized arguments",
            )),
        }
    }
}