lazify backtrace formatting for delayed diagnostics

This defers backtrace formatting to the point where we
actually want to flush delayed diagnostics. If they are discarded
before that point then we can avoid invoking the backtrace formatting
machinery which will parse debug info and symbol tables.

for debuginfo=2 this leads to a 20% walltime reduction of the UI testsuite

compiler/rustc_errors/src/lib.rs

@@ -294,6 +294,7 @@ impl error::Error for ExplicitBug {}
 
 pub use diagnostic::{Diagnostic, DiagnosticId, DiagnosticStyledString, SubDiagnostic};
 pub use diagnostic_builder::DiagnosticBuilder;
+use std::backtrace::Backtrace;
 
 /// A handler deals with errors and other compiler output.
 /// Certain errors (fatal, bug, unimpl) may cause immediate exit,
@@ -317,7 +318,7 @@ struct HandlerInner {
     deduplicated_err_count: usize,
     emitter: Box<dyn Emitter + sync::Send>,
     delayed_span_bugs: Vec<Diagnostic>,
-    delayed_good_path_bugs: Vec<Diagnostic>,
+    delayed_good_path_bugs: Vec<DelayedDiagnostic>,
 
     /// This set contains the `DiagnosticId` of all emitted diagnostics to avoid
     /// emitting the same diagnostic with extended help (`--teach`) twice, which
@@ -388,7 +389,7 @@ impl Drop for HandlerInner {
         if !self.has_any_message() {
             let bugs = std::mem::replace(&mut self.delayed_good_path_bugs, Vec::new());
             self.flush_delayed(
-                bugs,
+                bugs.into_iter().map(DelayedDiagnostic::decorate).collect(),
                 "no warnings or errors encountered even though `delayed_good_path_bugs` issued",
             );
         }
@@ -968,12 +969,12 @@ impl HandlerInner {
     }
 
     fn delay_good_path_bug(&mut self, msg: &str) {
-        let mut diagnostic = Diagnostic::new(Level::Bug, msg);
+        let diagnostic = Diagnostic::new(Level::Bug, msg);
         if self.flags.report_delayed_bugs {
             self.emit_diagnostic(&diagnostic);
         }
-        diagnostic.note(&format!("delayed at {}", std::backtrace::Backtrace::force_capture()));
-        self.delayed_good_path_bugs.push(diagnostic);
+        let backtrace = std::backtrace::Backtrace::force_capture();
+        self.delayed_good_path_bugs.push(DelayedDiagnostic::with_backtrace(diagnostic, backtrace));
     }
 
     fn failure(&mut self, msg: &str) {
@@ -1042,6 +1043,22 @@ impl HandlerInner {
     }
 }
 
+struct DelayedDiagnostic {
+    inner: Diagnostic,
+    note: Backtrace,
+}
+
+impl DelayedDiagnostic {
+    fn with_backtrace(diagnostic: Diagnostic, backtrace: Backtrace) -> Self {
+        DelayedDiagnostic { inner: diagnostic, note: backtrace }
+    }
+
+    fn decorate(mut self) -> Diagnostic {
+        self.inner.note(&format!("delayed at {}", self.note));
+        self.inner
+    }
+}
+
 #[derive(Copy, PartialEq, Clone, Hash, Debug, Encodable, Decodable)]
 pub enum Level {
     Bug,