Don't consider delayed bugs for -Ztreat-err-as-bug.

`-Ztreat-err-as-bug` treats normal errors and delayed bugs equally,
which can lead to some really surprising results.

This commit changes `-Ztreat-err-as-bug` so it ignores delayed bugs,
unless they get promoted to proper bugs and are printed.

This feels to me much simpler and more logical. And it simplifies the
implementation:
- The `-Ztreat-err-as-bug` check is removed from in
  `DiagCtxt::{delayed_bug,span_delayed_bug}`.
- `treat_err_as_bug` doesn't need to count delayed bugs.
- The `-Ztreat-err-as-bug` panic message is simpler, because it doesn't
  have to mention delayed bugs.

Output of delayed bugs is now more consistent. They're always printed
the same way. Previously when they triggered `-Ztreat-err-as-bug` they
would be printed slightly differently, via `span_bug` in
`span_delayed_bug` or `delayed_bug`.

A minor behaviour change: the "no errors encountered even though
`span_delayed_bug` issued" printed before delayed bugs is now a note
rather than a bug. This is done so it doesn't get counted as an error
that might trigger `-Ztreat-err-as-bug`, which would be silly.
This means that if you use `-Ztreat-err-as-bug=1` and there are no
normal errors but there are delayed bugs, the first delayed bug will be
shown (and the panic will happen after it's printed).

Also, I have added a second note saying "those delayed bugs will now be
shown as internal compiler errors". I think this makes it clearer what
is happening, because the whole concept of delayed bugs is non-obvious.

There are some test changes.
- equality-in-canonical-query.rs: Minor output changes, and the error
  count reduces by one because the "no errors encountered even though
  `span_delayed_bug` issued" message is no longer counted as an error.
- rpit_tait_equality_in_canonical_query.rs: Ditto.
- storage-live.rs: The query stack disappears because these delayed bugs
  are now printed at the end, rather than when they are created.
- storage-return.rs, span_delayed_bug.rs: now need
  `-Zeagerly-emit-delayed-bugs` because they need the delayed bugs
  emitted immediately to preserve behaviour.

compiler/rustc_errors/src/lib.rs

@@ -865,10 +865,6 @@ impl DiagCtxt {
     /// directly).
     #[track_caller]
     pub fn delayed_bug(&self, msg: impl Into<DiagnosticMessage>) -> ErrorGuaranteed {
-        let treat_next_err_as_bug = self.inner.borrow().treat_next_err_as_bug();
-        if treat_next_err_as_bug {
-            self.bug(msg);
-        }
         DiagnosticBuilder::<ErrorGuaranteed>::new(self, DelayedBug(DelayedBugKind::Normal), msg)
             .emit()
     }
@@ -883,10 +879,6 @@ impl DiagCtxt {
         sp: impl Into<MultiSpan>,
         msg: impl Into<DiagnosticMessage>,
     ) -> ErrorGuaranteed {
-        let treat_next_err_as_bug = self.inner.borrow().treat_next_err_as_bug();
-        if treat_next_err_as_bug {
-            self.span_bug(sp, msg);
-        }
         DiagnosticBuilder::<ErrorGuaranteed>::new(self, DelayedBug(DelayedBugKind::Normal), msg)
             .with_span(sp)
             .emit()
@@ -1259,10 +1251,6 @@ impl DiagCtxtInner {
     }
 
     fn emit_diagnostic(&mut self, mut diagnostic: Diagnostic) -> Option<ErrorGuaranteed> {
-        if matches!(diagnostic.level, Error | Fatal) && self.treat_next_err_as_bug() {
-            diagnostic.level = Bug;
-        }
-
         // The `LintExpectationId` can be stable or unstable depending on when it was created.
         // Diagnostics created before the definition of `HirId`s are unstable and can not yet
         // be stored. Instead, they are buffered until the `LintExpectationId` is replaced by
@@ -1298,6 +1286,12 @@ impl DiagCtxtInner {
             _ => {}
         }
 
+        // This must come after the possible promotion of `DelayedBug` to
+        // `Error` above.
+        if matches!(diagnostic.level, Error | Fatal) && self.treat_next_err_as_bug() {
+            diagnostic.level = Bug;
+        }
+
         if diagnostic.has_future_breakage() {
             // Future breakages aren't emitted if they're Level::Allow,
             // but they still need to be constructed and stashed below,
@@ -1387,20 +1381,14 @@ impl DiagCtxtInner {
     }
 
     fn treat_err_as_bug(&self) -> bool {
-        self.flags.treat_err_as_bug.is_some_and(|c| {
-            self.err_count + self.lint_err_count + self.delayed_bug_count() >= c.get()
-        })
+        self.flags.treat_err_as_bug.is_some_and(|c| self.err_count + self.lint_err_count >= c.get())
     }
 
     // Use this one before incrementing `err_count`.
     fn treat_next_err_as_bug(&self) -> bool {
-        self.flags.treat_err_as_bug.is_some_and(|c| {
-            self.err_count + self.lint_err_count + self.delayed_bug_count() + 1 >= c.get()
-        })
-    }
-
-    fn delayed_bug_count(&self) -> usize {
-        self.span_delayed_bugs.len() + self.good_path_delayed_bugs.len()
+        self.flags
+            .treat_err_as_bug
+            .is_some_and(|c| self.err_count + self.lint_err_count + 1 >= c.get())
     }
 
     fn has_errors(&self) -> bool {
@@ -1412,7 +1400,7 @@ impl DiagCtxtInner {
     }
 
     fn flush_delayed(&mut self, kind: DelayedBugKind) {
-        let (bugs, explanation) = match kind {
+        let (bugs, note1) = match kind {
             DelayedBugKind::Normal => (
                 std::mem::take(&mut self.span_delayed_bugs),
                 "no errors encountered even though `span_delayed_bug` issued",
@@ -1422,6 +1410,7 @@ impl DiagCtxtInner {
                 "no warnings or errors encountered even though `good_path_delayed_bugs` issued",
             ),
         };
+        let note2 = "those delayed bugs will now be shown as internal compiler errors";
 
         if bugs.is_empty() {
             return;
@@ -1447,8 +1436,11 @@ impl DiagCtxtInner {
 
             if i == 0 {
                 // Put the overall explanation before the `DelayedBug`s, to
-                // frame them better (e.g. separate warnings from them).
-                self.emit_diagnostic(Diagnostic::new(Bug, explanation));
+                // frame them better (e.g. separate warnings from them). Also,
+                // make it a note so it doesn't count as an error, because that
+                // could trigger `-Ztreat-err-as-bug`, which we don't want.
+                self.emit_diagnostic(Diagnostic::new(Note, note1));
+                self.emit_diagnostic(Diagnostic::new(Note, note2));
             }
 
             let mut bug =
@@ -1474,22 +1466,12 @@ impl DiagCtxtInner {
 
     fn panic_if_treat_err_as_bug(&self) {
         if self.treat_err_as_bug() {
-            match (
-                self.err_count + self.lint_err_count,
-                self.delayed_bug_count(),
-                self.flags.treat_err_as_bug.map(|c| c.get()).unwrap(),
-            ) {
-                (1, 0, 1) => panic!("aborting due to `-Z treat-err-as-bug=1`"),
-                (0, 1, 1) => panic!("aborting due delayed bug with `-Z treat-err-as-bug=1`"),
-                (count, delayed_count, val) => {
-                    if delayed_count > 0 {
-                        panic!(
-                            "aborting after {count} errors and {delayed_count} delayed bugs due to `-Z treat-err-as-bug={val}`",
-                        )
-                    } else {
-                        panic!("aborting after {count} errors due to `-Z treat-err-as-bug={val}`")
-                    }
-                }
+            let n = self.flags.treat_err_as_bug.map(|c| c.get()).unwrap();
+            assert_eq!(n, self.err_count + self.lint_err_count);
+            if n == 1 {
+                panic!("aborting due to `-Z treat-err-as-bug=1`");
+            } else {
+                panic!("aborting after {n} errors due to `-Z treat-err-as-bug={n}`");
             }
         }
     }