Fix reborrows of &mut pointers

[dotdash]

Fixes #28839

[rust-highfive]

r? @nikomatsakis

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[nikomatsakis]

r=me with comment

[dotdash]

@bors r=nikomatsakis 73396b5

[Manishearth]

---- [compile-fail] compile-fail/borrowck-loan-of-static-data-issue-27616.rs stdout ----

error: unexpected compiler error or warning: '/Users/rustbuild/src/rust-buildbot/slave/auto-mac-64-nopt-t/build/src/test/compile-fail/borrowck-loan-of-static-data-issue-27616.rs:26:5: 26:23 error: cannot assign to `*s` because it is borrowed [E0506]'
status: exit code: 101
command: x86_64-apple-darwin/stage2/bin/rustc /Users/rustbuild/src/rust-buildbot/slave/auto-mac-64-nopt-t/build/src/test/compile-fail/borrowck-loan-of-static-data-issue-27616.rs -L x86_64-apple-darwin/test/compile-fail/ --target=x86_64-apple-darwin -L x86_64-apple-darwin/test/compile-fail/borrowck-loan-of-static-data-issue-27616.stage2-x86_64-apple-darwin.compile-fail.libaux -C prefer-dynamic -o x86_64-apple-darwin/test/compile-fail/borrowck-loan-of-static-data-issue-27616.stage2-x86_64-apple-darwin --cfg rtopt -L x86_64-apple-darwin/rt
stdout:
------------------------------------------

------------------------------------------
stderr:
------------------------------------------
/Users/rustbuild/src/rust-buildbot/slave/auto-mac-64-nopt-t/build/src/test/compile-fail/borrowck-loan-of-static-data-issue-27616.rs:26:5: 26:23 error: cannot assign to `*s` because it is borrowed [E0506]
/Users/rustbuild/src/rust-buildbot/slave/auto-mac-64-nopt-t/build/src/test/compile-fail/borrowck-loan-of-static-data-issue-27616.rs:26     *s = String::new(); //~ ERROR use of moved value
                                                                                                                                           ^~~~~~~~~~~~~~~~~~
/Users/rustbuild/src/rust-buildbot/slave/auto-mac-64-nopt-t/build/src/test/compile-fail/borrowck-loan-of-static-data-issue-27616.rs:23:38: 23:39 note: borrow of `*s` occurs here
/Users/rustbuild/src/rust-buildbot/slave/auto-mac-64-nopt-t/build/src/test/compile-fail/borrowck-loan-of-static-data-issue-27616.rs:23     let alias: &'static mut String = s;
                                                                                                                                                                            ^
error: aborting due to previous error

------------------------------------------

[bors]

⌛ Testing commit 73396b5 with merge 7de3c81...

[bors]

💔 Test failed - auto-mac-64-opt

[dotdash]

@bors r=nikomatsakis d96ef0b

The test originally said cannot assign but was in the wrong directory. It was adjusted to accept the new, wrong error message in #28519. I reverted it back to the original check.

[bors]

⌛ Testing commit d96ef0b with merge eb5076c...

[bors]

💔 Test failed - auto-linux-32-opt

[dotdash]

The failure on bors looks retry worthy, but I'm also getting a pretty failure locally, which I don't know how to fix.

running 1 test
test [pretty] run-pass/issue-28839.rs ... FAILED

failures:

---- [pretty] run-pass/issue-28839.rs stdout ----

error: pretty-printed source does not match expected source

expected:
------------------------------------------
// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
// file at the top-level directory of this distribution and at
// http://rust-lang.org/COPYRIGHT.
//
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.

pub struct Foo;

pub fn get_foo2<'a>(foo: &'a mut Option<&'a mut Foo>) -> &'a mut Foo {
    match foo {

        // Ensure that this is not considered a move, but rather a reborrow.
        &mut Some(ref mut x) => *x,
        &mut None => panic!(),
    }
}

fn main() { }

------------------------------------------
actual:
------------------------------------------
// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
// file at the top-level directory of this distribution and at
// http://rust-lang.org/COPYRIGHT.
//
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.

pub struct Foo;

pub fn get_foo2<'a>(foo: &'a mut Option<&'a mut Foo>) -> &'a mut Foo {
    match foo {


        // Ensure that this is not considered a move, but rather a reborrow.
        &mut Some(ref mut x) => *x,
        &mut None => panic!(),
    }
}

fn main() { }

------------------------------------------


thread '[pretty] run-pass/issue-28839.rs' panicked at 'explicit panic', /home/bs/src/rust/src/compiletest/runtest.rs:298

[dotdash]

@bors r=nikomatsakis 4da796f

Marked the test to be ignored by pretty, same bug as #23623

[bors]

⌛ Testing commit 4da796f with merge 3cee9e2...

[bors]

☀️ Test successful - auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-x-android-t, auto-mac-32-opt, auto-mac-64-nopt-t, auto-mac-64-opt, auto-win-gnu-32-nopt-t, auto-win-gnu-32-opt, auto-win-gnu-64-nopt-t, auto-win-gnu-64-opt, auto-win-msvc-32-opt, auto-win-msvc-64-opt