Rollup merge of rust-lang#104612 - Swatinem:async-ret-y, r=estebank

Lower return type outside async block creation

This allows feeding a different output type to async blocks with a different `ImplTraitContext`. Spotted this while working on rust-lang#104321

compiler/rustc_ast_lowering/src/expr.rs

@@ -588,17 +588,12 @@ impl<'hir> LoweringContext<'_, 'hir> {
  &mut self,
  capture_clause: CaptureBy,
  closure_node_id: NodeId,
- ret_ty: Option<AstP<Ty>>,
+ ret_ty: Option<hir::FnRetTy<'hir>>,
  span: Span,
  async_gen_kind: hir::AsyncGeneratorKind,
  body: impl FnOnce(&mut Self) -> hir::Expr<'hir>,
  ) -> hir::ExprKind<'hir> {
- let output = match ret_ty {
- Some(ty) => hir::FnRetTy::Return(
- self.lower_ty(&ty, &ImplTraitContext::Disallowed(ImplTraitPosition::AsyncBlock)),
- ),
- None => hir::FnRetTy::DefaultReturn(self.lower_span(span)),
- };
+ let output = ret_ty.unwrap_or_else(|| hir::FnRetTy::DefaultReturn(self.lower_span(span)));
 
  // Resume argument type. We let the compiler infer this to simplify the lowering. It is
  // fully constrained by `future::from_generator`.
@@ -1003,8 +998,13 @@ impl<'hir> LoweringContext<'_, 'hir> {
  // Transform `async |x: u8| -> X { ... }` into
  // `|x: u8| future_from_generator(|| -> X { ... })`.
  let body_id = this.lower_fn_body(&outer_decl, |this| {
- let async_ret_ty =
- if let FnRetTy::Ty(ty) = &decl.output { Some(ty.clone()) } else { None };
+ let async_ret_ty = if let FnRetTy::Ty(ty) = &decl.output {
+ let itctx = ImplTraitContext::Disallowed(ImplTraitPosition::AsyncBlock);
+ Some(hir::FnRetTy::Return(this.lower_ty(&ty, &itctx)))
+ } else {
+ None
+ };
+
  let async_body = this.make_async_expr(
  capture_clause,
  inner_closure_id,