`break` in consteval has strange const promotion behavior

[theemathas]

I don't know what the correct behavior is. However, this combination of behaviors with these variations in the code seem rather inconsistent.

The below code compiles, seemingly due to const promotion or temporary lifetime extension:

#![allow(unreachable_code, unused)]

const A: () = {
    let _x: &i32 = 'a: {
        break 'a &one();
        panic!()
    };
};

const fn one() -> i32 {
    1
}

Adding an irrelevant if true {} makes it not compile, even without the &'static annotation:

#![allow(unreachable_code, unused)]

const A: () = {
    let _x = 'a: {
        if true {}
        break 'a &one();
        panic!()
    };
};

const fn one() -> i32 {
    1
}

error[E0716]: temporary value dropped while borrowed
 --> src/lib.rs:6:19
  |
4 |     let _x = 'a: {
  |         -- borrow later stored here
5 |         if true { }
6 |         break 'a &one();
  |                   ^^^^^- temporary value is freed at the end of this statement
  |                   |
  |                   creates a temporary value which is freed while still in use
  |
  = note: consider using a `let` binding to create a longer lived value

For more information about this error, try `rustc --explain E0716`.
error: could not compile `playground` (lib) due to 1 previous error

Adding a break inside the condition makes the error blame the panic:

#![allow(unreachable_code, unused)]

const A: () = {
    let _x = 'a: {
        if true {
            break 'a &1;
        }
        break 'a &one();
        panic!()
    };
};

const fn one() -> i32 {
    1
}

error[E0716]: temporary value dropped while borrowed
 --> src/lib.rs:8:19
  |
8 |         break 'a &one();
  |                   ^^^^^- temporary value is freed at the end of this statement
  |                   |
  |                   creates a temporary value which is freed while still in use
9 |         panic!()
  |         -------- borrow later used here
  |
  = note: consider using a `let` binding to create a longer lived value

For more information about this error, try `rustc --explain E0716`.
error: could not compile `playground` (lib) due to 1 previous error

Putting the initial code inside a fn instead of a const makes it not compile, even without the &'static annotation:

#![allow(unreachable_code, unused)]

fn foo() {
    let _x: &i32 = 'a: {
        break 'a &one();
        panic!()
    };
}

const fn one() -> i32 {
    1
}

error[E0716]: temporary value dropped while borrowed
 --> src/lib.rs:5:19
  |
4 |     let _x: &i32 = 'a: {
  |         -- borrow later stored here
5 |         break 'a &one();
  |                   ^^^^^- temporary value is freed at the end of this statement
  |                   |
  |                   creates a temporary value which is freed while still in use
  |
  = note: consider using a `let` binding to create a longer lived value

For more information about this error, try `rustc --explain E0716`.
error: could not compile `playground` (lib) due to 1 previous error

Putting the initial code inside if false makes it not compile, even without the &'static annotation:

#![allow(unreachable_code, unused)]

const A: () = if false {
    let _x: &i32 = 'a: {
        break 'a &one();
        panic!()
    };
};

const fn one() -> i32 {
    1
}

playground)
error[E0716]: temporary value dropped while borrowed
 --> src/lib.rs:5:19
  |
4 |     let _x: &i32 = 'a: {
  |         -- borrow later stored here
5 |         break 'a &one();
  |                   ^^^^^- temporary value is freed at the end of this statement
  |                   |
  |                   creates a temporary value which is freed while still in use
  |
  = note: consider using a `let` binding to create a longer lived value

For more information about this error, try `rustc --explain E0716`.
error: could not compile `playground` (lib) due to 1 previous error

@rustbot labels +A-const-eval

CC @traviscross

Meta

All code snippets tested on the playground with version 1.89.0.

[traviscross]

This is due to #121557. Correspondingly, your second, third, and fifth example indeed regressed in Rust 1.79. The fourth one fails because we don't promote calls to arbitrary constant functions except inside initializers as noted in #144939 (comment).

[theemathas]

That's wild.

Why does the third example have such a weird error message though?

[theemathas]

Filed a separate diagnostic issue at #145530. Closing this issue, since it's working as intended, albeit strangely.

[RalfJung]

since it's working as intended, albeit strangely.

Yeah, I'd love to clean this up, but it requires a non-trivial edition transition and I don't think I'll have the time and energy to implement that. See #124328. If someone is willing to figure out how to invoke the borrow checker twice on two different variants of a MIR body to provide a migration lint, I'm happy to explain the general strategy of how this could work. :D