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Changing mutable borrow check error when it occurs in a loop #2080

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ivanbakel opened this issue Jul 25, 2017 · 4 comments
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Changing mutable borrow check error when it occurs in a loop #2080

ivanbakel opened this issue Jul 25, 2017 · 4 comments
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@ivanbakel
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I'm happy to write up a full RFC for this, but I'd prefer to float the idea and understand if more contextual messages are a good thing. Inspired by my own experience in rust-lang/rust#43437, the message for two mutable borrows doesn't account for the case of loops - producing an error message that looks strange and doesn't really communicate what the borrow checker is having a problem with.

I'd like to change it so that the formatted message is changed based on it occuring inside a loop, similar to the use of moved value error message, which specifies when the move occurs in a loop iteration.

The code is already written, in ivanbakel/rust@53fb2a5 .
@Mark-Simulacrum
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I think that a PR would be sufficient for this. In general, if we can detect a particular case accurately (and with little overhead for the non-erroring path) more specific error messages are greatly preferable.

@ivanbakel
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What do you mean by a PR - a RFC PR, or a PR directly to the rust-lang/rust repo?

@Mark-Simulacrum
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Sorry, a PR directly to rust-lang/rust.

@ivanbakel ivanbakel mentioned this issue Jul 25, 2017
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bors added a commit to rust-lang/rust that referenced this issue Jul 27, 2017
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Auto merge of #43479 - ivanbakel:loop_borrow_msg, r=estebank
da4d490 
Extended error message for mut borrow conflicts in loops

RFC issue: rust-lang/rfcs#2080

The error message for multiple mutable borrows on the same value over loop iterations now makes it clear that the conflict comes from the borrow outlasting the loop. The wording of the error is based on the special case of the moved-value error for a value moved in a loop. Following the example of that error, the code remains the same for the special case.

This is mainly because I felt the current message is confusing in the loop case : #43437. It's not clear that the two conflicting borrows are in different iterations of the loop, and instead it just looks like the compiler has an issue with a single line.
@ivanbakel
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My PR was merged, so I'm closing this.

@Centril Centril added the T-lang Relevant to the language team, which will review and decide on the RFC. label Feb 23, 2018
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