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

clippy_lints/src/redundant_closure_call.rs

@@ -10,6 +10,7 @@ use rustc_hir::intravisit::{Visitor as HirVisitor, Visitor};
 use rustc_lint::{LateContext, LateLintPass};
 use rustc_middle::hir::nested_filter;
 use rustc_middle::lint::in_external_macro;
+use rustc_middle::ty;
 use rustc_session::{declare_lint_pass, declare_tool_lint};
 
 declare_clippy_lint! {
@@ -84,7 +85,7 @@ fn find_innermost_closure<'tcx>(
     cx: &LateContext<'tcx>,
     mut expr: &'tcx hir::Expr<'tcx>,
     mut steps: usize,
-) -> Option<(&'tcx hir::Expr<'tcx>, &'tcx hir::FnDecl<'tcx>, hir::IsAsync)> {
+) -> Option<(&'tcx hir::Expr<'tcx>, &'tcx hir::FnDecl<'tcx>, ty::Asyncness)> {
     let mut data = None;
 
     while let hir::ExprKind::Closure(closure) = expr.kind
@@ -98,9 +99,9 @@ fn find_innermost_closure<'tcx>(
     {
         expr = body.value;
         data = Some((body.value, closure.fn_decl, if is_async_closure(body) {
-            hir::IsAsync::Async
+            ty::Asyncness::Yes
         } else {
-            hir::IsAsync::NotAsync
+            ty::Asyncness::No
         }));
         steps -= 1;
     }

clippy_utils/src/lib.rs

@@ -90,7 +90,7 @@ use rustc_hir::intravisit::{walk_expr, FnKind, Visitor};
 use rustc_hir::LangItem::{OptionNone, OptionSome, ResultErr, ResultOk};
 use rustc_hir::{
     self as hir, def, Arm, ArrayLen, BindingAnnotation, Block, BlockCheckMode, Body, Closure, Destination, Expr,
-    ExprField, ExprKind, FnDecl, FnRetTy, GenericArgs, HirId, Impl, ImplItem, ImplItemKind, ImplItemRef, IsAsync, Item,
+    ExprField, ExprKind, FnDecl, FnRetTy, GenericArgs, HirId, Impl, ImplItem, ImplItemKind, ImplItemRef, Item,
     ItemKind, LangItem, Local, MatchSource, Mutability, Node, OwnerId, Param, Pat, PatKind, Path, PathSegment, PrimTy,
     QPath, Stmt, StmtKind, TraitItem, TraitItemKind, TraitItemRef, TraitRef, TyKind, UnOp,
 };
@@ -1958,8 +1958,8 @@ pub fn if_sequence<'tcx>(mut expr: &'tcx Expr<'tcx>) -> (Vec<&'tcx Expr<'tcx>>,
 /// Checks if the given function kind is an async function.
 pub fn is_async_fn(kind: FnKind<'_>) -> bool {
     match kind {
-        FnKind::ItemFn(_, _, header) => header.asyncness == IsAsync::Async,
-        FnKind::Method(_, sig) => sig.header.asyncness == IsAsync::Async,
+        FnKind::ItemFn(_, _, header) => header.asyncness .is_async(),
+        FnKind::Method(_, sig) => sig.header.asyncness.is_async(),
         FnKind::Closure => false,
     }
 }