Consider Scalar to be a bool only if its unsigned

[nagisa]

This seems right, given that conceptually bools are unsigned, but the
implications of this change may have more action at distance that I'm
not sure how to exhaustively consider.

For instance there are a number of cases where code attaches range
metadata if scalar.is_bool() holds. Supposedly it would no longer be
attached to the repr(i8) enums? Though I'm not sure why booleans are
being special-cased here in the first place...

Fixes #80556

cc @eddyb

[rust-highfive]

r? @varkor

(rust-highfive has picked a reviewer for you, use r? to override)

[nagisa]

An alternative fix could be to set sext here if the scalar is signed here.

[varkor]

r? @eddyb, who is more familiar with the subtleties here. There already seems to be special casing regarding boolean values for signed integers, e.g.

rust/compiler/rustc_codegen_ssa/src/mir/place.rs

Lines 241 to 248 in 1d27267

	let signed = match tag_scalar.value {
	// We use `i1` for bytes that are always `0` or `1`,
	// e.g., `#[repr(i8)] enum E { A, B }`, but we can't
	// let LLVM interpret the `i1` as signed, because
	// then `i1 1` (i.e., `E::B`) is effectively `i8 -1`.
	Int(_, signed) => !tag_scalar.is_bool() && signed,
	_ => false,
	};

which would presumably become vacuous if signed integers are never treated as booleans.

[eddyb]

Sorry for the delay, r=me with @camelid's suggestion applied.

[nagisa]

@bors r=eddyb

[bors]

📌 Commit 915a04e has been approved by eddyb