Only use special async fn case for actual async fns in borrowck diagnostics.

compiler/rustc_borrowck/src/diagnostics/region_name.rs

@@ -254,6 +254,7 @@ impl<'tcx> MirBorrowckCtxt<'_, 'tcx> {
     /// *user* has a name for. In that case, we'll be able to map
     /// `fr` to a `Region<'tcx>`, and that region will be one of
     /// named variants.
+    #[tracing::instrument(level = "trace", skip(self))]
     fn give_name_from_error_region(&self, fr: RegionVid) -> Option<RegionName> {
         let error_region = self.to_error_region(fr)?;
 
@@ -290,7 +291,7 @@ 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 {
+                    } else if let hir::IsAsync::Async = tcx.asyncness(self.mir_hir_id().owner) {
                         // 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
@@ -301,6 +302,8 @@ impl<'tcx> MirBorrowckCtxt<'_, 'tcx> {
                             name,
                             source: RegionNameSource::AnonRegionFromAsyncFn(span),
                         })
+                    } else {
+                        None
                     }
                 }
 
@@ -362,6 +365,7 @@ impl<'tcx> MirBorrowckCtxt<'_, 'tcx> {
     ///  | fn foo(x: &u32) { .. }
     ///           ------- fully elaborated type of `x` is `&'1 u32`
     /// ```
+    #[tracing::instrument(level = "trace", skip(self))]
     fn give_name_if_anonymous_region_appears_in_arguments(
         &self,
         fr: RegionVid,
@@ -651,6 +655,7 @@ impl<'tcx> MirBorrowckCtxt<'_, 'tcx> {
     ///  | let x = Some(&22);
     ///        - fully elaborated type of `x` is `Option<&'1 u32>`
     /// ```
+    #[tracing::instrument(level = "trace", skip(self))]
     fn give_name_if_anonymous_region_appears_in_upvars(&self, fr: RegionVid) -> Option<RegionName> {
         let upvar_index = self.regioncx.get_upvar_index_for_region(self.infcx.tcx, fr)?;
         let (upvar_name, upvar_span) = self.regioncx.get_upvar_name_and_span_for_region(
@@ -670,6 +675,7 @@ impl<'tcx> MirBorrowckCtxt<'_, 'tcx> {
     /// must be a closure since, in a free fn, such an argument would
     /// have to either also appear in an argument (if using elision)
     /// or be early bound (named, not in argument).
+    #[tracing::instrument(level = "trace", skip(self))]
     fn give_name_if_anonymous_region_appears_in_output(&self, fr: RegionVid) -> Option<RegionName> {
         let tcx = self.infcx.tcx;
         let hir = tcx.hir();
@@ -801,6 +807,7 @@ impl<'tcx> MirBorrowckCtxt<'_, 'tcx> {
         }
     }
 
+    #[tracing::instrument(level = "trace", skip(self))]
     fn give_name_if_anonymous_region_appears_in_yield_ty(
         &self,
         fr: RegionVid,

src/test/ui/issues/issue-37884.stderr

@@ -11,7 +11,7 @@ LL | |     }
    |
    = note: expected fn pointer `fn(&mut RepeatMut<'a, T>) -> Option<_>`
               found fn pointer `fn(&'a mut RepeatMut<'a, T>) -> Option<_>`
-note: the anonymous lifetime as defined here...
+note: the anonymous lifetime #1 defined here...
   --> $DIR/issue-37884.rs:6:5
    |
 LL |     fn next(&'a mut self) -> Option<Self::Item>