document -Zmiri-backtrace; dont print backtrace when there's only one frame

Author: RalfJung

README.md

@@ -292,6 +292,9 @@ Try running `cargo miri clean`.
 Miri adds its own set of `-Z` flags, which are usually set via the `MIRIFLAGS`
 environment variable. We first document the most relevant and most commonly used flags:
 
+* `-Zmiri-backtrace=<0|1|full>` configures how Miri prints backtraces: `1` is the default,
+  where backtraces are printed in pruned form; `full` prints backtraces without pruning, and `0`
+  disables backtraces entirely.
 * `-Zmiri-deterministic-concurrency` makes Miri's concurrency-related behavior fully deterministic.
   Strictly speaking, Miri is always fully deterministic when isolation is enabled (the default
   mode), but this determinism is achieved by using an RNG with a fixed seed. Seemingly harmless

src/diagnostics.rs

@@ -604,28 +604,30 @@ pub fn report_msg<'tcx>(
     }
 
     // Add backtrace
-    let mut backtrace_title = String::from("BACKTRACE");
-    if extra_span {
-        write!(backtrace_title, " (of the first span)").unwrap();
-    }
-    if let Some(thread) = thread {
-        let thread_name = machine.threads.get_thread_display_name(thread);
-        if thread_name != "main" {
-            // Only print thread name if it is not `main`.
-            write!(backtrace_title, " on thread `{thread_name}`").unwrap();
-        };
-    }
-    write!(backtrace_title, ":").unwrap();
-    err.note(backtrace_title);
-    for (idx, frame_info) in stacktrace.iter().enumerate() {
-        let is_local = machine.is_local(frame_info);
-        // No span for non-local frames and the first frame (which is the error site).
-        if is_local && idx > 0 {
-            err.subdiagnostic(frame_info.as_note(machine.tcx));
-        } else {
-            let sm = sess.source_map();
-            let span = sm.span_to_embeddable_string(frame_info.span);
-            err.note(format!("{frame_info} at {span}"));
+    if stacktrace.len() > 1 {
+        let mut backtrace_title = String::from("BACKTRACE");
+        if extra_span {
+            write!(backtrace_title, " (of the first span)").unwrap();
+        }
+        if let Some(thread) = thread {
+            let thread_name = machine.threads.get_thread_display_name(thread);
+            if thread_name != "main" {
+                // Only print thread name if it is not `main`.
+                write!(backtrace_title, " on thread `{thread_name}`").unwrap();
+            };
+        }
+        write!(backtrace_title, ":").unwrap();
+        err.note(backtrace_title);
+        for (idx, frame_info) in stacktrace.iter().enumerate() {
+            let is_local = machine.is_local(frame_info);
+            // No span for non-local frames and the first frame (which is the error site).
+            if is_local && idx > 0 {
+                err.subdiagnostic(frame_info.as_note(machine.tcx));
+            } else {
+                let sm = sess.source_map();
+                let span = sm.span_to_embeddable_string(frame_info.span);
+                err.note(format!("{frame_info} at {span}"));
+            }
         }
     }
 

tests/fail-dep/concurrency/apple_os_unfair_lock_assert_not_owner.stderr

@@ -3,9 +3,6 @@ error: abnormal termination: called os_unfair_lock_assert_not_owner on an os_unf
    |
 LL |         libc::os_unfair_lock_assert_not_owner(lock.get());
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ abnormal termination occurred here
-   |
-   = note: BACKTRACE:
-   = note: inside `main` at tests/fail-dep/concurrency/apple_os_unfair_lock_assert_not_owner.rs:LL:CC
 
 note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace
 

tests/fail-dep/concurrency/apple_os_unfair_lock_assert_owner.stderr

@@ -3,9 +3,6 @@ error: abnormal termination: called os_unfair_lock_assert_owner on an os_unfair_
    |
 LL |         libc::os_unfair_lock_assert_owner(lock.get());
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ abnormal termination occurred here
-   |
-   = note: BACKTRACE:
-   = note: inside `main` at tests/fail-dep/concurrency/apple_os_unfair_lock_assert_owner.rs:LL:CC
 
 note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace
 

tests/fail-dep/concurrency/apple_os_unfair_lock_move_deadlock.stderr

@@ -3,9 +3,6 @@ error: the evaluated program deadlocked
    |
 LL |     unsafe { libc::os_unfair_lock_lock(lock.get()) };
    |                                                  ^ this thread got stuck here
-   |
-   = note: BACKTRACE:
-   = note: inside `main` at tests/fail-dep/concurrency/apple_os_unfair_lock_move_deadlock.rs:LL:CC
 
 note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace
 

tests/fail-dep/concurrency/apple_os_unfair_lock_reentrant.stderr

@@ -3,9 +3,6 @@ error: abnormal termination: attempted to lock an os_unfair_lock that is already
    |
 LL |         libc::os_unfair_lock_lock(lock.get());
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ abnormal termination occurred here
-   |
-   = note: BACKTRACE:
-   = note: inside `main` at tests/fail-dep/concurrency/apple_os_unfair_lock_reentrant.rs:LL:CC
 
 note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace
 

tests/fail-dep/concurrency/apple_os_unfair_lock_unowned.stderr

@@ -3,9 +3,6 @@ error: abnormal termination: attempted to unlock an os_unfair_lock not owned by
    |
 LL |         libc::os_unfair_lock_unlock(lock.get());
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ abnormal termination occurred here
-   |
-   = note: BACKTRACE:
-   = note: inside `main` at tests/fail-dep/concurrency/apple_os_unfair_lock_unowned.rs:LL:CC
 
 note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace
 

tests/fail-dep/concurrency/libc_pthread_cond_double_destroy.stderr

@@ -6,8 +6,6 @@ LL |         libc::pthread_cond_destroy(cond.as_mut_ptr());
    |
    = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
    = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information
-   = note: BACKTRACE:
-   = note: inside `main` at tests/fail-dep/concurrency/libc_pthread_cond_double_destroy.rs:LL:CC
 
 Uninitialized memory occurred at ALLOC[0xX..0xY], in this allocation:
 ALLOC DUMP

tests/fail-dep/concurrency/libc_pthread_condattr_double_destroy.stderr

@@ -6,8 +6,6 @@ LL |         libc::pthread_condattr_destroy(attr.as_mut_ptr());
    |
    = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
    = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information
-   = note: BACKTRACE:
-   = note: inside `main` at tests/fail-dep/concurrency/libc_pthread_condattr_double_destroy.rs:LL:CC
 
 Uninitialized memory occurred at ALLOC[0xX..0xY], in this allocation:
 ALLOC DUMP

tests/fail-dep/concurrency/libc_pthread_create_too_few_args.stderr

@@ -4,7 +4,6 @@ error: Undefined Behavior: calling a function with more arguments than it expect
    = note: (no span available)
    = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
    = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information
-   = note: BACKTRACE on thread `unnamed-ID`:
 
 error: aborting due to 1 previous error