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Detect iterators going out of scope and suggest collecting #81584

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estebank opened this issue Jan 31, 2021 · 4 comments · Fixed by #81939
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Detect iterators going out of scope and suggest collecting #81584

estebank opened this issue Jan 31, 2021 · 4 comments · Fixed by #81939
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@kper
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A-diagnostics Area: Messages for errors, warnings, and lints A-iterators Area: Iterators A-lifetimes Area: lifetime related A-suggestion-diagnostics Area: Suggestions generated by the compiler applied by `cargo fix`. C-enhancement Category: An issue proposing an enhancement or a PR with one. D-newcomer-roadblock Diagnostics: Confusing error or lint; hard to understand for new users. P-low Low priority T-compiler Relevant to the compiler team, which will review and decide on the PR/issue.

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@estebank
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estebank commented Jan 31, 2021 •
edited by rustbot
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Given a situation where an iterator is constructed inside of a scope where the data is being returned, we currently emit:

error[E0515]: cannot return value referencing function parameter `y`
 --> src/main.rs:4:18
  |
4 |         .map(|y| y.iter().map(|x| x + 1))
  |                  -^^^^^^^^^^^^^^^^^^^^^^
  |                  |
  |                  returns a value referencing data owned by the current function
  |                  `y` is borrowed here

We should suggest using .collect() in these cases.
@estebank estebank added A-diagnostics Area: Messages for errors, warnings, and lints A-lifetimes Area: lifetime related P-low Low priority T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. A-suggestion-diagnostics Area: Suggestions generated by the compiler applied by `cargo fix`. D-newcomer-roadblock Diagnostics: Confusing error or lint; hard to understand for new users. labels Jan 31, 2021
@leonardo-m
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Or flatten/flat_map?

@estebank
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I want for us to detect cases of trying to operate on iterators of iterators because the only option then is to allocate or completely rework the logic. The former will help newcomers, experts will know enough to try the later.

@camelid camelid added A-iterators Area: Iterators C-enhancement Category: An issue proposing an enhancement or a PR with one. labels Feb 1, 2021
@kper
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kper commented Feb 9, 2021

@rustbot claim

@kper kper mentioned this issue Feb 9, 2021
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@the8472
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the8472 commented Feb 9, 2021

Couldn't we still recommend both? Recommending bad practices seems suboptimal.

Dylan-DPC-zz pushed a commit to Dylan-DPC-zz/rust that referenced this issue Mar 5, 2021
@Dylan-DPC
Rollup merge of rust-lang#81939 - kper:fixing-81584-allocate-in-iter,…
d89038f 
… r=davidtwco

Add suggestion `.collect()` for iterators in iterators

Closes rust-lang#81584

```
error[E0515]: cannot return value referencing function parameter `y`
 --> main3.rs:4:38
  |
4 | ...                   .map(|y| y.iter().map(|x| x + 1))
  |                                -^^^^^^^^^^^^^^^^^^^^^^
  |                                |
  |                                returns a value referencing data owned by the current function
  |                                `y` is borrowed here
  |                                help: Maybe use `.collect()` to allocate the iterator
```

Added the suggestion: `help: Maybe use `.collect()` to allocate the iterator`
m-ou-se added a commit to m-ou-se/rust that referenced this issue Mar 5, 2021
@m-ou-se
Rollup merge of rust-lang#81939 - kper:fixing-81584-allocate-in-iter,…
2032fc7 
… r=davidtwco

Add suggestion `.collect()` for iterators in iterators

Closes rust-lang#81584

```
error[E0515]: cannot return value referencing function parameter `y`
 --> main3.rs:4:38
  |
4 | ...                   .map(|y| y.iter().map(|x| x + 1))
  |                                -^^^^^^^^^^^^^^^^^^^^^^
  |                                |
  |                                returns a value referencing data owned by the current function
  |                                `y` is borrowed here
  |                                help: Maybe use `.collect()` to allocate the iterator
```

Added the suggestion: `help: Maybe use `.collect()` to allocate the iterator`
@bors bors closed this as completed in 20887b7 Mar 5, 2021
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@kper kper

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A-diagnostics Area: Messages for errors, warnings, and lints A-iterators Area: Iterators A-lifetimes Area: lifetime related A-suggestion-diagnostics Area: Suggestions generated by the compiler applied by `cargo fix`. C-enhancement Category: An issue proposing an enhancement or a PR with one. D-newcomer-roadblock Diagnostics: Confusing error or lint; hard to understand for new users. P-low Low priority T-compiler Relevant to the compiler team, which will review and decide on the PR/issue.
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Add suggestion .collect() for iterators in iterators kper/rust
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@the8472 @estebank @kper @leonardo-m @camelid