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Add Frames Iterator for Backtrace #81022

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Feb 2, 2021
Merged

Add Frames Iterator for Backtrace #81022

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21 changes: 20 additions & 1 deletion library/std/src/backtrace.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -147,11 +147,14 @@ fn _assert_send_sync() {
_assert::<Backtrace>();
}

struct BacktraceFrame {
/// A single frame of a backtrace.
#[unstable(feature = "backtrace_frames", issue = "79676")]
pub struct BacktraceFrame {
frame: RawFrame,
symbols: Vec<BacktraceSymbol>,
}

#[derive(Debug)]
enum RawFrame {
Actual(backtrace_rs::Frame),
#[cfg(test)]
Expand Down Expand Up @@ -196,6 +199,14 @@ impl fmt::Debug for Backtrace {
}
}

impl fmt::Debug for BacktraceFrame {
fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
let mut dbg = fmt.debug_list();
dbg.entries(&self.symbols);
dbg.finish()
}
}

impl fmt::Debug for BacktraceSymbol {
fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
// FIXME: improve formatting: https://github.com/rust-lang/rust/issues/65280
Expand Down Expand Up @@ -353,6 +364,14 @@ impl Backtrace {
}
}

impl<'a> Backtrace {
/// Returns an iterator over the backtrace frames.
#[unstable(feature = "backtrace_frames", issue = "79676")]
pub fn frames(&'a self) -> &'a [BacktraceFrame] {
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Wouldn't it be better if this were an iterator? That could make lazily walking the stack cheaper at some point in the future.

E.g. Java introduced a StackWalker API to make partial stack traces cheaper.

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This was indeed discussed. There's some additional discussion and context that isn't apparent because it's in the previous iteration of this PR. Apologies for that.

You can read some of that discussion here.

if let Inner::Captured(c) = &self.inner { &c.force().frames } else { &[] }
}
}

impl fmt::Display for Backtrace {
fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
let capture = match &self.inner {
Expand Down
109 changes: 72 additions & 37 deletions library/std/src/backtrace/tests.rs
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Original file line number Diff line number Diff line change
@@ -1,48 +1,52 @@
use super::*;

fn generate_fake_frames() -> Vec<BacktraceFrame> {
vec![
BacktraceFrame {
frame: RawFrame::Fake,
symbols: vec![BacktraceSymbol {
name: Some(b"std::backtrace::Backtrace::create".to_vec()),
filename: Some(BytesOrWide::Bytes(b"rust/backtrace.rs".to_vec())),
lineno: Some(100),
colno: None,
}],
},
BacktraceFrame {
frame: RawFrame::Fake,
symbols: vec![BacktraceSymbol {
name: Some(b"__rust_maybe_catch_panic".to_vec()),
filename: None,
lineno: None,
colno: None,
}],
},
BacktraceFrame {
frame: RawFrame::Fake,
symbols: vec![
BacktraceSymbol {
name: Some(b"std::rt::lang_start_internal".to_vec()),
filename: Some(BytesOrWide::Bytes(b"rust/rt.rs".to_vec())),
lineno: Some(300),
colno: Some(5),
},
BacktraceSymbol {
name: Some(b"std::rt::lang_start".to_vec()),
filename: Some(BytesOrWide::Bytes(b"rust/rt.rs".to_vec())),
lineno: Some(400),
colno: None,
},
],
},
]
}

#[test]
fn test_debug() {
let backtrace = Backtrace {
inner: Inner::Captured(LazilyResolvedCapture::new(Capture {
actual_start: 1,
resolved: true,
frames: vec![
BacktraceFrame {
frame: RawFrame::Fake,
symbols: vec![BacktraceSymbol {
name: Some(b"std::backtrace::Backtrace::create".to_vec()),
filename: Some(BytesOrWide::Bytes(b"rust/backtrace.rs".to_vec())),
lineno: Some(100),
colno: None,
}],
},
BacktraceFrame {
frame: RawFrame::Fake,
symbols: vec![BacktraceSymbol {
name: Some(b"__rust_maybe_catch_panic".to_vec()),
filename: None,
lineno: None,
colno: None,
}],
},
BacktraceFrame {
frame: RawFrame::Fake,
symbols: vec![
BacktraceSymbol {
name: Some(b"std::rt::lang_start_internal".to_vec()),
filename: Some(BytesOrWide::Bytes(b"rust/rt.rs".to_vec())),
lineno: Some(300),
colno: Some(5),
},
BacktraceSymbol {
name: Some(b"std::rt::lang_start".to_vec()),
filename: Some(BytesOrWide::Bytes(b"rust/rt.rs".to_vec())),
lineno: Some(400),
colno: None,
},
],
},
],
frames: generate_fake_frames(),
})),
};

Expand All @@ -58,3 +62,34 @@ fn test_debug() {
// Format the backtrace a second time, just to make sure lazily resolved state is stable
assert_eq!(format!("{:#?}", backtrace), expected);
}

#[test]
fn test_frames() {
let backtrace = Backtrace {
inner: Inner::Captured(LazilyResolvedCapture::new(Capture {
actual_start: 1,
resolved: true,
frames: generate_fake_frames(),
})),
};

let frames = backtrace.frames();

#[rustfmt::skip]
let expected = vec![
"[
{ fn: \"std::backtrace::Backtrace::create\", file: \"rust/backtrace.rs\", line: 100 },
]",
"[
{ fn: \"__rust_maybe_catch_panic\" },
]",
"[
{ fn: \"std::rt::lang_start_internal\", file: \"rust/rt.rs\", line: 300 },
{ fn: \"std::rt::lang_start\", file: \"rust/rt.rs\", line: 400 },
]"
];

let mut iter = frames.iter().zip(expected.iter());

assert!(iter.all(|(f, e)| format!("{:#?}", f) == *e));
}