Auto merge of #115864 - compiler-errors:rpitit-sugg, r=estebank

Suggest desugaring to return-position `impl Future` when an `async fn` in trait fails an auto trait bound

First commit allows us to store the span of the `async` keyword in HIR.

Second commit implements a suggestion to desugar an `async fn` to a return-position `impl Future` in trait to slightly improve the `Send` situation being discussed in #115822.

This suggestion is only made when `#![feature(return_type_notation)]` is not enabled -- if it is, we should instead suggest an appropriate where-clause bound.

Author: bors

compiler/rustc_ast_lowering/src/item.rs

@@ -1308,7 +1308,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
 
     fn lower_asyncness(&mut self, a: Async) -> hir::IsAsync {
         match a {
-            Async::Yes { .. } => hir::IsAsync::Async,
+            Async::Yes { span, .. } => hir::IsAsync::Async(span),
             Async::No => hir::IsAsync::NotAsync,
         }
     }

compiler/rustc_borrowck/src/diagnostics/region_name.rs

@@ -302,7 +302,7 @@ impl<'tcx> MirBorrowckCtxt<'_, 'tcx> {
                     if free_region.bound_region.is_named() {
                         // A named region that is actually named.
                         Some(RegionName { name, source: RegionNameSource::NamedFreeRegion(span) })
-                    } else if let hir::IsAsync::Async = tcx.asyncness(self.mir_hir_id().owner) {
+                    } else if tcx.asyncness(self.mir_hir_id().owner).is_async() {
                         // If we spuriously thought that the region is named, we should let the
                         // system generate a true name for error messages. Currently this can
                         // happen if we have an elided name in an async fn for example: the

compiler/rustc_hir/src/hir.rs

@@ -2853,13 +2853,13 @@ impl ImplicitSelfKind {
 #[derive(Copy, Clone, PartialEq, Eq, Encodable, Decodable, Debug)]
 #[derive(HashStable_Generic)]
 pub enum IsAsync {
-    Async,
+    Async(Span),
     NotAsync,
 }
 
 impl IsAsync {
     pub fn is_async(self) -> bool {
-        self == IsAsync::Async
+        matches!(self, IsAsync::Async(_))
     }
 }
 
@@ -3296,7 +3296,7 @@ pub struct FnHeader {
 
 impl FnHeader {
     pub fn is_async(&self) -> bool {
-        matches!(&self.asyncness, IsAsync::Async)
+        matches!(&self.asyncness, IsAsync::Async(_))
     }
 
     pub fn is_const(&self) -> bool {
@@ -4091,10 +4091,10 @@ mod size_asserts {
     static_assert_size!(GenericBound<'_>, 48);
     static_assert_size!(Generics<'_>, 56);
     static_assert_size!(Impl<'_>, 80);
-    static_assert_size!(ImplItem<'_>, 80);
-    static_assert_size!(ImplItemKind<'_>, 32);
-    static_assert_size!(Item<'_>, 80);
-    static_assert_size!(ItemKind<'_>, 48);
+    static_assert_size!(ImplItem<'_>, 88);
+    static_assert_size!(ImplItemKind<'_>, 40);
+    static_assert_size!(Item<'_>, 88);
+    static_assert_size!(ItemKind<'_>, 56);
     static_assert_size!(Local<'_>, 64);
     static_assert_size!(Param<'_>, 32);
     static_assert_size!(Pat<'_>, 72);
@@ -4105,8 +4105,8 @@ mod size_asserts {
     static_assert_size!(Res, 12);
     static_assert_size!(Stmt<'_>, 32);
     static_assert_size!(StmtKind<'_>, 16);
-    static_assert_size!(TraitItem<'_>, 80);
-    static_assert_size!(TraitItemKind<'_>, 40);
+    static_assert_size!(TraitItem<'_>, 88);
+    static_assert_size!(TraitItemKind<'_>, 48);
     static_assert_size!(Ty<'_>, 48);
     static_assert_size!(TyKind<'_>, 32);
     // tidy-alphabetical-end

compiler/rustc_hir_analysis/src/check/compare_impl_item.rs

@@ -595,7 +595,7 @@ fn compare_asyncness<'tcx>(
     trait_m: ty::AssocItem,
     delay: bool,
 ) -> Result<(), ErrorGuaranteed> {
-    if tcx.asyncness(trait_m.def_id) == hir::IsAsync::Async {
+    if tcx.asyncness(trait_m.def_id).is_async() {
         match tcx.fn_sig(impl_m.def_id).skip_binder().skip_binder().output().kind() {
             ty::Alias(ty::Opaque, ..) => {
                 // allow both `async fn foo()` and `fn foo() -> impl Future`

compiler/rustc_hir_analysis/src/check/entry.rs

@@ -112,7 +112,7 @@ fn check_main_fn_ty(tcx: TyCtxt<'_>, main_def_id: DefId) {
     }
 
     let main_asyncness = tcx.asyncness(main_def_id);
-    if let hir::IsAsync::Async = main_asyncness {
+    if main_asyncness.is_async() {
         let asyncness_span = main_fn_asyncness_span(tcx, main_def_id);
         tcx.sess.emit_err(errors::MainFunctionAsync { span: main_span, asyncness: asyncness_span });
         error = true;
@@ -212,7 +212,7 @@ fn check_start_fn_ty(tcx: TyCtxt<'_>, start_def_id: DefId) {
                         });
                         error = true;
                     }
-                    if let hir::IsAsync::Async = sig.header.asyncness {
+                    if sig.header.asyncness.is_async() {
                         let span = tcx.def_span(it.owner_id);
                         tcx.sess.emit_err(errors::StartAsync { span: span });
                         error = true;

compiler/rustc_hir_analysis/src/collect/resolve_bound_vars.rs

@@ -1213,7 +1213,7 @@ impl<'a, 'tcx> BoundVarContext<'a, 'tcx> {
                         && let Some(generics) = self.tcx.hir().get_generics(self.tcx.local_parent(param_id))
                         && let Some(param) = generics.params.iter().find(|p| p.def_id == param_id)
                         && param.is_elided_lifetime()
-                        && let hir::IsAsync::NotAsync = self.tcx.asyncness(lifetime_ref.hir_id.owner.def_id)
+                        && !self.tcx.asyncness(lifetime_ref.hir_id.owner.def_id).is_async()
                         && !self.tcx.features().anonymous_lifetime_in_impl_trait
                     {
                         let mut diag =  rustc_session::parse::feature_err(

compiler/rustc_hir_pretty/src/lib.rs

@@ -2304,7 +2304,7 @@ impl<'a> State<'a> {
 
         match header.asyncness {
             hir::IsAsync::NotAsync => {}
-            hir::IsAsync::Async => self.word_nbsp("async"),
+            hir::IsAsync::Async(_) => self.word_nbsp("async"),
         }
 
         self.print_unsafety(header.unsafety);

compiler/rustc_hir_typeck/src/fn_ctxt/suggestions.rs

@@ -987,10 +987,10 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             let bound_vars = self.tcx.late_bound_vars(fn_id);
             let ty = self.tcx.erase_late_bound_regions(Binder::bind_with_vars(ty, bound_vars));
             let ty = match self.tcx.asyncness(fn_id.owner) {
-                hir::IsAsync::Async => self.get_impl_future_output_ty(ty).unwrap_or_else(|| {
+                ty::Asyncness::Yes => self.get_impl_future_output_ty(ty).unwrap_or_else(|| {
                     span_bug!(fn_decl.output.span(), "failed to get output type of async function")
                 }),
-                hir::IsAsync::NotAsync => ty,
+                ty::Asyncness::No => ty,
             };
             let ty = self.normalize(expr.span, ty);
             if self.can_coerce(found, ty) {

compiler/rustc_lint/src/builtin.rs

@@ -41,7 +41,6 @@ use crate::{
     },
     EarlyContext, EarlyLintPass, LateContext, LateLintPass, Level, LintContext,
 };
-use hir::IsAsync;
 use rustc_ast::attr;
 use rustc_ast::tokenstream::{TokenStream, TokenTree};
 use rustc_ast::visit::{FnCtxt, FnKind};
@@ -1294,7 +1293,7 @@ impl<'tcx> LateLintPass<'tcx> for UngatedAsyncFnTrackCaller {
         span: Span,
         def_id: LocalDefId,
     ) {
-        if fn_kind.asyncness() == IsAsync::Async
+        if fn_kind.asyncness().is_async()
             && !cx.tcx.features().async_fn_track_caller
             // Now, check if the function has the `#[track_caller]` attribute
             && let Some(attr) = cx.tcx.get_attr(def_id, sym::track_caller)

compiler/rustc_metadata/src/rmeta/mod.rs

@@ -439,7 +439,7 @@ define_tables! {
     coerce_unsized_info: Table<DefIndex, LazyValue<ty::adjustment::CoerceUnsizedInfo>>,
     mir_const_qualif: Table<DefIndex, LazyValue<mir::ConstQualifs>>,
     rendered_const: Table<DefIndex, LazyValue<String>>,
-    asyncness: Table<DefIndex, hir::IsAsync>,
+    asyncness: Table<DefIndex, ty::Asyncness>,
     fn_arg_names: Table<DefIndex, LazyArray<Ident>>,
     generator_kind: Table<DefIndex, LazyValue<hir::GeneratorKind>>,
     trait_def: Table<DefIndex, LazyValue<ty::TraitDef>>,