Suggest using ptr::null_mut when user provided ptr::null to a function expecting ptr::null_mut

[jieyouxu]

error[E0308]: mismatched types
  --> $DIR/ptr-null-mutability-suggestions.rs:9:24
   |
LL |     expecting_null_mut(ptr::null());
   |     ------------------ ^^^^^^^^^^^
   |     |                  |
   |     |                  types differ in mutability
   |     |                  help: consider using `core::ptr::null_mut` instead: `core::ptr::null_mut()`
   |     arguments to this function are incorrect
   |
   = note: expected raw pointer `*mut u8`
              found raw pointer `*const _`
note: function defined here
  --> $DIR/ptr-null-mutability-suggestions.rs:6:4
   |
LL | fn expecting_null_mut(_: *mut u8) {}
   |    ^^^^^^^^^^^^^^^^^^ ----------

Closes #85184.

[rustbot]

r? @TaKO8Ki

(rustbot has picked a reviewer for you, use r? to override)

[WaffleLapkin]

You can simplify the condition quite a lot, maybe something like

Suggested change

	if let ty::RawPtr(ty::TypeAndMut { mutbl: expected_mutbl, .. }) = expected_ty.kind()
	&& let ty::RawPtr(ty::TypeAndMut { mutbl: provided_mutbl, .. }) = provided_ty.kind()
	&& let provided_arg_expr = *provided_args[provided_idx]
	&& let hir::ExprKind::Call(callee, _) = provided_arg_expr.kind
	&& let hir::ExprKind::Path(hir::QPath::Resolved(_, path)) = callee.kind
	&& let Res::Def(_, def_id) = path.res
	{
	match (expected_mutbl, provided_mutbl) {
	(hir::Mutability::Mut, hir::Mutability::Not) => {
	if let Some(ptr_null_def_id) = self.tcx.get_diagnostic_item(sym::ptr_null)
	&& def_id == ptr_null_def_id
	{
	// The user provided `ptr::null()`, but the function expects
	// `ptr::null_mut()`.
	err.subdiagnostic(SuggestPtrNullMut {
	span: provided_arg_expr.span
	});
	}
	},
	_ => {},
	}
	}
	if let ty::RawPtr(ty::TypeAndMut { mutbl: hir::Mutability::Mut, .. }) = expected_ty.kind()
	&& let ty::RawPtr(ty::TypeAndMut { mutbl: hir::Mutability::Not, .. }) = provided_ty.kind()
	&& let provided_arg_expr = *provided_args[provided_idx]
	&& let hir::ExprKind::Call(callee, _) = provided_arg_expr.kind
	&& let hir::ExprKind::Path(hir::QPath::Resolved(_, path)) = callee.kind
	&& let Res::Def(_, def_id) = path.res
	&& Some(def_id) == self.tcx.get_diagnostic_item(sym::ptr_null)
	{
	// The user provided `ptr::null()`, but the function expects
	// `ptr::null_mut()`.
	err.subdiagnostic(SuggestPtrNullMut {
	span: provided_arg_expr.span
	});
	}

[jieyouxu]

@rustbot ready

[WaffleLapkin]

I'm not sure why you don't want to just ty::RawPtr(ty::TypeAndMut { mutbl: hir::Mutability::Mut, .. }) in the pattern above, as imo it reads quite good: expected_ty is a raw pointer with mut mutability...

But if you don't like to do it in the pattern, at least use .is_mut()/.is_not() and move those two checks to the top, just after the corresponding let raw_ptr = ty.kind()-ish code.

[WaffleLapkin]

Suggested change

	&& let Some(ptr_null_def_id) = self.tcx.get_diagnostic_item(sym::ptr_null)
	&& def_id == ptr_null_def_id
	&& Some(def_id) == self.tcx.get_diagnostic_item(sym::ptr_null)

or maybe

Suggested change

	&& let Some(ptr_null_def_id) = self.tcx.get_diagnostic_item(sym::ptr_null)
	&& def_id == ptr_null_def_id
	&& self.tcx.get_diagnostic_item(sym::ptr_null) == Some(def_id)

[WaffleLapkin]

@bors r+ rollup

[bors]

📌 Commit 432ce39 has been approved by WaffleLapkin

It is now in the queue for this repository.