Miscompilation of mutable reference to const temporary

[jethrogb]

trait XLen {
    fn xlen(&mut self) -> usize;
}

impl<'a> XLen for &'a [u8] {
    fn xlen(&mut self) -> usize {
        self.len()
    }
}

fn main() {
    const A: &'static [u8] = b"x";
    println!("{} {}",A.len(),A.xlen());
}

This code compiles, but yields:

1 2097865012304223517

The second number may vary.

This used to lead to segfaults (in 1.5.0 and earlier) in code such as:

use std::io::Read;
fn main() {
    const A: &'static [u8] = b"x";
    let mut a=[0u8;32];
    A.read(&mut a);
}

This doesn't segfault in 1.6.0 anymore, but that is due to an unrelated change.

[eddyb]

@nikomatsakis I wish there was an easy way to dump the const qualifications as annotations on expressions.

My guess is that we're missing the borrow somehow, or if const qualification is correct, then maybe the const handling code in trans::expr isn't.

[jethrogb]

Using {A} instead of A yields the correct results in both examples.

[eddyb]

Using &'static usize instead of a slice, I've found this IR (simplified for presentation):

@ref = constant i64 0
@const = constant i64* @ref
...
  %1 = load i64**, i64*** bitcast (i64** @const to i64***)
  %2 = call i64 @method(i64** %1)

Which is roughly the same as:

@ref = constant i64 0
...
  %2 = call i64 @method(i64** bitcast (i64* @ref to i64**))

So trans::expr is taking an A: &T constant and casting it to &mut &T without actually performing the autoref (which involves taking a &mut to a stack temporary containing the &T).

cc @dotdash

[nikomatsakis]

Just to be clear: the problem is in trans, right? That is, I expect this example to compile, but it seems like the temporary stack slot is being dropped too early?

[eddyb]

@nikomatsakis Yes, it's supposed to compile, and no, there is no temporary stack slot, &'static T is effectively transmuted to &mut &T by some incorrect adjustment handling.

[nikomatsakis]

@eddyb

no, there is no temporary stack slot,

But there ought to be one, right? The second call requires an autoref (it is an &self method defined on a &[T]) -- this creates a temporary (which contains an &'static value).

[eddyb]

@nikomatsakis There should be, but there isn't. That's the bug.

[jethrogb]

Ping. Can someone at least assign some labels to this issue?

[nikomatsakis]

In theory this is probably fixed with -Z orbit, we should test.

[nikomatsakis]

Given that this works with -Z orbit, assigning low priority. Will be fixed in MIR transition.

[nikomatsakis]

triage: P-low

[sanmai-NL]

It appears this issue is related to one I have. Why is it possible to make mutable references to const items but not to let-bound items? This is very counterintuitive (at least to a beginner) and seems hardly productive to me.

See this playground: https://is.gd/6eVLzl

[eddyb]

You shouldn't be able to get a mutable reference that you can use to mutate the constant but instead a copy (e.g. just like you can do &mut 0).

[jethrogb]

This issue has been fixed in the MIR rewrite.

@sanmai-NL What you're seeing is not related and is also working as intended. A const does not define a variable but rather you can think of it as an “alias” for writing out the const value in place. Whereever you are using MY_STRUCT in your code, you are creating a temporary with the value specified. Temporaries are mutable.

[petrochenkov]

There are two issues about issuing warnings for this situation - https://github.com/Manishearth/rust-clippy/issues/829 and rust-lang/rfcs#885.

[sanmai-NL]

@eddyb: In my understanding, the struct in my example has move semantics, so creating a temporary in the way shown should move it and I should be unable to use the constant after creating the mutable reference to the temporary, no? (I'm just sounding off my intuition here, I expect this analysis to be wrong.)

[jethrogb]

Every time you write MY_STRUCT you're instantiating a new temporary with the value MyStruct { title: "Original" }. Again, it's exactly the same as if you would just write MyStruct { title: "Original" } instead.

[sanmai-NL]

@jethrogb: Thanks for explaining.

From what I read in the Rust book:

const items are not necessarily guaranteed to refer to the same memory address

, which is misleading if they practically always refer to a different address in memory. Your issue is not a docs issue, but I hope I may take the opportunity to point out another confusing or misleading sentence there:

Constants live for the entire lifetime of a program.

They don't, they do not have a single lifetime at all, they have lifetimes corresponding to each scope in which they are written (not ‘used’).

I really wonder what the use of const items is when it is alternatively possible to write a placeholder macro that provides a literal? const items cannot be used as literals with e.g. std::format!, they are distinct from literals in disadvantageous ways to programmers, but in the ‘mutably referencing constants’ case they behave similarly to literals.

(Okay now, I won't discuss this side issue further even in response.)