From 70980929b480d6c13e23d79ad8de2ff0afbce60c Mon Sep 17 00:00:00 2001
From: Nilstrieb <48135649+Nilstrieb@users.noreply.github.com>
Date: Tue, 6 Jun 2023 19:24:33 +0000
Subject: [PATCH] Respect `RUST_BACKTRACE` for delayed bugs

Sometimes, especially with MIR validation, the backtraces from delayed
bugs are noise and make it harder to look at them. Respect the
environment variable and don't print it when the user doesn't want it.
---
 compiler/rustc_errors/src/lib.rs | 18 ++++++++++++++----
 1 file changed, 14 insertions(+), 4 deletions(-)

diff --git a/compiler/rustc_errors/src/lib.rs b/compiler/rustc_errors/src/lib.rs
index bf77ed81f9bd0..7a297ea0d5f82 100644
--- a/compiler/rustc_errors/src/lib.rs
+++ b/compiler/rustc_errors/src/lib.rs
@@ -383,7 +383,7 @@ pub use diagnostic_builder::{DiagnosticBuilder, EmissionGuarantee, Noted};
 pub use diagnostic_impls::{
     DiagnosticArgFromDisplay, DiagnosticSymbolList, LabelKind, SingleLabelManySpans,
 };
-use std::backtrace::Backtrace;
+use std::backtrace::{Backtrace, BacktraceStatus};
 
 /// A handler deals with errors and other compiler output.
 /// Certain errors (fatal, bug, unimpl) may cause immediate exit,
@@ -1331,7 +1331,7 @@ impl HandlerInner {
             // once *any* errors were emitted (and truncate `delayed_span_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::force_capture();
+            let backtrace = std::backtrace::Backtrace::capture();
             self.delayed_span_bugs
                 .push(DelayedDiagnostic::with_backtrace(diagnostic.clone(), backtrace));
 
@@ -1620,7 +1620,7 @@ impl HandlerInner {
         if self.flags.report_delayed_bugs {
             self.emit_diagnostic(&mut diagnostic);
         }
-        let backtrace = std::backtrace::Backtrace::force_capture();
+        let backtrace = std::backtrace::Backtrace::capture();
         self.delayed_good_path_bugs.push(DelayedDiagnostic::with_backtrace(diagnostic, backtrace));
     }
 
@@ -1739,7 +1739,17 @@ impl DelayedDiagnostic {
     }
 
     fn decorate(mut self) -> Diagnostic {
-        self.inner.note(format!("delayed at {}\n{}", self.inner.emitted_at, self.note));
+        match self.note.status() {
+            BacktraceStatus::Captured => {
+                self.inner.note(format!("delayed at {}\n{}", self.inner.emitted_at, self.note));
+            }
+            // Avoid the needless newline when no backtrace has been captured,
+            // the display impl should just be a single line.
+            _ => {
+                self.inner.note(format!("delayed at {} - {}", self.inner.emitted_at, self.note));
+            }
+        }
+
         self.inner
     }
 }