Auto merge of rust-lang#101224 - compiler-errors:rpitit, r=oli-obk

Initial implementation of return-position `impl Trait` in traits

* Create a new item kind, called `ImplTraitPlaceholder`, which is used to lower `ast::TyKind::ImplTrait` in trait method returns.
    * This item is the child of the trait method, which simplifies the way we calculate bounds and stuff.
* Use the def-id of this type to construct a projection type that we use during astconv for the return type of the trait method signature.
* Implement logic to normalize this `ImplTraitPlaceholder` projection type when we know its concrete impl (this is pretty limited currently, but really there are no other selection candidates that make sense -- for now!)
* Check that the `impl Trait`'s bounds are satisfied on the concrete type provided in the impl.

This is obviously nowhere near complete, but I wanted to at least get some initial support landed so we can start playing around with it.

What works:
* async fn in trait and RPITIT, including multiple `impl Trait`s and `impl Trait` nested in associated type bindings, like `impl Future<Output = impl Sized>`

compiler/rustc_ast/src/ast.rs

@@ -2367,9 +2367,9 @@ impl Async {
     }
 
     /// In this case this is an `async` return, the `NodeId` for the generated `impl Trait` item.
-    pub fn opt_return_id(self) -> Option<NodeId> {
+    pub fn opt_return_id(self) -> Option<(NodeId, Span)> {
         match self {
-            Async::Yes { return_impl_trait_id, .. } => Some(return_impl_trait_id),
+            Async::Yes { return_impl_trait_id, span, .. } => Some((return_impl_trait_id, span)),
             Async::No => None,
         }
     }

compiler/rustc_ast_lowering/src/errors.rs

@@ -334,3 +334,14 @@ pub struct InclusiveRangeWithNoEnd {
     #[primary_span]
     pub span: Span,
 }
+
+#[derive(SessionDiagnostic, Clone, Copy)]
+#[diag(ast_lowering::trait_fn_async, code = "E0706")]
+#[note]
+#[note(ast_lowering::note2)]
+pub struct TraitFnAsync {
+    #[primary_span]
+    pub fn_span: Span,
+    #[label]
+    pub span: Span,
+}

compiler/rustc_ast_lowering/src/expr.rs

@@ -851,7 +851,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
 
         self.lower_lifetime_binder(closure_id, generic_params, |lctx, bound_generic_params| {
             // Lower outside new scope to preserve `is_in_loop_condition`.
-            let fn_decl = lctx.lower_fn_decl(decl, None, FnDeclKind::Closure, None);
+            let fn_decl = lctx.lower_fn_decl(decl, None, fn_decl_span, FnDeclKind::Closure, None);
 
             let c = lctx.arena.alloc(hir::Closure {
                 binder: binder_clause,
@@ -955,7 +955,8 @@ impl<'hir> LoweringContext<'_, 'hir> {
             // We need to lower the declaration outside the new scope, because we
             // have to conserve the state of being inside a loop condition for the
             // closure argument types.
-            let fn_decl = lctx.lower_fn_decl(&outer_decl, None, FnDeclKind::Closure, None);
+            let fn_decl =
+                lctx.lower_fn_decl(&outer_decl, None, fn_decl_span, FnDeclKind::Closure, None);
 
             let c = lctx.arena.alloc(hir::Closure {
                 binder: binder_clause,

compiler/rustc_ast_lowering/src/item.rs

@@ -269,7 +269,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                     let mut itctx = ImplTraitContext::Universal;
                     let (generics, decl) = this.lower_generics(generics, id, &mut itctx, |this| {
                         let ret_id = asyncness.opt_return_id();
-                        this.lower_fn_decl(&decl, Some(id), FnDeclKind::Fn, ret_id)
+                        this.lower_fn_decl(&decl, Some(id), fn_sig_span, FnDeclKind::Fn, ret_id)
                     });
                     let sig = hir::FnSig {
                         decl,
@@ -661,7 +661,13 @@ impl<'hir> LoweringContext<'_, 'hir> {
                         self.lower_generics(generics, i.id, &mut itctx, |this| {
                             (
                                 // Disallow `impl Trait` in foreign items.
-                                this.lower_fn_decl(fdec, None, FnDeclKind::ExternFn, None),
+                                this.lower_fn_decl(
+                                    fdec,
+                                    None,
+                                    sig.span,
+                                    FnDeclKind::ExternFn,
+                                    None,
+                                ),
                                 this.lower_fn_params_to_names(fdec),
                             )
                         });
@@ -771,9 +777,15 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 (hir::Generics::empty(), hir::TraitItemKind::Const(ty, body), body.is_some())
             }
             AssocItemKind::Fn(box Fn { ref sig, ref generics, body: None, .. }) => {
+                let asyncness = sig.header.asyncness;
                 let names = self.lower_fn_params_to_names(&sig.decl);
-                let (generics, sig) =
-                    self.lower_method_sig(generics, sig, i.id, FnDeclKind::Trait, None);
+                let (generics, sig) = self.lower_method_sig(
+                    generics,
+                    sig,
+                    i.id,
+                    FnDeclKind::Trait,
+                    asyncness.opt_return_id(),
+                );
                 (generics, hir::TraitItemKind::Fn(sig, hir::TraitFn::Required(names)), false)
             }
             AssocItemKind::Fn(box Fn { ref sig, ref generics, body: Some(ref body), .. }) => {
@@ -1240,12 +1252,12 @@ impl<'hir> LoweringContext<'_, 'hir> {
         sig: &FnSig,
         id: NodeId,
         kind: FnDeclKind,
-        is_async: Option<NodeId>,
+        is_async: Option<(NodeId, Span)>,
     ) -> (&'hir hir::Generics<'hir>, hir::FnSig<'hir>) {
         let header = self.lower_fn_header(sig.header);
         let mut itctx = ImplTraitContext::Universal;
         let (generics, decl) = self.lower_generics(generics, id, &mut itctx, |this| {
-            this.lower_fn_decl(&sig.decl, Some(id), kind, is_async)
+            this.lower_fn_decl(&sig.decl, Some(id), sig.span, kind, is_async)
         });
         (generics, hir::FnSig { header, decl, span: self.lower_span(sig.span) })
     }