3 files changed, 38 insertions, 23 deletions
diff --git a/compiler/rustc_errors/src/diagnostic.rs b/compiler/rustc_errors/src/diagnostic.rs
index 1763c355069..03717a4d654 100644
--- a/compiler/rustc_errors/src/diagnostic.rs
+++ b/compiler/rustc_errors/src/diagnostic.rs
@@ -77,11 +77,12 @@ where
 
     /// Add a subdiagnostic to an existing diagnostic where `f` is invoked on every message used
     /// (to optionally perform eager translation).
-    fn add_to_diagnostic_with<F>(self, diag: &mut Diagnostic, f: F)
-    where
-        F: Fn(&mut Diagnostic, SubdiagnosticMessage) -> SubdiagnosticMessage;
+    fn add_to_diagnostic_with<F: SubdiagnosticMessageOp>(self, diag: &mut Diagnostic, f: F);
 }
 
+pub trait SubdiagnosticMessageOp =
+    Fn(&mut Diagnostic, SubdiagnosticMessage) -> SubdiagnosticMessage;
+
 /// Trait implemented by lint types. This should not be implemented manually. Instead, use
 /// `#[derive(LintDiagnostic)]` -- see [rustc_macros::LintDiagnostic].
 #[rustc_diagnostic_item = "DecorateLint"]
diff --git a/compiler/rustc_errors/src/diagnostic_impls.rs b/compiler/rustc_errors/src/diagnostic_impls.rs
index 15effd3cbec..e936ebc7185 100644
--- a/compiler/rustc_errors/src/diagnostic_impls.rs
+++ b/compiler/rustc_errors/src/diagnostic_impls.rs
@@ -2,7 +2,7 @@ use crate::diagnostic::DiagnosticLocation;
 use crate::{fluent_generated as fluent, AddToDiagnostic};
 use crate::{
     DiagCtxt, DiagnosticArgValue, DiagnosticBuilder, EmissionGuarantee, ErrCode, IntoDiagnostic,
-    IntoDiagnosticArg, Level,
+    IntoDiagnosticArg, Level, SubdiagnosticMessageOp,
 };
 use rustc_ast as ast;
 use rustc_ast_pretty::pprust;
@@ -299,7 +299,7 @@ pub struct SingleLabelManySpans {
     pub label: &'static str,
 }
 impl AddToDiagnostic for SingleLabelManySpans {
-    fn add_to_diagnostic_with<F>(self, diag: &mut crate::Diagnostic, _: F) {
+    fn add_to_diagnostic_with<F: SubdiagnosticMessageOp>(self, diag: &mut crate::Diagnostic, _: F) {
         diag.span_labels(self.spans, self.label);
     }
 }
diff --git a/compiler/rustc_errors/src/lib.rs b/compiler/rustc_errors/src/lib.rs
index ec5029e505f..7216fa8f5e4 100644
--- a/compiler/rustc_errors/src/lib.rs
+++ b/compiler/rustc_errors/src/lib.rs
@@ -5,6 +5,8 @@
 // tidy-alphabetical-start
 #![allow(incomplete_features)]
 #![allow(internal_features)]
+#![allow(rustc::diagnostic_outside_of_impl)]
+#![allow(rustc::untranslatable_diagnostic)]
 #![doc(html_root_url = "https://doc.rust-lang.org/nightly/nightly-rustc/")]
 #![doc(rust_logo)]
 #![feature(array_windows)]
@@ -19,6 +21,7 @@
 #![feature(never_type)]
 #![feature(rustc_attrs)]
 #![feature(rustdoc_internals)]
+#![feature(trait_alias)]
 #![feature(try_blocks)]
 #![feature(yeet_expr)]
 // tidy-alphabetical-end
@@ -35,6 +38,7 @@ pub use codes::*;
 pub use diagnostic::{
     AddToDiagnostic, DecorateLint, Diagnostic, DiagnosticArg, DiagnosticArgName,
     DiagnosticArgValue, DiagnosticStyledString, IntoDiagnosticArg, StringPart, SubDiagnostic,
+    SubdiagnosticMessageOp,
 };
 pub use diagnostic_builder::{
     BugAbort, DiagnosticBuilder, EmissionGuarantee, FatalAbort, IntoDiagnostic,
@@ -429,6 +433,10 @@ struct DiagCtxtInner {
     /// The number of non-lint errors that have been emitted, including duplicates.
     err_count: usize,
 
+    /// The number of stashed errors. Unlike the other counts, this can go up
+    /// and down, so it doesn't guarantee anything.
+    stashed_err_count: usize,
+
     /// The error count shown to the user at the end.
     deduplicated_err_count: usize,
     /// The warning count shown to the user at the end.
@@ -598,6 +606,7 @@ impl DiagCtxt {
                 flags: DiagCtxtFlags { can_emit_warnings: true, ..Default::default() },
                 lint_err_count: 0,
                 err_count: 0,
+                stashed_err_count: 0,
                 deduplicated_err_count: 0,
                 deduplicated_warn_count: 0,
                 has_printed: false,
@@ -654,6 +663,7 @@ impl DiagCtxt {
         let mut inner = self.inner.borrow_mut();
         inner.lint_err_count = 0;
         inner.err_count = 0;
+        inner.stashed_err_count = 0;
         inner.deduplicated_err_count = 0;
         inner.deduplicated_warn_count = 0;
         inner.has_printed = false;
@@ -675,10 +685,8 @@ impl DiagCtxt {
         let key = (span.with_parent(None), key);
 
         if diag.is_error() {
-            if diag.is_lint.is_some() {
-                inner.lint_err_count += 1;
-            } else {
-                inner.err_count += 1;
+            if diag.is_lint.is_none() {
+                inner.stashed_err_count += 1;
             }
         }
 
@@ -694,10 +702,8 @@ impl DiagCtxt {
         let key = (span.with_parent(None), key);
         let diag = inner.stashed_diagnostics.remove(&key)?;
         if diag.is_error() {
-            if diag.is_lint.is_some() {
-                inner.lint_err_count -= 1;
-            } else {
-                inner.err_count -= 1;
+            if diag.is_lint.is_none() {
+                inner.stashed_err_count -= 1;
             }
         }
         Some(DiagnosticBuilder::new_diagnostic(self, diag))
@@ -845,6 +851,7 @@ impl DiagCtxt {
         self.struct_span_warn(span, msg).emit()
     }
 
+    #[track_caller]
     pub fn span_bug(&self, span: impl Into<MultiSpan>, msg: impl Into<DiagnosticMessage>) -> ! {
         self.struct_span_bug(span, msg).emit()
     }
@@ -922,13 +929,22 @@ impl DiagCtxt {
         self.struct_bug(msg).emit()
     }
 
-    /// This excludes lint errors and delayed bugs.
+    /// This excludes lint errors, delayed bugs, and stashed errors.
     #[inline]
     pub fn err_count(&self) -> usize {
         self.inner.borrow().err_count
     }
 
-    /// This excludes lint errors and delayed bugs.
+    /// This excludes normal errors, lint errors and delayed bugs. Unless
+    /// absolutely necessary, avoid using this. It's dubious because stashed
+    /// errors can later be cancelled, so the presence of a stashed error at
+    /// some point of time doesn't guarantee anything -- there are no
+    /// `ErrorGuaranteed`s here.
+    pub fn stashed_err_count(&self) -> usize {
+        self.inner.borrow().stashed_err_count
+    }
+
+    /// This excludes lint errors, delayed bugs, and stashed errors.
     pub fn has_errors(&self) -> Option<ErrorGuaranteed> {
         self.inner.borrow().has_errors().then(|| {
             // FIXME(nnethercote) find a way to store an `ErrorGuaranteed`.
@@ -937,8 +953,8 @@ impl DiagCtxt {
         })
     }
 
-    /// This excludes delayed bugs. Unless absolutely necessary, prefer
-    /// `has_errors` to this method.
+    /// This excludes delayed bugs and stashed errors. Unless absolutely
+    /// necessary, prefer `has_errors` to this method.
     pub fn has_errors_or_lint_errors(&self) -> Option<ErrorGuaranteed> {
         let inner = self.inner.borrow();
         let result = inner.has_errors() || inner.lint_err_count > 0;
@@ -949,8 +965,8 @@ impl DiagCtxt {
         })
     }
 
-    /// Unless absolutely necessary, prefer `has_errors` or
-    /// `has_errors_or_lint_errors` to this method.
+    /// This excludes stashed errors. Unless absolutely necessary, prefer
+    /// `has_errors` or `has_errors_or_lint_errors` to this method.
     pub fn has_errors_or_lint_errors_or_delayed_bugs(&self) -> Option<ErrorGuaranteed> {
         let inner = self.inner.borrow();
         let result =
@@ -1224,10 +1240,8 @@ impl DiagCtxtInner {
         for (_, diag) in std::mem::take(&mut self.stashed_diagnostics).into_iter() {
             // Decrement the count tracking the stash; emitting will increment it.
             if diag.is_error() {
-                if diag.is_lint.is_some() {
-                    self.lint_err_count -= 1;
-                } else {
-                    self.err_count -= 1;
+                if diag.is_lint.is_none() {
+                    self.stashed_err_count -= 1;
                 }
             } else {
                 // Unless they're forced, don't flush stashed warnings when