Incorrect promotion for projection in a `const`

[ecstatic-morse]

Consider the following code:

Playground

const _: &i32 = {
    let x = &(5, false).0;
    x
};

fn main() {
    let _: &'static i32 = &(5, false).0;
}

The borrow of the tuple field in main is promoted out along with the tuple itself in accordance with the rules for rvalue static promotion. However, in the const, the mechanism for promotion does not remove Drop/StorageDead for the tuple, causing x to dangle. On stable, this is only caught during const-evaluation and results in a const_err warning. If this warning is supressed, an ICE occurs.

error: internal compiler error: tried to intern dangling pointer
 --> src/main.rs:2:1
  |
2 | / const _: &i32 = {
3 | |     let x = &(5, false).0;
4 | |     x
5 | | };
  | |__^

thread 'rustc' panicked at 'no errors encountered even though `delay_span_bug` issued', src/librustc_errors/lib.rs:361:17
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace.

On nightly, the MIR borrow checker prevents this from happening, and this code is rejected instead. It should be accepted in all contexts.