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.
	}

[ada4a]

rust-analyzer/crates/hir-def/src/expr_store/lower.rs

Lines 2478 to 2502 in c2a4de0

	fn collect_extern_fn_param(&mut self, pat: Option<ast::Pat>) -> PatId {
	// `extern` functions cannot have pattern-matched parameters, and furthermore, the identifiers
	// in their parameters are always interpreted as bindings, even if in a normal function they
	// won't be, because they would refer to a path pattern.
	let Some(pat) = pat else { return self.missing_pat() };
	match &pat {
	ast::Pat::IdentPat(bp) => {
	// FIXME: Emit an error if `!bp.is_simple_ident()`.
	let name = bp.name().map(|nr| nr.as_name()).unwrap_or_else(Name::missing);
	let hygiene = bp
	.name()
	.map(|name| self.hygiene_id_for(name.syntax().text_range()))
	.unwrap_or(HygieneId::ROOT);
	let binding = self.alloc_binding(name, BindingAnnotation::Unannotated, hygiene);
	let pat =
	self.alloc_pat(Pat::Bind { id: binding, subpat: None }, AstPtr::new(&pat));
	self.add_definition_to_binding(binding, pat);
	pat
	}
	// FIXME: Emit an error.
	_ => self.missing_pat(),
	}
	}

[ada4a]

rust-analyzer/crates/hir-ty/src/method_resolution/confirm.rs

Lines 574 to 581 in 2103408

	fn check_for_illegal_method_calls(&self) {
	// Disallow calls to the method `drop` defined in the `Drop` trait.
	if let ItemContainerId::TraitId(trait_def_id) = self.candidate.loc(self.db()).container
	&& self.ctx.lang_items.Drop.is_some_and(|drop_trait| drop_trait == trait_def_id)
	{
	// FIXME: Report an error.
	}
	}

[MavenRain]

Picking up the duplicate-field FIXME at https://github.com/rust-lang/rust-analyzer/blob/HEAD/crates/hir-ty/src/infer/expr.rs#L1072 (the struct-literal duplicate-field case in check_record_expr_fields). PR incoming.