const_eval: JoinSemiLattice::join short-circuits with || skipping borrow state update

[SebTardif]

Bug

In compiler/rustc_const_eval/src/check_consts/resolver.rs:321-322, the JoinSemiLattice::join implementation for State uses short-circuit ||:

fn join(&mut self, other: &Self) -> bool {
    self.qualif.join(&other.qualif) || self.borrow.join(&other.borrow)
}

When self.qualif.join() returns true (qualif state changed), the || operator short-circuits and self.borrow.join() is never called. This means borrow state from other is silently dropped at dataflow merge points.

The JoinSemiLattice::join contract says: "Computes the least upper bound of two elements, storing the result in self and returning true if self has changed." The borrow join has a side effect (mutating self.borrow) that must always execute.

Fix

Replace || with | (non-short-circuit or), or use separate let bindings:

let a = self.qualif.join(&other.qualif);
let b = self.borrow.join(&other.borrow);
a | b

Impact

Const fn with branching control flow may miss borrow constraints at merge points, potentially allowing const evaluation to proceed with incomplete borrow tracking.