Make it into a structured suggestion, maybe-incorrect

compiler/rustc_lint/messages.ftl

@@ -189,6 +189,7 @@ lint_check_name_unknown_tool = unknown lint tool: `{$tool_name}`
 
 lint_closure_returning_async_block = closure returning async block can be made into an async closure
     .label = this async block can be removed, and the closure can be turned into an async closure
+    .suggestion = turn this into an async closure
 
 lint_command_line_source = `forbid` lint level was set on command line
 

compiler/rustc_lint/src/async_closures.rs

@@ -1,5 +1,5 @@
 use rustc_hir as hir;
-use rustc_macros::LintDiagnostic;
+use rustc_macros::{LintDiagnostic, Subdiagnostic};
 use rustc_session::{declare_lint, declare_lint_pass};
 use rustc_span::Span;
 
@@ -91,11 +91,19 @@ impl<'tcx> LateLintPass<'tcx> for AsyncClosureUsage {
             return;
         };
 
+        let deletion_span = cx.tcx.sess.source_map().span_extend_while_whitespace(async_decl_span);
+
         cx.tcx.emit_node_span_lint(
             CLOSURE_RETURNING_ASYNC_BLOCK,
             expr.hir_id,
             fn_decl_span,
-            ClosureReturningAsyncBlock { async_decl_span },
+            ClosureReturningAsyncBlock {
+                async_decl_span,
+                sugg: AsyncClosureSugg {
+                    deletion_span,
+                    insertion_span: fn_decl_span.shrink_to_lo(),
+                },
+            },
         );
     }
 }
@@ -105,4 +113,15 @@ impl<'tcx> LateLintPass<'tcx> for AsyncClosureUsage {
 struct ClosureReturningAsyncBlock {
     #[label]
     async_decl_span: Span,
+    #[subdiagnostic]
+    sugg: AsyncClosureSugg,
+}
+
+#[derive(Subdiagnostic)]
+#[multipart_suggestion(lint_suggestion, applicability = "maybe-incorrect")]
+struct AsyncClosureSugg {
+    #[suggestion_part(code = "")]
+    deletion_span: Span,
+    #[suggestion_part(code = "async ")]
+    insertion_span: Span,
 }

tests/ui/async-await/async-closures/lint-closure-returning-async-block.rs

@@ -0,0 +1,21 @@
+//@ edition: 2021
+
+#![feature(async_closure)]
+#![deny(closure_returning_async_block)]
+
+fn main() {
+    let x = || async {};
+    //~^ ERROR closure returning async block can be made into an async closure
+
+    let x = || async move {};
+    //~^ ERROR closure returning async block can be made into an async closure
+
+    let x = move || async move {};
+    //~^ ERROR closure returning async block can be made into an async closure
+
+    let x = move || async {};
+    //~^ ERROR closure returning async block can be made into an async closure
+
+    let x = || {{ async {} }};
+    //~^ ERROR closure returning async block can be made into an async closure
+}

tests/ui/async-await/async-closures/lint-closure-returning-async-block.stderr

@@ -0,0 +1,67 @@
+error: closure returning async block can be made into an async closure
+  --> $DIR/lint-closure-returning-async-block.rs:7:13
+   |
+LL |     let x = || async {};
+   |             ^^ ----- this async block can be removed, and the closure can be turned into an async closure
+   |
+note: the lint level is defined here
+  --> $DIR/lint-closure-returning-async-block.rs:4:9
+   |
+LL | #![deny(closure_returning_async_block)]
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+help: turn this into an async closure
+   |
+LL -     let x = || async {};
+LL +     let x = async || {};
+   |
+
+error: closure returning async block can be made into an async closure
+  --> $DIR/lint-closure-returning-async-block.rs:10:13
+   |
+LL |     let x = || async move {};
+   |             ^^ ---------- this async block can be removed, and the closure can be turned into an async closure
+   |
+help: turn this into an async closure
+   |
+LL -     let x = || async move {};
+LL +     let x = async || {};
+   |
+
+error: closure returning async block can be made into an async closure
+  --> $DIR/lint-closure-returning-async-block.rs:13:13
+   |
+LL |     let x = move || async move {};
+   |             ^^^^^^^ ---------- this async block can be removed, and the closure can be turned into an async closure
+   |
+help: turn this into an async closure
+   |
+LL -     let x = move || async move {};
+LL +     let x = async move || {};
+   |
+
+error: closure returning async block can be made into an async closure
+  --> $DIR/lint-closure-returning-async-block.rs:16:13
+   |
+LL |     let x = move || async {};
+   |             ^^^^^^^ ----- this async block can be removed, and the closure can be turned into an async closure
+   |
+help: turn this into an async closure
+   |
+LL -     let x = move || async {};
+LL +     let x = async move || {};
+   |
+
+error: closure returning async block can be made into an async closure
+  --> $DIR/lint-closure-returning-async-block.rs:19:13
+   |
+LL |     let x = || {{ async {} }};
+   |             ^^    ----- this async block can be removed, and the closure can be turned into an async closure
+   |
+help: turn this into an async closure
+   |
+LL -     let x = || {{ async {} }};
+LL +     let x = async || {{ {} }};
+   |
+
+error: aborting due to 5 previous errors
+