Account for missing lifetime in opaque and trait object return types #72543

estebank · 2020-05-24T17:57:00Z

When encountering an opaque closure return type that needs to bound a
lifetime to the function's arguments, including borrows and type params,
provide appropriate suggestions that lead to working code.

Get the user from

fn foo<G, T>(g: G, dest: &mut T) -> impl FnOnce()
where
    G: Get<T>
{
    move || {
        *dest = g.get();
    }
}

to

fn foo<'a, G: 'a, T>(g: G, dest: &'a mut T) -> impl FnOnce() +'a
where
    G: Get<T>
{
    move || {
        *dest = g.get();
    }
}

estebank · 2020-05-24T20:25:49Z

The nll output is as good as the new regular output. I'll take a look at that later.

src/test/ui/underscore-lifetime/dyn-trait-underscore.stderr

src/test/ui/suggestions/lifetimes/missing-lifetimes-in-signature.stderr

nikomatsakis · 2020-05-26T21:54:23Z

src/test/ui/issues/issue-16922.stderr

+   |     ---------^^^^^-
+   |     |        |
+   |     |        ...but this borrow...
+   |     this evaluates to the `'static` lifetime...


hmm, this kind of prints like a "jumble" to me...

I would prefer to keep output closer to E0621 but have trouble not "hiding" conceptual information that is actually necessary to ever learn why the + '_ fixes it.

Do you think we could print this?

error[E0621]: non-static reference captured in `dyn Any` type without explicit bound --> $DIR/issue-16922.rs:4:5 | LL | fn foo<T: Any>(value: &T) -> Box<dyn Any> { | -- data with this lifetime... LL | Box::new(value) as Box<dyn Any> | ^^^^^^^^^^^^^^^ is captured in a `dyn Any` type here with the default `'static` bound help: for a `dyn` type to capture a non-static reference, it needs an explicit bound, as shown | LL | fn foo<T: Any>(value: &T) -> Box<dyn Any + '_> { |

It seems like we might know everything we need to know to "special case" this scenario, no?

I think it should be possible, yes, probably as a new nice_region_error impl.

What about the following?

error: cannot infer an appropriate lifetime --> src/test/ui/issues/issue-16922.rs:4:14 | 3 | fn foo<T: Any>(value: &T) -> Box<dyn Any> { | -- data with this lifetime... 4 | Box::new(value) as Box<dyn Any> | ---------^^^^^- | | | | | ...is captured here... | ...and required to be `'static` by this | help: to permit non-static references in a `dyn Trait` value, you can add an explicit `the anonymous lifetime #1 defined on the function body at 3:1` bound | 3 | fn foo<T: Any>(value: &T) -> Box<dyn Any + '_> { | ^^^^

this looks good too, though it still has the "jumbled" look I was hoping to avoid

it does give more information

I think it looks good enough

but I could further tweak it to be

error: cannot infer an appropriate lifetime --> src/test/ui/issues/issue-16922.rs:4:14 | 3 | fn foo<T: Any>(value: &T) -> Box<dyn Any> { | -- data with this lifetime... 4 | Box::new(value) as Box<dyn Any> | ^^^^^ ...is captured here... note: ...and required to be `'static` by this | 4 | Box::new(value) as Box<dyn Any> | ^^^^^^^^^^^^^^^ help: to permit non-static references in a `dyn Trait` value, you can add an explicit `the anonymous lifetime #1 defined on the function body at 3:1` bound | 3 | fn foo<T: Any>(value: &T) -> Box<dyn Any + '_> { | ^^^^

src/test/ui/issues/issue-16922.stderr

src/librustc_infer/infer/error_reporting/mod.rs

src/test/ui/suggestions/lifetimes/missing-lifetimes-in-signature.stderr

src/librustc_infer/infer/error_reporting/mod.rs

estebank · 2020-05-27T22:48:21Z

src/test/ui/regions/region-object-lifetime-in-coercion.stderr

+help: you can add a bound to the returned `dyn Trait` to make it last less than `'static` and match the anonymous lifetime #1 defined on the function body at 17:1
+   |
+LL | fn c(v: &[u8]) -> Box<dyn Foo + '_> {
+   |                               ^^^^


Should I change this to be "to make it require less than 'static" instead?

I might say

help: to permit non-static references in a dyn Trait value, you need to add an explicit bound, as shown

or

help: consider adding an explicit bound to the dyn Trait as shown, which is used to declare that the dyn Trait may capture non-static references

Is the "as shown" something you'd like to add to all suggestions? In my mind it is implied always, and might not work nicely with the presentation in 3rd party tools (non-CLI uses).

I'm cooking up a commit to make this be "to permit non-static references in a dyn Trait value, you can add an explicit bound". WDYT?

you can leave out the "as shown", I'm not sure how implicit it really is, but I can believe it

src/test/ui/type-alias-impl-trait/generic_type_does_not_live_long_enough.stderr

estebank · 2020-05-28T22:52:15Z

Check the last commit with some of your review comments addressed.

nikomatsakis · 2020-05-29T18:23:03Z

@bors r+

bors · 2020-05-29T18:23:04Z

📌 Commit ff82af1b25443de4e3af42403b0d89c770126169 has been approved by nikomatsakis

bors · 2020-05-30T03:41:58Z

☔ The latest upstream changes (presumably #72756) made this pull request unmergeable. Please resolve the merge conflicts.

When encountering an opaque closure return type that needs to bound a lifetime to the function's arguments, including borrows and type params, provide appropriate suggestions that lead to working code. Get the user from ```rust fn foo<G, T>(g: G, dest: &mut T) -> impl FnOnce() where G: Get<T> { move || { *dest = g.get(); } } ``` to ```rust fn foo<'a, G: 'a, T>(g: G, dest: &'a mut T) -> impl FnOnce() +'a where G: Get<T> { move || { *dest = g.get(); } } ```

…ing opaque return type Go from ``` error[E0495]: cannot infer an appropriate lifetime due to conflicting requirements --> file8.rs:22:5 | 22 | / move || { 23 | | *dest = g.get(); 24 | | } | |_____^ | note: first, the lifetime cannot outlive the anonymous lifetime rust-lang#1 defined on the function body at 18:1... --> file8.rs:18:1 | 18 | / fn bat<'a, G: 'a, T>(g: G, dest: &mut T) -> impl FnOnce() + '_ + 'a 19 | | where 20 | | G: Get<T> 21 | | { ... | 24 | | } 25 | | } | |_^ note: ...so that the types are compatible --> file8.rs:22:5 | 22 | / move || { //~ ERROR cannot infer an appropriate lifetime 23 | | *dest = g.get(); 24 | | } | |_____^ = note: expected `&mut T` found `&mut T` note: but, the lifetime must be valid for the lifetime `'a` as defined on the function body at 18:8... --> file8.rs:18:8 | 18 | fn bat<'a, G: 'a, T>(g: G, dest: &mut T) -> impl FnOnce() + '_ + 'a | ^^ note: ...so that return value is valid for the call --> file8.rs:18:45 | 18 | fn bat<'a, G: 'a, T>(g: G, dest: &mut T) -> impl FnOnce() + '_ + 'a | ^^^^^^^^^^^^^^^^^^^^^^^ ``` to ``` error[E0621]: explicit lifetime required in the type of `dest` --> file8.rs:18:45 | 18 | fn bat<'a, G: 'a, T>(g: G, dest: &mut T) -> impl FnOnce() + '_ + 'a | ------ ^^^^^^^^^^^^^^^^^^^^^^^ lifetime `'a` required | | | help: add explicit lifetime `'a` to the type of `dest`: `&'a mut T` ```

Provide a suggestion for `dyn Trait + '_` when possible.

…ments

estebank · 2020-05-30T19:49:03Z

@bors r=nikomatsakis

rebased

bors · 2020-05-30T19:49:04Z

📌 Commit 83f6f22 has been approved by nikomatsakis

@ghost

Rollup of 13 pull requests Successful merges: - rust-lang#72543 (Account for missing lifetime in opaque and trait object return types) - rust-lang#72625 (Improve inline asm error diagnostics) - rust-lang#72637 (expand `env!` with def-site context) - rust-lang#72650 (Sort sidebar elements) - rust-lang#72657 (Allow types (with lifetimes/generics) in impl_lint_pass) - rust-lang#72666 (Add -Z profile-emit=<path> for Gcov gcda output.) - rust-lang#72668 (Fix missing parentheses Fn notation error) - rust-lang#72669 (rustc_session: Cleanup session creation) - rust-lang#72728 (Make bootstrap aware of relative libdir in stage0 compiler) - rust-lang#72757 (rustc_lexer: Optimize shebang detection slightly) - rust-lang#72772 (miri validation: clarify valid values of 'char') - rust-lang#72773 (Fix is_char_boundary documentation) - rust-lang#72777 (rustdoc: remove calls to `local_def_id_from_node_id`) Failed merges: r? @ghost

@nikomatsakis

…, r=nikomatsakis Further tweak lifetime errors involving `dyn Trait` and `impl Trait` in return position * Suggest substituting `'static` lifetime in impl/dyn `Trait + 'static` instead of `Trait + 'static + '_` * When `'static` is explicit, also suggest constraining argument with it * Reduce verbosity of suggestion message and mention lifetime in label * Tweak output for overlapping required/captured spans * Give these errors an error code Follow up to rust-lang#72543. r? @nikomatsakis

@nikomatsakis

…, r=nikomatsakis Further tweak lifetime errors involving `dyn Trait` and `impl Trait` in return position * Suggest substituting `'static` lifetime in impl/dyn `Trait + 'static` instead of `Trait + 'static + '_` * When `'static` is explicit, also suggest constraining argument with it * Reduce verbosity of suggestion message and mention lifetime in label * Tweak output for overlapping required/captured spans * Give these errors an error code Follow up to rust-lang#72543. r? @nikomatsakis

@nikomatsakis

…, r=nikomatsakis Further tweak lifetime errors involving `dyn Trait` and `impl Trait` in return position * Suggest substituting `'static` lifetime in impl/dyn `Trait + 'static` instead of `Trait + 'static + '_` * When `'static` is explicit, also suggest constraining argument with it * Reduce verbosity of suggestion message and mention lifetime in label * Tweak output for overlapping required/captured spans * Give these errors an error code Follow up to rust-lang#72543. r? @nikomatsakis

@nikomatsakis

…, r=nikomatsakis Further tweak lifetime errors involving `dyn Trait` and `impl Trait` in return position * Suggest substituting `'static` lifetime in impl/dyn `Trait + 'static` instead of `Trait + 'static + '_` * When `'static` is explicit, also suggest constraining argument with it * Reduce verbosity of suggestion message and mention lifetime in label * Tweak output for overlapping required/captured spans * Give these errors an error code Follow up to rust-lang#72543. r? @nikomatsakis

@nikomatsakis

…, r=nikomatsakis Further tweak lifetime errors involving `dyn Trait` and `impl Trait` in return position * Suggest substituting `'static` lifetime in impl/dyn `Trait + 'static` instead of `Trait + 'static + '_` * When `'static` is explicit, also suggest constraining argument with it * Reduce verbosity of suggestion message and mention lifetime in label * Tweak output for overlapping required/captured spans * Give these errors an error code Follow up to rust-lang#72543. r? @nikomatsakis

@nikomatsakis

…, r=nikomatsakis Further tweak lifetime errors involving `dyn Trait` and `impl Trait` in return position * Suggest substituting `'static` lifetime in impl/dyn `Trait + 'static` instead of `Trait + 'static + '_` * When `'static` is explicit, also suggest constraining argument with it * Reduce verbosity of suggestion message and mention lifetime in label * Tweak output for overlapping required/captured spans * Give these errors an error code Follow up to rust-lang#72543. r? @nikomatsakis

@nikomatsakis

…, r=nikomatsakis Further tweak lifetime errors involving `dyn Trait` and `impl Trait` in return position * Suggest substituting `'static` lifetime in impl/dyn `Trait + 'static` instead of `Trait + 'static + '_` * When `'static` is explicit, also suggest constraining argument with it * Reduce verbosity of suggestion message and mention lifetime in label * Tweak output for overlapping required/captured spans * Give these errors an error code Follow up to rust-lang#72543. r? @nikomatsakis

@nikomatsakis

…, r=nikomatsakis Further tweak lifetime errors involving `dyn Trait` and `impl Trait` in return position * Suggest substituting `'static` lifetime in impl/dyn `Trait + 'static` instead of `Trait + 'static + '_` * When `'static` is explicit, also suggest constraining argument with it * Reduce verbosity of suggestion message and mention lifetime in label * Tweak output for overlapping required/captured spans * Give these errors an error code Follow up to rust-lang#72543. r? @nikomatsakis

@nikomatsakis

…, r=nikomatsakis Further tweak lifetime errors involving `dyn Trait` and `impl Trait` in return position * Suggest substituting `'static` lifetime in impl/dyn `Trait + 'static` instead of `Trait + 'static + '_` * When `'static` is explicit, also suggest constraining argument with it * Reduce verbosity of suggestion message and mention lifetime in label * Tweak output for overlapping required/captured spans * Give these errors an error code Follow up to rust-lang#72543. r? @nikomatsakis

@nikomatsakis

…, r=nikomatsakis Further tweak lifetime errors involving `dyn Trait` and `impl Trait` in return position * Suggest substituting `'static` lifetime in impl/dyn `Trait + 'static` instead of `Trait + 'static + '_` * When `'static` is explicit, also suggest constraining argument with it * Reduce verbosity of suggestion message and mention lifetime in label * Tweak output for overlapping required/captured spans * Give these errors an error code Follow up to rust-lang#72543. r? @nikomatsakis

@nikomatsakis

…, r=nikomatsakis Further tweak lifetime errors involving `dyn Trait` and `impl Trait` in return position * Suggest substituting `'static` lifetime in impl/dyn `Trait + 'static` instead of `Trait + 'static + '_` * When `'static` is explicit, also suggest constraining argument with it * Reduce verbosity of suggestion message and mention lifetime in label * Tweak output for overlapping required/captured spans * Give these errors an error code Follow up to rust-lang#72543. r? @nikomatsakis

@nikomatsakis

…, r=nikomatsakis Further tweak lifetime errors involving `dyn Trait` and `impl Trait` in return position * Suggest substituting `'static` lifetime in impl/dyn `Trait + 'static` instead of `Trait + 'static + '_` * When `'static` is explicit, also suggest constraining argument with it * Reduce verbosity of suggestion message and mention lifetime in label * Tweak output for overlapping required/captured spans * Give these errors an error code Follow up to rust-lang#72543. r? @nikomatsakis

@nikomatsakis

…, r=nikomatsakis Further tweak lifetime errors involving `dyn Trait` and `impl Trait` in return position * Suggest substituting `'static` lifetime in impl/dyn `Trait + 'static` instead of `Trait + 'static + '_` * When `'static` is explicit, also suggest constraining argument with it * Reduce verbosity of suggestion message and mention lifetime in label * Tweak output for overlapping required/captured spans * Give these errors an error code Follow up to rust-lang#72543. r? @nikomatsakis

@nikomatsakis

…, r=nikomatsakis Further tweak lifetime errors involving `dyn Trait` and `impl Trait` in return position * Suggest substituting `'static` lifetime in impl/dyn `Trait + 'static` instead of `Trait + 'static + '_` * When `'static` is explicit, also suggest constraining argument with it * Reduce verbosity of suggestion message and mention lifetime in label * Tweak output for overlapping required/captured spans * Give these errors an error code Follow up to rust-lang#72543. r? @nikomatsakis

@nikomatsakis

…, r=nikomatsakis Further tweak lifetime errors involving `dyn Trait` and `impl Trait` in return position * Suggest substituting `'static` lifetime in impl/dyn `Trait + 'static` instead of `Trait + 'static + '_` * When `'static` is explicit, also suggest constraining argument with it * Reduce verbosity of suggestion message and mention lifetime in label * Tweak output for overlapping required/captured spans * Give these errors an error code Follow up to rust-lang#72543. r? @nikomatsakis

@nikomatsakis

…, r=nikomatsakis Further tweak lifetime errors involving `dyn Trait` and `impl Trait` in return position * Suggest substituting `'static` lifetime in impl/dyn `Trait + 'static` instead of `Trait + 'static + '_` * When `'static` is explicit, also suggest constraining argument with it * Reduce verbosity of suggestion message and mention lifetime in label * Tweak output for overlapping required/captured spans * Give these errors an error code Follow up to rust-lang#72543. r? @nikomatsakis

@nikomatsakis

…, r=nikomatsakis Further tweak lifetime errors involving `dyn Trait` and `impl Trait` in return position * Suggest substituting `'static` lifetime in impl/dyn `Trait + 'static` instead of `Trait + 'static + '_` * When `'static` is explicit, also suggest constraining argument with it * Reduce verbosity of suggestion message and mention lifetime in label * Tweak output for overlapping required/captured spans * Give these errors an error code Follow up to rust-lang#72543. r? @nikomatsakis

@nikomatsakis

…, r=nikomatsakis Further tweak lifetime errors involving `dyn Trait` and `impl Trait` in return position * Suggest substituting `'static` lifetime in impl/dyn `Trait + 'static` instead of `Trait + 'static + '_` * When `'static` is explicit, also suggest constraining argument with it * Reduce verbosity of suggestion message and mention lifetime in label * Tweak output for overlapping required/captured spans * Give these errors an error code Follow up to rust-lang#72543. r? @nikomatsakis

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estebank commented May 26, 2020

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estebank changed the title ~~Account for missing lifetime in opaque return type~~ Account for missing lifetime in opaque and trait object return types May 26, 2020

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estebank added 5 commits May 30, 2020 10:19

Fix NLL output

a724d9a

Account for returned dyn Trait evaluating to 'static lifetime

65f492b

Provide a suggestion for `dyn Trait + '_` when possible.

review comment: tweak wording and account for span overlap

731ea85

Tweak wording and spans of 'static dyn Trait/impl Trait require…

83f6f22

…ments

estebank force-pushed the opaque-missing-lts-in-fn branch from ff82af1 to 83f6f22 Compare May 30, 2020 17:22

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RalfJung mentioned this pull request May 30, 2020

Rollup of 13 pull requests #72794

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bors merged commit f1661d2 into rust-lang:master May 31, 2020

estebank mentioned this pull request May 31, 2020

Further tweak lifetime errors involving dyn Trait and impl Trait in return position #72804

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estebank mentioned this pull request Jul 15, 2020

confusing error message around trait object bound #54779

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Account for missing lifetime in opaque and trait object return types #72543

Account for missing lifetime in opaque and trait object return types #72543

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Account for missing lifetime in opaque and trait object return types #72543

Account for missing lifetime in opaque and trait object return types #72543

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