Overhaul how stashed diagnostics work, again.

Stashed errors used to be counted as errors, but could then be
cancelled, leading to `ErrorGuaranteed` soundness holes. #120828 changed
that, closing the soundness hole. But it introduced other difficulties
because you sometimes have to account for pending stashed errors when
making decisions about whether errors have occured/will occur and it's
easy to overlook these.

This commit aims for a middle ground.
- Stashed errors (not warnings) are counted immediately as emitted
  errors, avoiding the possibility of forgetting to consider them.
- The ability to cancel (or downgrade) stashed errors is eliminated, by
  disallowing the use of `steal_diagnostic` with errors, and introducing
  the more restrictive methods `try_steal_{modify,replace}_and_emit_err`
  that can be used instead.

Other things:
- `DiagnosticBuilder::stash` and `DiagCtxt::stash_diagnostic` now both
  return `Option<ErrorGuaranteed>`, which enables the removal of two
  `delayed_bug` calls and one `Ty::new_error_with_message` call. This is
  possible because we store error guarantees in
  `DiagCtxt::stashed_diagnostics`.
- Storing the guarantees also saves us having to maintain a counter.
- Calls to the `stashed_err_count` method are no longer necessary
  alongside calls to `has_errors`, which is a nice simplification, and
  eliminates two more `span_delayed_bug` calls and one FIXME comment.
- Tests are added for three of the four fixed PRs mentioned below.
- `issue-121108.rs`'s output improved slightly, omitting a non-useful
  error message.

Fixes #121451.
Fixes #121477.
Fixes #121504.
Fixes #121508.

compiler/rustc_infer/src/infer/at.rs

@@ -87,7 +87,6 @@ impl<'tcx> InferCtxt<'tcx> {
             reported_signature_mismatch: self.reported_signature_mismatch.clone(),
             tainted_by_errors: self.tainted_by_errors.clone(),
             err_count_on_creation: self.err_count_on_creation,
-            stashed_err_count_on_creation: self.stashed_err_count_on_creation,
             universe: self.universe.clone(),
             intercrate,
             next_trait_solver: self.next_trait_solver,

compiler/rustc_infer/src/infer/mod.rs

@@ -306,12 +306,6 @@ pub struct InferCtxt<'tcx> {
     // FIXME(matthewjasper) Merge into `tainted_by_errors`
     err_count_on_creation: usize,
 
-    /// Track how many errors were stashed when this infcx is created.
-    /// Used for the same purpose as `err_count_on_creation`, even
-    /// though it's weaker because the count can go up and down.
-    // FIXME(matthewjasper) Merge into `tainted_by_errors`
-    stashed_err_count_on_creation: usize,
-
     /// What is the innermost universe we have created? Starts out as
     /// `UniverseIndex::root()` but grows from there as we enter
     /// universal quantifiers.
@@ -717,7 +711,6 @@ impl<'tcx> InferCtxtBuilder<'tcx> {
             reported_signature_mismatch: Default::default(),
             tainted_by_errors: Cell::new(None),
             err_count_on_creation: tcx.dcx().err_count_excluding_lint_errs(),
-            stashed_err_count_on_creation: tcx.dcx().stashed_err_count(),
             universe: Cell::new(ty::UniverseIndex::ROOT),
             intercrate,
             next_trait_solver,
@@ -1274,14 +1267,6 @@ impl<'tcx> InferCtxt<'tcx> {
             let guar = self.dcx().has_errors().unwrap();
             self.set_tainted_by_errors(guar);
             Some(guar)
-        } else if self.dcx().stashed_err_count() > self.stashed_err_count_on_creation {
-            // Errors stashed since this infcx was made. Not entirely reliable
-            // because the count of stashed errors can go down. But without
-            // this case we get a moderate number of uninteresting and
-            // extraneous "type annotations needed" errors.
-            let guar = self.dcx().delayed_bug("tainted_by_errors: stashed bug awaiting emission");
-            self.set_tainted_by_errors(guar);
-            Some(guar)
         } else {
             None
         }