Improve diagnostics to guide users past Borrow checking regression in brotli

[SimonSapin]

I mean regression in the sense that code that used to compile doesn’t anymore. Maybe that is an intentional soundness fix.

Compiling https://crates.io/crates/brotli / https://github.com/dropbox/rust-brotli with:

• rustc 1.25.0-nightly (61452e5 2018-01-09): OK
• rustc 1.25.0-nightly (f62f774 2018-01-10): borrowck error:

error[E0502]: cannot borrow `self.buckets_` as immutable because it is also borrowed as mutable
   --> src/enc/backward_references.rs:339:51
    |
338 |     (*self).buckets_.slice_mut()[(key as (usize)).wrapping_add((cur_ix >> 3)
    |     ---------------- mutable borrow occurs here
339 |                                     .wrapping_rem(self.buckets_.BUCKET_SWEEP() as
    |                                                   ^^^^^^^^^^^^^ immutable borrow occurs here
340 |                                                   usize))] = cur_ix as (u32);
    |                                                          - mutable borrow ends here

error: aborting due to previous error

error: Could not compile `brotli`.

Commit range: 61452e5...f62f774. #47167 seems relevant. CC @ivanbakel @eddyb

[eddyb]

cc @nikomatsakis

[nikomatsakis]

Hmm.

[nikomatsakis]

triage: P-high

Have to get to the bottom of this.

[nikomatsakis]

Standalone test case:

trait Sweep {
    fn sweep(&self) -> usize;
}

trait SliceWrapper<T> {
    fn slice(&self) -> &[T];
}

trait SliceWrapperMut<T> {
    fn slice_mut(&mut self) -> &mut [T];
}

fn foo<B: SliceWrapper<u32> + SliceWrapperMut<u32> + Sweep>(mut buckets: B) {
    let key = 0u32;
    buckets.slice_mut()[(key as usize).wrapping_add(22).wrapping_rem(buckets.sweep())] = 22;
}

fn main() { }

changing the function foo to this version, bar, that operates on slices makes the error go away:

fn bar(buckets: &mut [u32]) {
    let key = 0u32;
    buckets[(key as usize).wrapping_add(22).wrapping_rem(buckets.len())] = 22;
}

Now to decide if it is legit. =)