Lint for returning a pointer to stack memory associated with a local variable

Code

pub fn foo() -> *const i32 {
    let x = 42;
    &x
}

Current output

rustc doesn't currently warn about this.

Desired output

A warning that this pointer is referencing a local variable and is going to be dangling immediately. E.g.

error: returning a pointer to stack memory associated with a local variable
  --> <source>:12:5
  |
LL|  &x
  |  ^^

Rationale and extra context

This code is very unlikely to be correct, which is exactly why a warning should be emitted. This would be similar to Clang's -Wreturn-stack-address warning.

In case of returning a reference, rustc would catch this and emit a compiler error. While it is fine to create such a pointer, it is very unlikely that the code should be written that way.

Rust Version

rustc 1.84.0-nightly (b8c8287a2 2024-11-03)
binary: rustc
commit-hash: b8c8287a229cd79604aa84c25e1235fc78cd5f2e
commit-date: 2024-11-03
host: x86_64-unknown-linux-gnu
release: 1.84.0-nightly
LLVM version: 19.1.3

Anything else?

If this becomes a Warn lint it will start warning immediately and potentially break users that configured warnings as errors. So maybe this should be Allow for the beginning?

Start by reproducing the Rust example and examining the compiler's existing raw-pointer and dangling-reference diagnostics. Trace the lint implementation and its UI-test conventions, using Clang's -Wreturn-stack-address behavior as context. Done means the example produces the requested diagnostic with an appropriate initial lint level, without warning on ordinary raw-pointer creation.