Reinstate `emit_stashed_diagnostics` in `DiagCtxtInner::drop`.

I removed it in #121206 because I thought thought it wasn't necessary.
But then I had to add an `emit_stashed_diagnostics` call elsewhere in
rustfmt to avoid the assertion failure (which took two attempts to get
right, #121487 and #121615), and now there's an assertion failure in
clippy as well (https://github.com/rust-lang/rust-clippy/issues/12364).

So this commit just reinstates the call in `DiagCtxtInner::drop`. It
also reverts the rustfmt changes from #121487 and #121615, though it
keeps the tests added for those PRs.

1 files changed, 6 insertions, 4 deletions

diff --git a/compiler/rustc_errors/src/lib.rs b/compiler/rustc_errors/src/lib.rs
index 60bda9f4c94..186ec41f7cd 100644
--- a/compiler/rustc_errors/src/lib.rs
+++ b/compiler/rustc_errors/src/lib.rs
@@ -555,12 +555,14 @@ pub struct DiagCtxtFlags {
 
 impl Drop for DiagCtxtInner {
     fn drop(&mut self) {
-        // Any stashed diagnostics should have been handled by
-        // `emit_stashed_diagnostics` by now.
+        // For tools using `interface::run_compiler` (e.g. rustc, rustdoc)
+        // stashed diagnostics will have already been emitted. But for others
+        // that don't use `interface::run_compiler` (e.g. rustfmt, some clippy
+        // lints) this fallback is necessary.
         //
         // Important: it is sound to produce an `ErrorGuaranteed` when stashing
-        // errors because they are guaranteed to have been emitted by here.
-        assert!(self.stashed_diagnostics.is_empty());
+        // errors because they are guaranteed to be emitted here or earlier.
+        self.emit_stashed_diagnostics();
 
         // Important: it is sound to produce an `ErrorGuaranteed` when emitting
         // delayed bugs because they are guaranteed to be emitted here if