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Don't do intra-pass validation on MIR shims #115005

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Aug 22, 2023
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Don't do intra-pass validation on MIR shims #115005

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6 changes: 5 additions & 1 deletion compiler/rustc_mir_transform/src/shim.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -90,7 +90,11 @@ fn make_shim<'tcx>(tcx: TyCtxt<'tcx>, instance: ty::InstanceDef<'tcx>) -> Body<'
};
debug!("make_shim({:?}) = untransformed {:?}", instance, result);

pm::run_passes(
// We don't validate MIR here because the shims may generate code that's
// only valid in a reveal-all param-env. However, since we do initial
// validation with the MirBuilt phase, which uses a user-facing param-env.
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What about setting body.phase to Runtime(Initial)?

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That probably works. I'm not sure if that breaks any other invariants of validation, let me try it.

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This does not work, because validation fails before the Derefer is run.

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I went down this path last week but wasn't very happy with the result as I had to run Derefer and then it seemed wise to run the other passes to actually put us in the same state as we should be when phase = Runtime(Initial) but that required either duplicating the list of passes or splitting up run_runtime_lowering_passes() since running ElaborateDrops again caused other issues. In the end, it felt pretty hacky and I think @compiler-errors's solution here is better.

// This causes validation errors when TAITs are involved.
pm::run_passes_no_validate(
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@compiler-errors compiler-errors Aug 19, 2023 •
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We could, perhaps, only do this for drop shims. Other shims are (probably?) alright to do validation on.

But I'm fine with ignoring all shims -- we still do final validation after passes are run since we're transitioning to MirPhase::Runtime(RuntimePhase::Optimized).

tcx,
&mut result,
&[
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21 changes: 21 additions & 0 deletions tests/ui/type-alias-impl-trait/auxiliary/drop-shim-relates-opaque-aux.rs
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@@ -0,0 +1,21 @@
// crate foo

#![feature(type_alias_impl_trait)]

type Tait = impl Sized;
fn _constrain() -> Tait {}

struct WrapperWithDrop<T>(T);
impl<T> Drop for WrapperWithDrop<T> {
fn drop(&mut self) {}
}

pub struct Foo(WrapperWithDrop<Tait>);

trait Id {
type Id: ?Sized;
}
impl<T: ?Sized> Id for T {
type Id = T;
}
pub struct Bar(WrapperWithDrop<<Tait as Id>::Id>);
10 changes: 10 additions & 0 deletions tests/ui/type-alias-impl-trait/drop-shim-relates-opaque-issue-114375.rs
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@@ -0,0 +1,10 @@
// aux-build:drop-shim-relates-opaque-aux.rs
// compile-flags: -Zvalidate-mir --crate-type=lib
// build-pass

extern crate drop_shim_relates_opaque_aux;

pub fn drop_foo(_: drop_shim_relates_opaque_aux::Foo) {}
pub fn drop_bar(_: drop_shim_relates_opaque_aux::Bar) {}

fn main() {}