MIR inlining breaks Location::caller()

[saethlin]

use core::panic::Location;

#[inline]
fn nested() -> &'static Location<'static> {
    Location::caller()
}

fn main() {
    println!("{:?}", nested());
}

In debug mode, this prints

Location { file: "src/main.rs", line: 5, col: 5 }

But with optimizations, this prints:

Location { file: "src/main.rs", line: 9, col: 22 }

With optimizations and -Zinline-mir=no, this prints:

Location { file: "src/main.rs", line: 5, col: 5 }

This check in the MIR inliner is hiding this problem from the track-caller UI tests:

rust/compiler/rustc_mir_transform/src/inline.rs

Lines 336 to 344 in 32da230

	// Only inline local functions if they would be eligible for cross-crate
	// inlining. This is to ensure that the final crate doesn't have MIR that
	// reference unexported symbols
	if callsite.callee.def_id().is_local() {
	let is_generic = callsite.callee.substs.non_erasable_generics().next().is_some();
	if !is_generic && !callee_attrs.requests_inline() {
	return Err("not exported");
	}
	}

@rustbot label +A-mir-opt +A-mir-opt-inlining

---

rustc --version --verbose:

rustc 1.68.0-nightly (dfe3fe710 2022-12-09)
binary: rustc
commit-hash: dfe3fe710181738a2cb3060c23ec5efb3c68ca09
commit-date: 2022-12-09
host: x86_64-unknown-linux-gnu
release: 1.68.0-nightly
LLVM version: 15.0.6

[Noratrieb]

This isn't specific to Location::caller but to any #[track_caller] function called in an inlined not-track-caller function.

#[track_caller]
fn another() {
    panic!("line 3, right?")
}

#[inline]
fn nested() {
    another();
}

fn main() {
    nested();
}

panics on line 8: thread 'main' panicked at 'line 3, right?', uwu.rs:8:5
Nevermind that, the panic here is correct actually :D

[tmiasko]

Ignoring changes in the standard library and optimization level at which MIR inliner is enabled, bisection points to #94427 cc @cjgillot.

[apiraino]

WG-prioritization assigning priority (Zulip discussion).

@rustbot label -I-prioritize +P-medium

[tmiasko]

use std::panic::Location;

macro_rules! f {
    () => { Location::caller() }
}

#[inline(always)]
fn g() -> &'static Location<'static> {
    f!()
}

fn main() {
    println!("{:?}", g());
}

$ rustc a.rs && ./a
Location { file: "a.rs", line: 9, col: 5 }
$ rustc a.rs -O && ./a
Location { file: "a.rs", line: 4, col: 13 }