clarify const_prop ICE protection comment

[RalfJung]

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 #64890; the only new commit is 05e4e6ba0d5.

[rust-highfive]

r? @davidtwco

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[RalfJung]

Hm, looks like changing the <= to a < makes a test fail.... oh it's because local 0 is the return place. Yeah that conditional needed a lot of comments that were never written.^^

[RalfJung]

Here's an example of taking a ref to an unsized local that's not a function argument... why does that not ICE in const-prop?

[wesleywiser]

r? @wesleywiser

[wesleywiser]

why does that not ICE in const-prop?

Currently, we only propagate temporaries, never variables:

rust/src/librustc_mir/transform/const_prop.rs

Lines 643 to 647 in e5b8c11

	// cannot use locals because if x < y { y - x } else { x - y } would
	// lint for x != y
	// FIXME(oli-obk): lint variables until they are used in a condition
	// FIXME(oli-obk): lint if return value is constant
	*val = body.local_kind(local) == LocalKind::Temp;

Assigning the argument to a variable thwarts ConstProp and bypasses the ICE.

[RalfJung]

I see. Still, it strikes me as a better test to check for "uninitialized + usized", which matches the actual error condition. Or even just "uninitialized", as that's already impossible code (though maybe there's something useful const-prop can do there?).

[wesleywiser]

That seems reasonable to me! Off-hand, I can't think of anything useful const prop can do with uninitialized values.

[RalfJung]

So, something like this? (Local build is sill ongoing.)

[RalfJung]

Okay, tests are passing locally. The PR is ready for review.

[wesleywiser]

Thanks @RalfJung!

@bors r+

[bors]

📌 Commit f907fbe has been approved by wesleywiser