Only force backtraces when a panic occurs in bootstrap

src/bootstrap/src/bin/main.rs

@@ -9,6 +9,7 @@ use std::fs::{self, OpenOptions, TryLockError};
 use std::io::{self, BufRead, BufReader, IsTerminal, Read, Write};
 use std::path::Path;
 use std::str::FromStr;
+use std::sync::Once;
 use std::time::Instant;
 use std::{env, process};
 
@@ -27,16 +28,23 @@ fn main() {
 
     let _start_time = Instant::now();
 
-    // Always print backtraces to provide richer errors, to help debug hard-to-reproduce panics
-    // when the user didn't specify RUST_BACKTRACE
-    if std::env::var("RUST_BACKTRACE").is_err() {
-        unsafe {
-            std::env::set_var("RUST_BACKTRACE", "1");
-        }
-    }
-
     let default_panic_hook = std::panic::take_hook();
     std::panic::set_hook(Box::new(move |info| {
+        static BACKTRACE_LOCK: Once = Once::new();
+
+        // Always print backtraces to provide richer errors, to help debug hard-to-reproduce panics
+        // when the user didn't specify RUST_BACKTRACE
+        // Note that we only override this variable in the panic handler, because bootstrap might
+        // manually capture backtraces when a command is executed, and in that case we do not want
+        // to always force backtraces.
+        BACKTRACE_LOCK.call_once(|| {
+            if std::env::var("RUST_BACKTRACE").is_err() {
+                unsafe {
+                    std::env::set_var("RUST_BACKTRACE", "1");
+                }
+            }
+        });
+
         default_panic_hook(info);
         StepStack::with_current(|stack| {
             eprintln!("\nBootstrap has panicked, currently active steps:");