remove Subtype projections
#133258
remove Subtype projections
#133258
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Some changes occurred to the CTFE / Miri interpreter cc @rust-lang/miri Some changes occurred to the CTFE machinery cc @rust-lang/wg-const-eval Some changes occurred in src/tools/clippy cc @rust-lang/clippy Some changes occurred to MIR optimizations cc @rust-lang/wg-mir-opt This PR changes MIR cc @oli-obk, @RalfJung, @JakobDegen, @davidtwco, @celinval, @vakaras Some changes occurred in compiler/rustc_codegen_cranelift cc @bjorn3 This PR changes Stable MIR |
| @@ -114,7 +114,6 @@ impl<'tcx, M: Machine<'tcx>> FnAbiOfHelpers<'tcx> for InterpCx<'tcx, M> { | |||
| } | |||
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| /// Test if it is valid for a MIR assignment to assign `src`-typed place to `dest`-typed value. | |||
| /// This test should be symmetric, as it is primarily about layout compatibility. | |||
| pub(super) fn mir_assign_valid_types<'tcx>( | |||
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we check for subtyping here which is not symmetric, so either this comment is wrong or the impl is. I would be open to alternative change this to try subtyping in either direction 🤷 but given that this already uses src and dest and the behavior is preexisting, this comment is not relied on rn
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I think I originally made the check symmetric since I didn't want to think about which direction it has to be. 😂 So it might be worth checking that all users of this function use it correctly.
I also find the comment extremely confusing. When doing an assignment, dest is a place and src is a value! So what is going on here?
What I expect is a comment like this:
/// Test if it is valid for a MIR assignment to store a `src`-typed value in a `dest`-typed place.
But I am not sure if that matches the implementation, or the users...
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Apparently I wrote that comment. Unfortunately I can't explain what I meant...
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yeah, it should be src-typed value and dest-typed place 😁
| .filter(|elem| !matches!(elem, ProjectionElem::OpaqueCast(_))) | ||
| .collect::<Vec<_>>(), | ||
| ); | ||
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| self.super_place(place, _context, _location); |
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while I don't fully understand in which cases we don't emit an OpaqueCast even though the type of a projection contains an opaque, it apparently can happen 🤔
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Yea there are bugs here. The system is fragile and I haven't been able to come up with sth better, so without redoing projections from scratch, it may be best to just forbid projecting into opaques. I wanted to so that originally, but there was pushback saying some stuff should compile
| @@ -682,20 +674,13 @@ impl<'a, 'tcx> Visitor<'tcx> for TypeChecker<'a, 'tcx> { | |||
| } | |||
| }; | |||
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| let kind = match parent_ty.ty.kind() { | |||
| &ty::Alias(ty::Opaque, ty::AliasTy { def_id, args, .. }) => { | |||
| self.tcx.type_of(def_id).instantiate(self.tcx, args).kind() | |||
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very much incorrect in case the type of an opaque is either not normalized with these args or yet another opaque :3
There you said we'd need explicit types in some places, replacing these projections. (Specifically, casts and calls.) How can this PR work without adding those explicit types? |
we currently still prevent optimizations from dropping assignments causing subtyping, as the subtype projections were never enough to actually fix the unsoundness so rn the fix for the unsoundness is: make sure no optimization merges locals which are subtypes of each other |
Okay. That sounds fragile, I guess #112651 is tracking the improvement of that situation? |
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cc #112651 closes #118478
see #112651 (comment) for the reasoning behind this change
r? types