Suggest using anonymous lifetime in `impl Trait` return #58919

estebank · 2019-03-04T19:52:49Z

estebank · 2019-03-04T19:53:59Z

This is a blatant hack, but wanted to have a PR out so we can talk about the desired output.

pnkfelix · 2019-03-14T20:13:29Z

Hmm that output is interesting.

I wonder, as a matter of coding style and/or readability, if people would prefer to see the following suggested:

 fn elided<'a>(x: &'a i32) -> impl Copy + 'a { x }

(but I don't know, I think I often err on the side of explicitness when people would prefer elision...)

pnkfelix · 2019-03-14T20:16:00Z

Oh; I guess that test case, in its full form, even includes an example where we do suggest the form with the explicit named lifetime, if you start with fn elided<'a>(x: &'a i32) -> impl Copy { x }.

So its really a matter of style, and I suspect the prevailing style is towards what @estebank is proposing here

estebank · 2019-03-14T20:24:35Z

We can expand this code to also suggest

help: you can add an explicit named lifetime `'a` to the type of `x` and the return type
   |
LL | fn elided<'a>(x: &'a i32) -> impl Copy + 'a { x }
   |          ^^^^     ^^                   ^^^^

but the code itself to make that suggestion might be quite brittle.

Centril · 2019-03-27T13:32:22Z

I think the form suggested here is quite reasonable and I suspect doing the more elaborate suggestion is more complex impl wise...

To reduce Niko's backlog, r? @pnkfelix

Centril · 2019-03-30T13:05:35Z

Ping from triage, @pnkfelix -- shall we go with @estebank's idea for now?

Fallback to `static_impl_trait` for nice error message by peeking at the return type and the lifetime type. Point at the return type instead of the return expr/stmt in NLL mode.

estebank · 2019-03-31T16:29:18Z

I had a talk with Niko and he clarified what a better approach would be.

I looked around and arrived to the current solution, with which I'm relatively happy with (even though I think we could improve the static_impl_trait output in a separate PR). I feel that this PR is now reasonable/reviewable/mergeable.

Centril · 2019-03-31T16:34:20Z

src/test/ui/impl-trait/must_outlive_least_region_or_bound.stderr

+   |
+LL | fn elided(x: &i32) -> impl Copy { x }
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+help: you can add a constraint to the return type to make it last less than `'static` and match the anonymous lifetime #1 defined on the function body at 3:1


Btw... "constraint" here is odd; we typically use "bound" for things in A + B + ....

We've been using that wording for 6 months, but I agree we could improve on the wording on these errors.

pnkfelix · 2019-04-01T11:21:34Z

src/librustc/infer/error_reporting/nice_region_error/named_anon_conflict.rs

@@ -84,6 +86,13 @@ impl<'a, 'gcx, 'tcx> NiceRegionError<'a, 'gcx, 'tcx> {
            {
                return None;
            }
+            if let FunctionRetTy::Return(ty) = &fndecl.output {
+                if let (TyKind::Def(_, _), ty::ReStatic) = (&ty.node, sub) {
+                    // This is an impl Trait return that evaluates de need of 'static.


"evaluates de need of 'static" ?

What is this?

A partial rewording of the prior comment 😬

I believe something along the lines of "that evaluates to a need of static".

pnkfelix · 2019-04-01T11:25:38Z

@bors r+

bors · 2019-04-01T11:25:39Z

📌 Commit 30c247f has been approved by pnkfelix

@nikomatsakis

…, r=pnkfelix Suggest using anonymous lifetime in `impl Trait` return Fix rust-lang#48467. r? @nikomatsakis

@nikomatsakis

…, r=pnkfelix Suggest using anonymous lifetime in `impl Trait` return Fix rust-lang#48467. r? @nikomatsakis

@ghost

Rollup of 7 pull requests Successful merges: - #58507 (Add a -Z time option which prints only passes which runs once) - #58919 (Suggest using anonymous lifetime in `impl Trait` return) - #59041 (fixes #56766) - #59586 (Fixed URL in cargotest::TEST_REPOS) - #59595 (Update rustc-guide submodule) - #59601 (Fix small typo) - #59603 (stabilize ptr::hash) Failed merges: r? @ghost

rust-highfive assigned nikomatsakis Mar 4, 2019

rust-highfive added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Mar 4, 2019

rust-highfive assigned pnkfelix and unassigned nikomatsakis Mar 27, 2019

Suggest using anonymous lifetime in impl Trait return

ee2a9d9

Suggest using anonymous lifetime in impl Trait return without hacks

30c247f

Fallback to `static_impl_trait` for nice error message by peeking at the return type and the lifetime type. Point at the return type instead of the return expr/stmt in NLL mode.

estebank force-pushed the impl-trait-return-lifetime branch from 68c28b6 to 30c247f Compare March 31, 2019 16:25

Centril reviewed Mar 31, 2019

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pnkfelix reviewed Apr 1, 2019

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bors added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Apr 1, 2019

Centril added a commit to Centril/rust that referenced this pull request Apr 1, 2019

Rollup merge of rust-lang#58919 - estebank:impl-trait-return-lifetime…

4a9e791

…, r=pnkfelix Suggest using anonymous lifetime in `impl Trait` return Fix rust-lang#48467. r? @nikomatsakis

Centril mentioned this pull request Apr 1, 2019

Rollup of 5 pull requests #59602

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Centril added a commit to Centril/rust that referenced this pull request Apr 1, 2019

Rollup merge of rust-lang#58919 - estebank:impl-trait-return-lifetime…

0d01fba

…, r=pnkfelix Suggest using anonymous lifetime in `impl Trait` return Fix rust-lang#48467. r? @nikomatsakis

Centril mentioned this pull request Apr 1, 2019

Rollup of 7 pull requests #59606

Merged

bors merged commit 30c247f into rust-lang:master Apr 1, 2019

Nemo157 mentioned this pull request Apr 10, 2019

arbitrary_self_types don't support lifetime elision #52675

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Suggest using anonymous lifetime in `impl Trait` return #58919

Suggest using anonymous lifetime in `impl Trait` return #58919

estebank commented Mar 4, 2019

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Suggest using anonymous lifetime in impl Trait return #58919

Suggest using anonymous lifetime in impl Trait return #58919

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Suggest using anonymous lifetime in `impl Trait` return #58919

Suggest using anonymous lifetime in `impl Trait` return #58919

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