Address FIXMEs for adding diagnostics

[ChayimFriedman2]

We have quite a lot of FIXMEs, mostly in hir-ty, for adding new diagnostics for things (and I'm constantly adding more). The shape of them is like FIXME: Emit an error ... (my own FIXMEs usually use these words, others might use similar). Most are pretty easy to do. The steps to resolve one (in inference) are:

1. Add a variant to InferenceDiagnostic with the necessary information.
2. Call self.push_diagnostic(diag) on the place.
3. In hir/src/diagnostics.rs, add a new struct for the diagnostic and convert the enum variant to it in the functions of that module. Note: this struct must not use hir-ty types, only hir types.
4. Create a new handler in ide-diagnostics/src/handlers/ for the diagnostic (follow existing handlers). A simple handler can take a few lines. Also add some tests.
5. Call the handler from ide-diagnostics/src/lib.rs.

That's it! You can also of course customize the error message to be clearer/clever, and/or add a fix, but that is good enough.

[ada4a]

I'll start with this one:

rust-analyzer/crates/hir-ty/src/lower/path.rs

Lines 302 to 306 in f45c1e2

	if self.ctx.lowering_param_default.is_some() {
	// Generic defaults are not allowed to refer to `Self`.
	// FIXME: Emit an error.
	return false;
	}

[ChayimFriedman2]

@ada4a That one is an error in types as well, so you need to add the diagnostic to PathLoweringDiagnostic instead. But the process is similar.

[ada4a]

rust-analyzer/crates/hir-ty/src/infer/expr.rs

Lines 251 to 258 in 7bf346e

	#[expect(clippy::needless_return)]
	pub(crate) fn check_lhs_assignable(&self, lhs: ExprId) {
	if self.is_syntactic_place_expr(lhs) {
	return;
	}
	// FIXME: Emit diagnostic.
	}