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nll: incorrect unused-mut warnings #51918

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nikomatsakis opened this issue Jun 29, 2018 · 4 comments
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nll: incorrect unused-mut warnings #51918

nikomatsakis opened this issue Jun 29, 2018 · 4 comments
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@matthewjasper
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A-NLL Area: Non-lexical lifetimes (NLL) C-enhancement Category: An issue proposing an enhancement or a PR with one. NLL-complete Working towards the "valid code works" goal T-compiler Relevant to the compiler team, which will review and decide on the PR/issue.
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Rust 2018 Preview 2

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@nikomatsakis
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nikomatsakis commented Jun 29, 2018 •
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There are a lot of incorrect unused_mut warnings coming up. I'm going to make this meta-bug for tracking them because it's very unclear how many underlying issues there are:
@nikomatsakis nikomatsakis added T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. WG-compiler-nll NLL-complete Working towards the "valid code works" goal labels Jun 29, 2018
@nikomatsakis nikomatsakis added this to the Rust 2018 Preview 2 milestone Jun 29, 2018
This was referenced Jun 29, 2018
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@matthewjasper
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There are at least two different issues here it seems:

fn foo(ref x: i32) {}

fn bar() {
    let mut x = 0;
    move || {
        x = 1;
    };
}

I'll try to have a PR for the first one over the weekend.
The second one requires move investigation, because the code is certainly trying to avoid linting in this case.

@matthewjasper matthewjasper mentioned this issue Jul 1, 2018
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@jkordish jkordish added C-enhancement Category: An issue proposing an enhancement or a PR with one. A-NLL Area: Non-lexical lifetimes (NLL) labels Jul 3, 2018
bors added a commit that referenced this issue Jul 5, 2018
@bors
Auto merge of #51964 - matthewjasper:unused-mut-mir-generation, r=nik…
b51ca20 
…omatsakis

[NLL] Fix various unused mut errors

Closes #51801
Closes #50897
Closes #51830
Closes #51904
cc #51918 - keeping this one open in case there are any more issues

This PR contains multiple changes. List of changes with examples of what they fix:

* Change mir generation so that the parameter variable doesn't get a name when a `ref` pattern is used as an argument
```rust
fn f(ref y: i32) {} // doesn't trigger lint
```
* Change mir generation so that by-move closure captures don't get first moved into a temporary.
```rust
let mut x = 0; // doesn't trigger lint
move || {
    x = 1;
};
```
* Treat generator upvars the same as closure upvars
```rust
let mut x = 0; // This mut is now necessary and is not linted against.
move || {
    x = 1;
    yield;
};
```

r? @nikomatsakis
@nikomatsakis
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@matthewjasper what remains now that your PR landed?

@matthewjasper
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I don't think there are any more bugs. I think this is just waiting for this to be on nightly a few days to be sure.

@nikomatsakis
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Closing as fixed then. @matthewjasper let me know if you think that's wrong!

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A-NLL Area: Non-lexical lifetimes (NLL) C-enhancement Category: An issue proposing an enhancement or a PR with one. NLL-complete Working towards the "valid code works" goal T-compiler Relevant to the compiler team, which will review and decide on the PR/issue.
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Rust 2018 Preview 2
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@jkordish @nikomatsakis @matthewjasper