Optimize delayed_bug handling.

Once we have emitted at least one error, delayed bugs won't be used. So
we can (a) we can (a) discard any existing delayed bugs, and (b) stop
recording any new delayed bugs.

This eliminates a longstanding `FIXME` comment. There should be no
soundness issues because it's not possible to un-emit an error.

compiler/rustc_errors/src/lib.rs

@@ -1275,6 +1275,9 @@ impl DiagCtxtInner {
             self.future_breakage_diagnostics.push(diagnostic.clone());
         }
 
+        // Note that because this comes before the `match` below,
+        // `-Zeagerly-emit-delayed-bugs` continues to work even after we've
+        // issued an error and stopped recording new delayed bugs.
         if diagnostic.level == DelayedBug && self.flags.eagerly_emit_delayed_bugs {
             diagnostic.level = Error;
         }
@@ -1286,18 +1289,20 @@ impl DiagCtxtInner {
                 diagnostic.level = Bug;
             }
             DelayedBug => {
-                // FIXME(eddyb) this should check for `has_errors` and stop pushing
-                // once *any* errors were emitted (and truncate `delayed_bugs`
-                // when an error is first emitted, also), but maybe there's a case
-                // in which that's not sound? otherwise this is really inefficient.
-                let backtrace = std::backtrace::Backtrace::capture();
-                // This `unchecked_error_guaranteed` is valid. It is where the
-                // `ErrorGuaranteed` for delayed bugs originates.
-                #[allow(deprecated)]
-                let guar = ErrorGuaranteed::unchecked_error_guaranteed();
-                self.delayed_bugs
-                    .push((DelayedDiagnostic::with_backtrace(diagnostic, backtrace), guar));
-                return Some(guar);
+                // If we have already emitted at least one error, we don't need
+                // to record the delayed bug, because it'll never be used.
+                return if let Some(guar) = self.has_errors_or_lint_errors() {
+                    Some(guar)
+                } else {
+                    let backtrace = std::backtrace::Backtrace::capture();
+                    // This `unchecked_error_guaranteed` is valid. It is where the
+                    // `ErrorGuaranteed` for delayed bugs originates.
+                    #[allow(deprecated)]
+                    let guar = ErrorGuaranteed::unchecked_error_guaranteed();
+                    self.delayed_bugs
+                        .push((DelayedDiagnostic::with_backtrace(diagnostic, backtrace), guar));
+                    Some(guar)
+                };
             }
             Warning if !self.flags.can_emit_warnings => {
                 if diagnostic.has_future_breakage() {
@@ -1363,6 +1368,16 @@ impl DiagCtxtInner {
             }
 
             if is_error {
+                // If we have any delayed bugs recorded, we can discard them
+                // because they won't be used. (This should only occur if there
+                // have been no errors previously emitted, because we don't add
+                // new delayed bugs once the first error is emitted.)
+                if !self.delayed_bugs.is_empty() {
+                    assert_eq!(self.lint_err_guars.len() + self.err_guars.len(), 0);
+                    self.delayed_bugs.clear();
+                    self.delayed_bugs.shrink_to_fit();
+                }
+
                 // This `unchecked_error_guaranteed` is valid. It is where the
                 // `ErrorGuaranteed` for errors and lint errors originates.
                 #[allow(deprecated)]