Rollup merge of #121669 - nnethercote:count-stashed-errs-again, r=estebank

Count stashed errors again

Stashed diagnostics are such a pain. Their "might be emitted, might not" semantics messes with lots of things.

#120828 and #121206 made some big changes to how they work, improving some things, but still leaving some problems, as seen by the issues caused by #121206. This PR aims to fix all of them by restricting them in a way that eliminates the "might be emitted, might not" semantics while still allowing 98% of their benefit. Details in the individual commit logs.

r? `@oli-obk`

compiler/rustc_middle/src/ty/mod.rs

@@ -847,13 +847,15 @@ impl<'tcx> OpaqueHiddenType<'tcx> {
         opaque_def_id: LocalDefId,
         tcx: TyCtxt<'tcx>,
     ) -> Result<Diag<'tcx>, ErrorGuaranteed> {
-        if let Some(diag) = tcx
-            .sess
-            .dcx()
-            .steal_diagnostic(tcx.def_span(opaque_def_id), StashKey::OpaqueHiddenTypeMismatch)
-        {
-            diag.cancel();
-        }
+        // We used to cancel here for slightly better error messages, but
+        // cancelling stashed diagnostics is no longer allowed because it
+        // causes problems when tracking whether errors have actually
+        // occurred.
+        tcx.sess.dcx().try_steal_modify_and_emit_err(
+            tcx.def_span(opaque_def_id),
+            StashKey::OpaqueHiddenTypeMismatch,
+            |_err| {},
+        );
         (self.ty, other.ty).error_reported()?;
         // Found different concrete types for the opaque type.
         let sub_diag = if self.span == other.span {

compiler/rustc_middle/src/ty/region.rs

@@ -91,8 +91,8 @@ impl<'tcx> Region<'tcx> {
 
     /// Constructs a `RegionKind::ReError` region.
     #[track_caller]
-    pub fn new_error(tcx: TyCtxt<'tcx>, reported: ErrorGuaranteed) -> Region<'tcx> {
-        tcx.intern_region(ty::ReError(reported))
+    pub fn new_error(tcx: TyCtxt<'tcx>, guar: ErrorGuaranteed) -> Region<'tcx> {
+        tcx.intern_region(ty::ReError(guar))
     }
 
     /// Constructs a `RegionKind::ReError` region and registers a delayed bug to ensure it gets

compiler/rustc_middle/src/ty/sty.rs

@@ -1528,8 +1528,8 @@ impl<'tcx> Ty<'tcx> {
     }
 
     /// Constructs a `TyKind::Error` type with current `ErrorGuaranteed`
-    pub fn new_error(tcx: TyCtxt<'tcx>, reported: ErrorGuaranteed) -> Ty<'tcx> {
-        Ty::new(tcx, Error(reported))
+    pub fn new_error(tcx: TyCtxt<'tcx>, guar: ErrorGuaranteed) -> Ty<'tcx> {
+        Ty::new(tcx, Error(guar))
     }
 
     /// Constructs a `TyKind::Error` type and registers a `span_delayed_bug` to ensure it gets used.