Make tests capture the error printed by a Result return

Author: dtolnay

library/std/src/io/mod.rs

@@ -262,6 +262,7 @@ use crate::sys_common::memchr;
 
 #[stable(feature = "bufwriter_into_parts", since = "1.56.0")]
 pub use self::buffered::WriterPanicked;
+pub(crate) use self::stdio::attempt_print_to_stderr;
 #[unstable(feature = "internal_output_capture", issue = "none")]
 #[doc(no_inline, hidden)]
 pub use self::stdio::set_output_capture;

library/std/src/io/stdio.rs

@@ -999,7 +999,18 @@ fn print_to<T>(args: fmt::Arguments<'_>, global_s: fn() -> T, label: &str)
 where
     T: Write,
 {
-    if OUTPUT_CAPTURE_USED.load(Ordering::Relaxed)
+    if print_to_buffer_if_capture_used(args) {
+        // Successfully wrote to capture buffer.
+        return;
+    }
+
+    if let Err(e) = global_s().write_fmt(args) {
+        panic!("failed printing to {label}: {e}");
+    }
+}
+
+fn print_to_buffer_if_capture_used(args: fmt::Arguments<'_>) -> bool {
+    OUTPUT_CAPTURE_USED.load(Ordering::Relaxed)
         && OUTPUT_CAPTURE.try_with(|s| {
             // Note that we completely remove a local sink to write to in case
             // our printing recursively panics/prints, so the recursive
@@ -1009,14 +1020,19 @@ where
                 s.set(Some(w));
             })
         }) == Ok(Some(()))
-    {
-        // Successfully wrote to capture buffer.
+}
+
+/// Used by impl Termination for Result to print error after `main` or a test
+/// has returned. Should avoid panicking, although we can't help it if one of
+/// the Display impls inside args decides to.
+pub(crate) fn attempt_print_to_stderr(args: fmt::Arguments<'_>) {
+    if print_to_buffer_if_capture_used(args) {
         return;
     }
 
-    if let Err(e) = global_s().write_fmt(args) {
-        panic!("failed printing to {label}: {e}");
-    }
+    // Ignore error if the write fails, for example because stderr is already
+    // closed. There is not much point panicking at this point.
+    let _ = stderr().write_fmt(args);
 }
 
 #[unstable(

library/std/src/process.rs

@@ -2200,9 +2200,7 @@ impl<T: Termination, E: fmt::Debug> Termination for Result<T, E> {
         match self {
             Ok(val) => val.report(),
             Err(err) => {
-                // Ignore error if the write fails, for example because stderr is
-                // already closed. There is not much point panicking at this point.
-                let _ = writeln!(io::stderr(), "Error: {err:?}");
+                io::attempt_print_to_stderr(format_args_nl!("Error: {err:?}"));
                 ExitCode::FAILURE
             }
         }