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Improve error message for lifetime error with dyn Trait #67378

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ohadravid
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@ohadravid ohadravid commented Dec 17, 2019 •
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This is an attempt to improve the errors in cases like #54779.
Because SomeType<dyn OtherTrait> and impl SomeTrait for dyn OtherTrait are implicitly dyn OtherTrait+ 'static, it can be quite confusing as to why things don't work. (I originally stumbled upon this because of this question - playground link).

This PR tries to handle those two cases, by:

  1. Pointing at the requiring implementation.
  2. Suggesting the dyn Trait + '_, like impl Trait does.

There seems to be a lot of places to handle these errors, and I can't really say I fully understand the sub/sup/origin region stuff, but I got it to mostly work and wanted to get some feedback on what should I do next (and if! I'm not sure if this is the best idea/approach).
Also there are almost no existing examples in the test suite which seems to cover this usecase.

Most noticeably, I think that if you already specify + 'static or some other lifetime, the suggestion will be both useless and incorrect, and if you have Box<&dyn Trait> you'll get Box<&dyn Trait + '_> which isn't really correct (should be &(dyn Trait + '_)).

r? @estebank

@rust-highfive rust-highfive added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Dec 17, 2019
estebank
estebank reviewed Dec 18, 2019
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src/librustc/infer/error_reporting/mod.rs
"you can add an explicit constraint to the implementation so that it applies \
to types with less than `'static` lifetime",
),
snippet.replace(dyn_trait_name, &format!("{} + '_", &dyn_trait_name)),
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I don't have time to do an in-depth review right now, but we should try to come up with a different way of doing this. Fiddling with the textual representation of the code is a recipe for pain later on. It produces output that isn't always correct (like the problem with the missing parens and some incorrect suggestions I noticed below), and is brittle and affected by whitespace. I'll take another look later to see if I can have more actionable feedback on what to do instead.

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bors commented Dec 22, 2019

☔ The latest upstream changes (presumably #67495) made this pull request unmergeable. Please resolve the merge conflicts.

@Dylan-DPC-zz Dylan-DPC-zz added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Dec 23, 2019
@joelpalmer
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Ping from Triage: Any updates @ohadravid or @estebank?

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@joelpalmer I'll close this and try something more minimal in a different PR

@ohadravid ohadravid closed this Jan 6, 2020
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