Compiler Warnings 2018-07-30

When building Clippy the compiler now emits the warnings below:

warning[E0502]: cannot borrow `reg.sess.target` as immutable because it is also borrowed as mutable
   --> clippy_lints/src/lib.rs:371:13
    |
369 |       reg.register_late_lint_pass(box trivially_copy_pass_by_ref::TriviallyCopyPassByRef::new(
    |       ---
    |       |
    |  _____mutable borrow occurs here
    | |
370 | |             conf.trivial_copy_size_limit,
371 | |             &reg.sess.target,
    | |             ^^^^^^^^^^^^^^^^ immutable borrow occurs here
372 | |     ));
    | |______- borrow later used here
    |
    = warning: This error has been downgraded to a warning for backwards compatibility with previous releases.
            It represents potential unsoundness in your code.
            This warning will become a hard error in the future.

warning: variable does not need to be mutable
   --> clippy_lints/src/consts.rs:244:21
    |
244 |                 let mut value = !value;
    |                     ----^^^^^
    |                     |
    |                     help: remove this `mut`
    |
    = note: #[warn(unused_mut)] on by default

warning[E0502]: cannot borrow `self.cx.tcx` as immutable because it is also borrowed as mutable
   --> clippy_lints/src/utils/hir_utils.rs:518:33
    |
518 |                 self.hash_expr(&self.cx.tcx.hir.body(l_id.body).value);
    |                 ----------------^^^^^^^^^^^---------------------------
    |                 |               |
    |                 |               immutable borrow occurs here
    |                 mutable borrow occurs here
    |                 borrow later used here
    |
    = warning: This error has been downgraded to a warning for backwards compatibility with previous releases.
            It represents potential unsoundness in your code.
            This warning will become a hard error in the future.

warning[E0502]: cannot borrow `self.cx.tcx` as immutable because it is also borrowed as mutable
   --> clippy_lints/src/utils/hir_utils.rs:451:33
    |
451 |                 self.hash_expr(&self.cx.tcx.hir.body(eid).value);
    |                 ----------------^^^^^^^^^^^---------------------
    |                 |               |
    |                 |               immutable borrow occurs here
    |                 mutable borrow occurs here
    |                 borrow later used here
    |
    = warning: This error has been downgraded to a warning for backwards compatibility with previous releases.
            It represents potential unsoundness in your code.
            This warning will become a hard error in the future.

I've already got a fix for the unused_mut error.
Is this a compiler issue or are these warnings genuine? Shouldn't the borrow on the argument end before the method is called?

[flip1995]

I guess these warnings are caused by this PR: rust-lang/rust#52681

https://github.com/rust-lang/rust/pull/52681/files#diff-5b4d01d26caf43976125ba0f877e78c0R337

[pnkfelix]

Hmm, i would have expected at least some of these cases to be allowed in NLL due to two-phase borrows...

Update: Oh, no, i think I misread the code. two-phase borrows let you do an immutable borrow during an expression that evaluated for a method call's parameter, but the borrow has to end before the method itself gets around to executing. In the cases that I misidentified as being potentially allowed, I think you have immutable borrows that are lasting long enough to leak into the method call execution.

[pnkfelix]

(Still this case is interesting because I don't think I was aware of an AST-borrowck soundness bug of this form, and yet this seems to be one...)

((Or maybe the borrows here are of derefed data, and it doesn’t actually conflict with the &mut. So that would bring me back to thinking this is a weakness in two phase borrows ))

[nikomatsakis]

I believe @pnkfelix that this could be rust-lang/rust#38899