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Rollup merge of #65592 - RalfJung:const-prop-comment, r=wesleywiser

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clarify const_prop ICE protection comment

This is based on discussion at https://github.com/rust-lang/rust/pull/64890/files#r334555787.

That said, why are function arguments the only unsized locals that could remain uninitialized? Couldn't we also fail to initialize some local but still go on with const_prop, and then hit a line that takes a reference to that? Cc @wesleywiser @oli-obk ; I don't know enough about const-prop to understand why this can happen only for function arguments.

~~The PR includes https://github.com/rust-lang/rust/pull/64890; the only new commit is 05e4e6ba0d5.~~
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Mazdak Farrokhzad 2019-10-21 01:39:13 +02:00 • committed by GitHub
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25
src/librustc_mir/transform/const_prop.rs
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@ -518,27 +518,30 @@ impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> {
}
}
// Work around: avoid ICE in miri.
// FIXME(wesleywiser) we don't currently handle the case where we try to make a ref
// from a function argument that hasn't been assigned to in this function. The main
// issue is if an arg is a fat-pointer, miri `expects()` to be able to read the value
// of that pointer to get size info. However, since this is `ConstProp`, that argument
// doesn't actually have a backing value and so this causes an ICE.
// Work around: avoid ICE in miri. FIXME(wesleywiser)
// The Miri engine ICEs when taking a reference to an uninitialized unsized
// local. There's nothing it can do here: taking a reference needs an allocation
// which needs to know the size. Normally that's okay as during execution
// (e.g. for CTFE) it can never happen. But here in const_prop
// unknown data is uninitialized, so if e.g. a function argument is unsized
// and has a reference taken, we get an ICE.
Rvalue::Ref(_, _, Place { base: PlaceBase::Local(local), projection: box [] }) => {
trace!("checking Ref({:?})", place);
let alive =
if let LocalValue::Live(_) = self.ecx.frame().locals[*local].value {
true
} else { false };
} else {
false
};
if local.as_usize() <= self.ecx.frame().body.arg_count && !alive {
trace!("skipping Ref({:?})", place);
if !alive {
trace!("skipping Ref({:?}) to uninitialized local", place);
return None;
}
}
// Work around: avoid extra unnecessary locals.
// FIXME(wesleywiser): const eval will turn this into a `const Scalar(<ZST>)` that
// Work around: avoid extra unnecessary locals. FIXME(wesleywiser)
// Const eval will turn this into a `const Scalar(<ZST>)` that
// `SimplifyLocals` doesn't know it can remove.
Rvalue::Aggregate(_, operands) if operands.len() == 0 => {
return None;