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unused_mut lint doesn't cover every case #43526

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CryZe opened this issue Jul 28, 2017 · 1 comment · Fixed by #43582
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unused_mut lint doesn't cover every case #43526

CryZe opened this issue Jul 28, 2017 · 1 comment · Fixed by #43582
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A-lints Area: Lints (warnings about flaws in source code) such as unused_mut. C-bug Category: This is a bug.

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@CryZe
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CryZe commented Jul 28, 2017 

fn main() {
    let mut x = &mut 5;
    *x = 4;
    println!("{}", x);
}

Playground

This does not print a warning that x doesn't need to be mutable, even though the binding never changes.
@sfackler sfackler added the A-lints Area: Lints (warnings about flaws in source code) such as unused_mut. label Jul 28, 2017
@Mark-Simulacrum Mark-Simulacrum added the C-bug Category: This is a bug. label Jul 28, 2017
@oli-obk
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oli-obk commented Jul 28, 2017

Isn't that essentially the same as #25049 ?

@ivanbakel ivanbakel mentioned this issue Aug 1, 2017
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bors added a commit that referenced this issue Aug 10, 2017
@bors
Auto merge of #43582 - ivanbakel:unused_mut_ref, r=arielb1
d21ec9b 
Fixed mutable vars being marked used when they weren't

#### NB : bootstrapping is slow on my machine, even with `keep-stage` - fixes for occurances in the current codebase are <s>in the pipeline</s> done. This PR is being put up for review of the fix of the issue.

Fixes #43526, Fixes #30280, Fixes #25049

### Issue
Whenever the compiler detected a mutable deref being used mutably, it marked an associated value as being used mutably as well. In the case of derefencing local variables which were mutable references, this incorrectly marked the reference itself being used mutably, instead of its contents - with the consequence of making the following code emit no warnings
```
fn do_thing<T>(mut arg : &mut T) {
    ... // don't touch arg - just deref it to access the T
}
```

### Fix
Make dereferences not be counted as a mutable use, but only when they're on borrows on local variables.
#### Why not on things other than local variables?
  * Whenever you capture a variable in a closure, it gets turned into a hidden reference - when you use it in the closure, it gets dereferenced. If the closure uses the variable mutably, that is actually a mutable use of the thing being dereffed to, so it has to be counted.
  * If you deref a mutable `Box` to access the contents mutably, you are using the `Box` mutably - so it has to be counted.
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A-lints Area: Lints (warnings about flaws in source code) such as unused_mut. C-bug Category: This is a bug.
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Fixed mutable vars being marked used when they weren't ivanbakel/rust
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@oli-obk @CryZe @sfackler @Mark-Simulacrum