Can we add a new `StableLintExpectationId` type so we can remove unstable Hash impls

[oli-obk]

Can we add a new StableLintExpectationId type so we can statically prevent the Unstable case? Then we'd have enum LintExpectationId { Unstable { attr_id, lint_index }, Stable(StableLintExpectationId) }, would be a slight pain to rewrite but mostly mechanical.

Might make sense to do it for the return type of normalize() too so we don't accidentally hash the attr_id; that seems worse since it will only invalidate the incremental cache but not be obvious that it's a bug.

Originally posted by @jyn514 in #101620 (comment)

[Urgau]

Seems to have been done.

rust/compiler/rustc_lint_defs/src/lib.rs

Lines 93 to 102 in 976487c

	pub enum LintExpectationId {
	/// Used for lints emitted during the `EarlyLintPass`. This id is not
	/// hash stable and should not be cached.
	Unstable { attr_id: AttrId, lint_index: Option<u16> },
	/// The [`HirId`] that the lint expectation is attached to. This id is
	/// stable and can be cached. The additional index ensures that nodes with
	/// several expectations can correctly match diagnostics to the individual
	/// expectation.
	Stable { hir_id: HirId, attr_index: u16, lint_index: Option<u16> },
	}