Add explanatory note to async block type mismatch error

Author: gurry

compiler/rustc_infer/src/infer/error_reporting/note_and_explain.rs

@@ -32,6 +32,12 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
                         diag.note("no two closures, even if identical, have the same type");
                         diag.help("consider boxing your closure and/or using it as a trait object");
                     }
+                    (ty::Coroutine(def_id1, ..), ty::Coroutine(def_id2, ..))
+                        if self.tcx.coroutine_is_async(def_id1)
+                            && self.tcx.coroutine_is_async(def_id2) =>
+                    {
+                        diag.note("every async block has a different type even when the output type is the same");
+                    }
                     (ty::Alias(ty::Opaque, ..), ty::Alias(ty::Opaque, ..)) => {
                         // Issue #63167
                         diag.note("distinct uses of `impl Trait` result in different opaque types");

tests/ui/async-await/coroutine-desc.stderr

@@ -10,6 +10,7 @@ LL |     fun(async {}, async {});
    |
    = note: expected `async` block `{async block@$DIR/coroutine-desc.rs:10:9: 10:17}`
               found `async` block `{async block@$DIR/coroutine-desc.rs:10:19: 10:27}`
+   = note: every async block has a different type even when the output type is the same
 note: function defined here
   --> $DIR/coroutine-desc.rs:8:4
    |
@@ -51,6 +52,7 @@ LL |     fun((async || {})(), (async || {})());
    |
    = note: expected `async` closure body `{async closure body@$DIR/coroutine-desc.rs:14:19: 14:21}`
               found `async` closure body `{async closure body@$DIR/coroutine-desc.rs:14:36: 14:38}`
+   = note: every async block has a different type even when the output type is the same
 note: function defined here
   --> $DIR/coroutine-desc.rs:8:4
    |