`borrow_as_ptr` false negative when borrowing a temporary literal

[xtqqczze]

Summary

The borrow_as_ptr lint is emitted for borrowing a local variable as a raw pointer, but it is not emitted when the borrowed expression is a temporary literal.

Lint Name

borrow_as_ptr

Reproducer

the lint does not fire for m1, but does for m2

#[inline(never)]
pub fn m1() {
    let _ptr = &42f32 as *const f32;
}

#[inline(never)]
pub fn m2() {
    let tmp = 42f32;
    let _ptr = &tmp as *const f32;
}

https://godbolt.org/z/xoPTzr6sx

Version

[Jarcho]

&raw const and ptr::addr_or can't be used on temporary values. Suggesting to add an additional local in order to avoid the cast is excessive.

[krobelus]

&raw const can't be used on temporary values

I wonder why not, since it seems to work fine with the cast version (let _ptr = &42f32 as *const f32;).
It's called lifetime extension IIRC.

[xtqqczze]

this could also be written as:

#[inline(never)]
pub fn m1() {
    let ptr = std::ptr::from_ref(&42f32);
}

[xtqqczze]

Related: #16214

[Jarcho]

You can't use from_ref for this since it doesn't extend the lifetime. Technically you can in that specific case due to const promotion, but you can't in general with a temporary.