Suggest using slice when encountering `let x = ""[..];` #46249

estebank · 2017-11-24T22:49:27Z

Fix #26319.

rust-highfive · 2017-11-24T22:49:30Z

r? @eddyb

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eddyb · 2017-11-24T23:37:42Z

I don't think the index type matters, we should just not check it and just emit the suggestion.

oli-obk · 2017-11-25T10:11:43Z

src/test/ui/suggestions/str-array-assignment.rs

+
+fn main() {
+  let s = "abc";
+  let t = if true { s[..2] } else { s };


You need to add //~ ERROR comments. Ui tests work like compile-fail now

oli-obk · 2017-11-25T10:13:38Z

src/librustc/traits/error_reporting.rs

@@ -821,6 +823,33 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
        err.emit();
    }

+    /// When encountering an assignment of an unsized trait, like `let x = ""[..];`, provide a


Why just for assignments and not all the cases in the test?

The other cases in the test go through type check instead. I'll see if I can expand #46256 to also suggest in these cases. I included all the cases originally mentioned in order to make sure we don't regress in any of the cases (without noticing).

estebank · 2017-11-25T20:15:40Z

@eddyb I believe you were right so removed the index type check.

eddyb · 2017-11-25T20:48:20Z

r? @nikomatsakis I still feel this is much more specific than it should be, but I don't have any good suggestions, other than perhaps that & isn't called "slice-ing" or "taking a slice".
a[..] does the slicing, & just borrows that slice. You can also have a[..].slice_method().

In general, both deref and indexing could use a suggestion to borrow the resulting lvalue.

arielb1 · 2017-11-27T12:32:57Z

src/test/ui/suggestions/str-array-assignment.stderr

+  --> $DIR/str-array-assignment.rs:17:27
+   |
+11 | fn main() { //~ NOTE expected `()` because of default return type
+   |           - expected `()` because of default return type


this looks dubious. Could you look into this?

Yeah, I noticed that too. Still digging.

it's also already the case on nightly. Feel free to ignore this and open an issue.

arielb1 · 2017-11-27T12:40:14Z

This feels hacky to me, but I don't see any better way of implementing this (the rhs of locals is an lvalue, so checks based on it being an rvalue won't quite work).

btw, you should be checking that the problem type is a slice or string - if the return type is a struct or trait object, you don't want the error message to mention "slices".

arielb1 · 2017-11-27T12:42:54Z

OTOH, having an Index implementation that returns trait objects is a bit weird - that would mean someone does some kind of indirection somewhere. But I think "consider borrowing here" would look better than "consider borrowing the slice" (also because str is not a "slice").

estebank · 2017-11-27T14:07:25Z

@arielb1 reworded.

arielb1 · 2017-11-27T14:29:24Z

@bors r+ rollup

bors · 2017-11-27T14:29:25Z

📌 Commit fa44927 has been approved by arielb1

Suggest using slice when encountering `let x = ""[..];` Fix rust-lang#26319.

Rollup of 10 pull requests - Successful merges: #45506, #46174, #46231, #46240, #46249, #46258, #46262, #46275, #46282, #46285 - Failed merges:

rust-highfive assigned eddyb Nov 24, 2017

kennytm added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Nov 25, 2017

oli-obk reviewed Nov 25, 2017

View reviewed changes

estebank added 2 commits November 25, 2017 06:08

Suggest using slice when encountering let x = ""[..];

fa6ae4c

Add compile-fail style comments to tests

cde0023

estebank force-pushed the suggest-slice branch 3 times, most recently from 134889a to 3b56558 Compare November 25, 2017 14:50

Remove index type check (review comment)

97d8d04

estebank force-pushed the suggest-slice branch from 3b56558 to 97d8d04 Compare November 25, 2017 15:51

rust-highfive assigned nikomatsakis and unassigned eddyb Nov 25, 2017

arielb1 reviewed Nov 27, 2017

View reviewed changes

arielb1 closed this Nov 27, 2017

arielb1 reopened this Nov 27, 2017

reword to "consider borrowing here: {suggestion}"

fa44927

estebank force-pushed the suggest-slice branch from c924646 to fa44927 Compare November 27, 2017 14:05

kennytm added a commit to kennytm/rust that referenced this pull request Nov 27, 2017

Rollup merge of rust-lang#46249 - estebank:suggest-slice, r=arielb1

a60ffa0

Suggest using slice when encountering `let x = ""[..];` Fix rust-lang#26319.

kennytm mentioned this pull request Nov 27, 2017

Rollup of 10 pull requests #46312

Merged

bors added a commit that referenced this pull request Nov 27, 2017

Auto merge of #46312 - kennytm:rollup, r=kennytm

b087a99

Rollup of 10 pull requests - Successful merges: #45506, #46174, #46231, #46240, #46249, #46258, #46262, #46275, #46282, #46285 - Failed merges:

bors merged commit fa44927 into rust-lang:master Nov 28, 2017

arielb1 mentioned this pull request Nov 30, 2017

Use suggestions instead of notes ref mismatches #46256

Merged

estebank deleted the suggest-slice branch November 9, 2023 05:25

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Suggest using slice when encountering `let x = ""[..];` #46249

Suggest using slice when encountering `let x = ""[..];` #46249

estebank commented Nov 24, 2017

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eddyb commented Nov 24, 2017

oli-obk Nov 25, 2017

estebank Nov 25, 2017

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bors commented Nov 27, 2017

Suggest using slice when encountering let x = ""[..]; #46249

Suggest using slice when encountering let x = ""[..]; #46249

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Suggest using slice when encountering `let x = ""[..];` #46249

Suggest using slice when encountering `let x = ""[..];` #46249

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