Auto merge of #93101 - Mark-Simulacrum:library-backtrace, r=yaahc

Support configuring whether to capture backtraces at runtime

Tracking issue: https://github.com/rust-lang/rust/issues/93346

This adds a new API to the `std::panic` module which configures whether and how the default panic hook will emit a backtrace when a panic occurs.

After discussion with `@yaahc` on [Zulip](https://rust-lang.zulipchat.com/#narrow/stream/219381-t-libs/topic/backtrace.20lib.20vs.2E.20panic), this PR chooses to avoid adjusting or seeking to provide a similar API for the (currently unstable) std::backtrace API. It seems likely that the users of that API may wish to expose more specific settings rather than just a global one (e.g., emulating the `env_logger`, `tracing` per-module configuration) to avoid the cost of capture in hot code. The API added here could plausibly be copied and/or re-exported directly from std::backtrace relatively easily, but I don't think that's the right call as of now.

```rust
mod panic {
    #[derive(Copy, Clone, Debug, PartialEq, Eq)]
    #[non_exhaustive]
    pub enum BacktraceStyle {
        Short,
        Full,
        Off,
    }
    fn set_backtrace_style(BacktraceStyle);
    fn get_backtrace_style() -> Option<BacktraceStyle>;
}
```

Several unresolved questions:

* Do we need to move to a thread-local or otherwise more customizable strategy for whether to capture backtraces? See [this comment](https://github.com/rust-lang/rust/pull/79085#issuecomment-727845826) for some potential use cases for this.
   * Proposed answer: no, leave this for third-party hooks.
* Bikeshed on naming of all the options, as usual.
* Should BacktraceStyle be moved into `std::backtrace`?
   * It's already somewhat annoying to import and/or re-type the `std::panic::` prefix necessary to use these APIs, probably adding a second module to the mix isn't worth it.

Note that PR #79085 proposed a much simpler API, but particularly in light of the desire to fully replace setting environment variables via `env::set_var` to control the backtrace API, a more complete API seems preferable. This PR likely subsumes that one.

4 files changed, 36 insertions, 0 deletions

diff --git a/src/test/ui/panics/runtime-switch.legacy.run.stderr b/src/test/ui/panics/runtime-switch.legacy.run.stderr
new file mode 100644
index 00000000000..979cc56b831
--- /dev/null
+++ b/src/test/ui/panics/runtime-switch.legacy.run.stderr
@@ -0,0 +1,5 @@
+thread 'main' panicked at 'explicit panic', $DIR/runtime-switch.rs:24:5
+stack backtrace:
+   0: std::panicking::begin_panic
+   1: runtime_switch::main
+note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.
diff --git a/src/test/ui/panics/runtime-switch.rs b/src/test/ui/panics/runtime-switch.rs
new file mode 100644
index 00000000000..c1634811406
--- /dev/null
+++ b/src/test/ui/panics/runtime-switch.rs
@@ -0,0 +1,25 @@
+// Test for std::panic::set_backtrace_style.
+
+// compile-flags: -O
+// run-fail
+// check-run-results
+// exec-env:RUST_BACKTRACE=0
+
+// ignore-msvc see #62897 and `backtrace-debuginfo.rs` test
+// ignore-android FIXME #17520
+// ignore-openbsd no support for libbacktrace without filename
+// ignore-wasm no panic or subprocess support
+// ignore-emscripten no panic or subprocess support
+// ignore-sgx no subprocess support
+
+// NOTE(eddyb) output differs between symbol mangling schemes
+// revisions: legacy v0
+// [legacy] compile-flags: -Zunstable-options -Csymbol-mangling-version=legacy
+//     [v0] compile-flags: -Csymbol-mangling-version=v0
+
+#![feature(panic_backtrace_config)]
+
+fn main() {
+    std::panic::set_backtrace_style(std::panic::BacktraceStyle::Short);
+    panic!()
+}
diff --git a/src/test/ui/panics/runtime-switch.v0.run.stderr b/src/test/ui/panics/runtime-switch.v0.run.stderr
new file mode 100644
index 00000000000..48f829c26d4
--- /dev/null
+++ b/src/test/ui/panics/runtime-switch.v0.run.stderr
@@ -0,0 +1,5 @@
+thread 'main' panicked at 'explicit panic', $DIR/runtime-switch.rs:24:5
+stack backtrace:
+   0: std::panicking::begin_panic::<&str>
+   1: runtime_switch::main
+note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.
diff --git a/src/tools/tidy/src/pal.rs b/src/tools/tidy/src/pal.rs
index 24a10018779..cb6d56a167d 100644
--- a/src/tools/tidy/src/pal.rs
+++ b/src/tools/tidy/src/pal.rs
@@ -58,6 +58,7 @@ const EXCEPTION_PATHS: &[&str] = &[
     "library/std/src/path.rs",
     "library/std/src/sys_common", // Should only contain abstractions over platforms
     "library/std/src/net/test.rs", // Utility helpers for tests
+    "library/std/src/panic.rs",   // fuchsia-specific panic backtrace handling
 ];
 
 pub fn check(path: &Path, bad: &mut bool) {