Incorrect unused lifetime warning after import failure

[RalfJung]

When building this commit with ./x.py build --stage 0 src/librustc_mir, I get the following output:

error[E0432]: unresolved import `crate::interpret::InterpretCx`
  --> src/librustc_mir/const_eval.rs:25:59
   |
25 |     InterpResult, InterpErrorInfo, InterpError, GlobalId, InterpretCx, StackPopCleanup,
   |                                                           ^^^^^^^^^^^
   |                                                           |
   |                                                           no `InterpretCx` in `interpret`
   |                                                           help: a similar name exists in the module: `InterpCx`

error[E0432]: unresolved import `crate::interpret::InterpretCx`
  --> src/librustc_mir/transform/const_prop.rs:26:11
   |
26 |     self, InterpretCx, ScalarMaybeUndef, Immediate, OpTy,
   |           ^^^^^^^^^^^
   |           |
   |           no `InterpretCx` in `interpret`
   |           help: a similar name exists in the module: `InterpCx`

error[E0392]: parameter `'mir` is never used
  --> src/librustc_mir/interpret/intern.rs:23:27
   |
23 | struct InternVisitor<'rt, 'mir, 'tcx> {
   |                           ^^^^ unused parameter
   |
   = help: consider removing `'mir` or using a marker such as `std::marker::PhantomData`

error: aborting due to 3 previous errors

The first two are expected, that's a mistake I made. The last one is wrong though, 'mir is used:

struct InternVisitor<'rt, 'mir, 'tcx> {
    /// previously encountered safe references
    ref_tracking: &'rt mut RefTracking<(MPlaceTy<'tcx>, Mutability, InternMode)>,
    ecx: &'rt mut CompileTimeEvalContext<'mir, 'tcx>, // <-- used here!
    /* ... */
}

The thing is, it gets used as argument to CompileTimeEvalContext, which is defined in const_eval.rs, which had an import failure.

So it looks like the "unused lifetime" lint is a bit too aggressive here and considers a lifetime unused because the way it got used referred to a type that was in a failing module -- or so. I tried to minimize this but failed, here's my last attempt. But anyway I think this is a bug -- we want the user to look at the actual failure, not at spurious warnings that were caused by the failure.

[Enselic]

@rustbot label E-needs-mcve A-lifetimes