Match guard can both move and static-promote a single constant

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I'm not sure if this is a bug or not.

I expected the below code to produce a compile error, but instead it compiles fine.

struct Thing;

fn foo() -> &'static Thing {
    let y;
    match Thing {
        x if { y = &x; true } => {
            // This drop should invalidate y
            drop(x);
            y
        }
        _ => {
            panic!();
        }
    }
}

It seems that the single Thing constant is both moved and static-promoted. This seems very strange.

Related to #144939. Discovered in https://github.com/rust-lang/rust/pull/141295#issuecomment-3173106871.

@rustbot labels +A-patterns +A-const-eval

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Reproduces on the playground with version 1.91.0-nightly (2025-08-09 ca77504943887037504c)

Start by reproducing the example on the Rust nightly version noted in the issue, then read related issue #144939 and the discussion in PR #141295. Trace how match guards, moves, and static promotion interact, and establish whether this should be rejected. Done means the behavior is resolved and covered by a compiler regression test.