fix: Yet another false positive invalid cast diagnostic #19432

ShoyuVanilla · 2025-03-23T17:35:12Z

Fixes #19431

ShoyuVanilla · 2025-03-23T17:37:52Z

crates/ide-diagnostics/src/handlers/invalid_cast.rs

    let t: *mut (dyn Trait + 'static) = 0 as *mut _;
-                                      //^^^^^^^^^^^ error: cannot cast `usize` to a fat pointer `*mut _`
+


We have disabled diagnostics on casts into *const dyn Trait here

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

Lines 125 to 130 in 37acea8

// Chalk doesn't support trait upcasting and fails to solve some obvious goals

// when the trait environment contains some recursive traits (See issue #18047)

// We skip cast checks for such cases for now, until the next-gen solver.

if contains_dyn_trait(&self.cast_ty) {

return Ok(());

}

but this somehow survived it by hiding behinde inference var 😅

ShoyuVanilla · 2025-03-23T17:46:02Z

crates/ide-diagnostics/src/handlers/invalid_cast.rs

+struct ZerocopyKnownLayoutMaybeUninit(<<Struct as Field>::Type as KnownLayout>::MaybeUninit);
+
+fn test(ptr: *mut ZerocopyKnownLayoutMaybeUninit) -> *mut <<Struct as Field>::Type as KnownLayout>::MaybeUninit {
+    ptr as *mut _


In zerocopy's proc macro expansion, there are so many nested projection types and inference vars and because of that the last field extracting stops along the way in the following function:

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

Lines 919 to 944 in 37acea8

/// Check if given type is `Sized` or not

pub(crate) fn is_sized(&mut self, ty: &Ty) -> bool {

let mut ty = ty.clone();

{

let mut structs = SmallVec::<[_; 8]>::new();

// Must use a loop here and not recursion because otherwise users will conduct completely

// artificial examples of structs that have themselves as the tail field and complain r-a crashes.

while let Some((AdtId::StructId(id), subst)) = ty.as_adt() {

let struct_data = self.db.variant_data(id.into());

if let Some((last_field, _)) = struct_data.fields().iter().next_back() {

let last_field_ty = self.db.field_types(id.into())[last_field]

.clone()

.substitute(Interner, subst);

if structs.contains(&ty) {

// A struct recursively contains itself as a tail field somewhere.

return true; // Don't overload the users with too many errors.

}

structs.push(ty);

// Structs can have DST as its last field and such cases are not handled

// as unsized by the chalk, so we do this manually.

ty = last_field_ty;

} else {

break;

};

}

}

Ideally, this should anyway return !Sized as the function body before trait solving is just a performance-related logic, but the chalk returns wrong unique solution to a goal struct Dst([u8]): Sized 🤔

So, I added a function eagerly resolves such nested inference vars and projection types.

fix: Yet another false positive invalid cast diagnostic

9cfea3e

rustbot added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Mar 23, 2025

ShoyuVanilla commented Mar 23, 2025

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fix: Yet another false positive invalid cast diagnostic #19432

fix: Yet another false positive invalid cast diagnostic #19432

ShoyuVanilla commented Mar 23, 2025

ShoyuVanilla Mar 23, 2025 •

edited

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ShoyuVanilla Mar 23, 2025

		let t: mut (dyn Trait + 'static) = 0 as mut _;
		//^^^^^^^^^^^ error: cannot cast `usize` to a fat pointer `*mut _`

	// Chalk doesn't support trait upcasting and fails to solve some obvious goals
	// when the trait environment contains some recursive traits (See issue #18047)
	// We skip cast checks for such cases for now, until the next-gen solver.
	if contains_dyn_trait(&self.cast_ty) {
	return Ok(());
	}

	/// Check if given type is `Sized` or not
	pub(crate) fn is_sized(&mut self, ty: &Ty) -> bool {
	let mut ty = ty.clone();
	{
	let mut structs = SmallVec::<[_; 8]>::new();
	// Must use a loop here and not recursion because otherwise users will conduct completely
	// artificial examples of structs that have themselves as the tail field and complain r-a crashes.
	while let Some((AdtId::StructId(id), subst)) = ty.as_adt() {
	let struct_data = self.db.variant_data(id.into());
	if let Some((last_field, _)) = struct_data.fields().iter().next_back() {
	let last_field_ty = self.db.field_types(id.into())[last_field]
	.clone()
	.substitute(Interner, subst);
	if structs.contains(&ty) {
	// A struct recursively contains itself as a tail field somewhere.
	return true; // Don't overload the users with too many errors.
	}
	structs.push(ty);
	// Structs can have DST as its last field and such cases are not handled
	// as unsized by the chalk, so we do this manually.
	ty = last_field_ty;
	} else {
	break;
	};
	}
	}

fix: Yet another false positive invalid cast diagnostic #19432

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fix: Yet another false positive invalid cast diagnostic #19432

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ShoyuVanilla commented Mar 23, 2025

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ShoyuVanilla Mar 23, 2025 •

edited

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