Indexing regression in beta

[Mark-Simulacrum]

seal-0.1.0 regressed with the following:

error[E0502]: cannot borrow `*self` as immutable because it is also borrowed as mutable
  --> src/pair/alignment_matrix.rs:79:26
   |
79 |         self.mut_slice()[self.offset(cursor)] = byte;
   |         ----             ^^^^               - mutable borrow ends here
   |         |                |
   |         |                immutable borrow occurs here
   |         mutable borrow occurs here

error: aborting due to previous error

error: Could not compile `seal`.

This looks vaguely familiar but looking at pull requests didn't seem to find anything apparent.

cc @rust-lang/compiler

[Mark-Simulacrum]

cc @regexident

[nikomatsakis]

I too remember something like this. I think it was a bug fix. Let me see if I can find it.

[nikomatsakis]

Maybe same problem as reported here: #47349

[Mark-Simulacrum]

Okay, we should investigate. I'm going to nominate and set to P-high for compiler team eyes, if it's the same reason as #47349 then let's just close -- I don't think we need to do much more than that if it's a bugfix.

[kennytm]

The regression is caused by #47167 according to bisection, as expected.

Test program:

#![crate_type = "rlib"]
struct Cursor;
struct AlignmentMatrix(Vec<u8>);
impl AlignmentMatrix {
    fn offset(&self, _: &Cursor) -> usize {
        0
    }
    fn mut_slice(&mut self) -> &mut [u8] {
        &mut self.0
    }
    pub fn set_at(&mut self, cursor: &Cursor) {
        self.mut_slice()[self.offset(cursor)] = 0;
    }
}

Test script:

#!/bin/sh
$RUSTC_RELATIVE -V
! $RUSTC_RELATIVE 1.rs                  

Result:

$ RUST_LOG=rust_sysroot=info target/release/bisect --start b65f0bedd2f22d9661ecb7092f07746dc2ccfb0d --end c83fa5d91c3b16459ab7b87c48ed18bd059a23af --test test.sh

INFO:rust_sysroot: Getting commits from the git checkout in b65f0bedd2f22d9661ecb7092f07746dc2ccfb0d...c83fa5d91c3b16459ab7b87c48ed18bd059a23af
INFO:rust_sysroot: Received 159 commits
Searching in 159 commits; about 8 steps
rustc 1.25.0-nightly (97520ccb1 2018-01-21)
rustc 1.25.0-nightly (27ede5541 2018-01-10)
rustc 1.25.0-nightly (bb345a0be 2018-01-15)
rustc 1.25.0-nightly (9b2f8ac29 2018-01-13)
rustc 1.25.0-nightly (0b90e4e8c 2018-01-12)
rustc 1.25.0-nightly (619ced057 2018-01-11)
rustc 1.25.0-nightly (c9c298073 2018-01-11)
rustc 1.25.0-nightly (f62f77403 2018-01-10)
searched commits b65f0bedd2f22d9661ecb7092f07746dc2ccfb0d through c83fa5d91c3b16459ab7b87c48ed18bd059a23af
regression in 40; Some(Commit { sha: "f62f774035735a06c880c48c0b9017fcc0577e33", date: 2018-01-10T12:29:05Z, summary: "Auto merge of #47167 - ivanbakel:builtin_indexing, r=nikomatsakis" })

[regexident]

So, what did I win? 😅

^{(Joking aside, any way I can be of help here?)}

[kennytm]

@regexident You could split that into two statements:

let offset = self.offset(cursor);
self.mut_slice()[offset] = byte;

[nikomatsakis]

Since this is due to a confirmed bug fix, I'm going to close the issue then. @regexident please let us know though if we can help in any other way!