compiler: suggest `.collect()` when `String` is expected and `Iterator` is found by thiago-fealves · Pull Request #156121 · rust-lang/rust

thiago-fealves · 2026-05-03T22:00:39Z

This commit adds a diagnostic suggestion to help users who forget to call .collect() when they have an iterator and the function or variable expects a String.

The logic checks if the expected type is std::string::String and if the found type implements the Iterator trait, if so the compiler provides a suggestion to add .collect()

Includes also a UI test to verify if the suggestion appears correctly

rustbot · 2026-05-03T22:00:43Z

r? @JohnTitor

rustbot has assigned @JohnTitor.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

Why was this reviewer chosen?

The reviewer was selected based on:

Owners of files modified in this PR: compiler
compiler expanded to 73 candidates
Random selection from 22 candidates

teor2345 · 2026-05-04T00:25:52Z

The job aarch64-gnu-llvm-21-1 failed!

Looks like you need to run ./x test --bless tests/ui to update the expected test outputs for existing tests with the new suggestion.

Most of the failures are:

help: consider using .collect() to convert the Iterator into a ()

Is this a useful suggestion?
We might want to skip the unit type if other suggestions are more useful, or put it lower down in the suggestion priority list.

thiago-fealves · 2026-05-04T03:17:37Z

The job aarch64-gnu-llvm-21-1 failed!

Looks like you need to run ./x test --bless tests/ui to update the expected test outputs for existing tests with the new suggestion.

Most of the failures are:

help: consider using .collect() to convert the Iterator into a ()

Is this a useful suggestion? We might want to skip the unit type if other suggestions are more useful, or put it lower down in the suggestion priority list.

I didn't anticipate this could happen, i was alway thinking about more concrete types like String and int, honestly i think in this case the suggestion is not being useful for the user anymore, i can't think of a situation where you would collect into () at least. What do you suggest in this case?

I'm gonna implement more tests for non-String objects in the meantime

teor2345 · 2026-05-04T04:25:42Z

I didn't anticipate this could happen, i was alway thinking about more concrete types like String and int, honestly i think in this case the suggestion is not being useful for the user anymore, i can't think of a situation where you would collect into () at least. What do you suggest in this case?

The docs say this is intended for cases like Result<(), E>:
https://doc.rust-lang.org/std/iter/trait.FromIterator.html#impl-FromIterator%3C()%3E-for-()

So skipping the suggestion when the expected type is just () seems like a good idea.

thiago-fealves · 2026-05-04T13:33:23Z

I didn't anticipate this could happen, i was alway thinking about more concrete types like String and int, honestly i think in this case the suggestion is not being useful for the user anymore, i can't think of a situation where you would collect into () at least. What do you suggest in this case?

The docs say this is intended for cases like Result<(), E>: https://doc.rust-lang.org/std/iter/trait.FromIterator.html#impl-FromIterator%3C()%3E-for-()

So skipping the suggestion when the expected type is just () seems like a good idea.

I agree, gonna finish implementing that and adding the UI tests.

…r` is found This commit adds a diagnostic suggestion to help users who forget to call `.collect()` when they have an iterator and the function or variable expects a `String`. The logic checks if the expected type is `std::string::String` and if the found type implements the `Iterator` trait, if so the compiler provides a suggestion to add `.collect()` Includes also a UI test to verify if the suggestion appears correctly

thiago-fealves · 2026-05-04T13:56:44Z

Just added a check for the unit type, let's see if the tests pass now

thiago-fealves · 2026-05-04T15:16:56Z

Looks like tests succeeded!

Co-authored-by: Qai Juang <qaijuang@gmail.com>

teor2345

This looks good to me, but I'm not a typeck expert.

Feel free to resolve my earlier feedback, I think GitHub is having issues and dropping my attempts to do it.

View changes since this review

estebank · 2026-05-15T22:51:20Z

@bors r+

rust-bors · 2026-05-15T22:51:23Z

📌 Commit e1941ff has been approved by estebank

It is now in the queue for this repository.

…bank compiler: suggest `.collect()` when `String` is expected and `Iterator` is found This commit adds a diagnostic suggestion to help users who forget to call `.collect()` when they have an iterator and the function or variable expects a `String`. The logic checks if the expected type is `std::string::String` and if the found type implements the `Iterator` trait, if so the compiler provides a suggestion to add `.collect()` Includes also a UI test to verify if the suggestion appears correctly

rust-bors · 2026-05-16T01:09:38Z

💔 Test for f2434d3 failed: CI. Failed job:

auto - dist-various-2 (web logs, enhanced plaintext logs)

rust-log-analyzer · 2026-05-16T01:24:08Z

A job failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

#13 DONE 32.2s

#14 [ 6/25] RUN apt-key adv --batch --yes --keyserver keyserver.ubuntu.com --recv-keys 74DA7924C5513486
#14 0.159 Warning: apt-key is deprecated. Manage keyring files in trusted.gpg.d instead (see apt-key(8)).
#14 0.176 Executing: /tmp/apt-key-gpghome.zToD4iKYCd/gpg.1.sh --batch --yes --keyserver keyserver.ubuntu.com --recv-keys 74DA7924C5513486
#14 1.978 gpg: key 74DA7924C5513486: public key "Igor Kozhukhov <igor@dilos.org>" imported
#14 1.982 gpg: Total number processed: 1
#14 1.982 gpg:               imported: 1
#14 DONE 2.1s

Kobzol · 2026-05-16T18:55:33Z

@bors retry

Spurious failure fixed by #156640.

…ing, r=estebank compiler: suggest `.collect()` when `String` is expected and `Iterator` is found This commit adds a diagnostic suggestion to help users who forget to call `.collect()` when they have an iterator and the function or variable expects a `String`. The logic checks if the expected type is `std::string::String` and if the found type implements the `Iterator` trait, if so the compiler provides a suggestion to add `.collect()` Includes also a UI test to verify if the suggestion appears correctly

Rollup of 6 pull requests Successful merges: - #152852 (Remove driver_lint_caps) - #156121 (compiler: suggest `.collect()` when `String` is expected and `Iterator` is found) - #156518 ([style] rustfmt `match`es with comments in or-patterns) - #156596 (Split `LintExpectationId`s) - #156577 (Test EII UI tests with prefer-dynamic) - #156633 (Add regression test for issue #41261)

…bank compiler: suggest `.collect()` when `String` is expected and `Iterator` is found This commit adds a diagnostic suggestion to help users who forget to call `.collect()` when they have an iterator and the function or variable expects a `String`. The logic checks if the expected type is `std::string::String` and if the found type implements the `Iterator` trait, if so the compiler provides a suggestion to add `.collect()` Includes also a UI test to verify if the suggestion appears correctly

rust-log-analyzer · 2026-05-17T01:27:04Z

The job x86_64-gnu-llvm-21-1 failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

test expr_store::scope::tests::test_shadow_variable ... ok
test expr_store::tests::body::array_element_cfg ... ok
test expr_store::tests::body::async_fn_weird_param_patterns ... ok
test expr_store::tests::body::block::inner_item_smoke ... ok
error: test failed, to rerun pass `-p hir-def --lib`

Caused by:
  process didn't exit successfully: `/checkout/obj/build/x86_64-unknown-linux-gnu/stage2-tools/x86_64-unknown-linux-gnu/release/deps/hir_def-f381e8bd2208ec8b '--skip=tests::smoke_test_real_sysroot_cargo' --skip=check_code_formatting -Z unstable-options --format json` (signal: 11, SIGSEGV: invalid memory reference)
Bootstrap failed while executing `--stage 2 test --skip tests --skip coverage-map --skip coverage-run --skip library --skip tidyselftest`
Build completed unsuccessfully in 1:04:34
  local time: Sun May 17 01:26:49 UTC 2026
  network time: Sun, 17 May 2026 01:26:49 GMT
##[error]Process completed with exit code 1.

rust-bors · 2026-05-17T01:27:35Z

💔 Test for c9ce8ae failed: CI. Failed job:

auto - x86_64-gnu-llvm-21-1 (web logs, enhanced plaintext logs)

JonathanBrouwer · 2026-05-17T07:29:13Z

@bors retry

…ing, r=estebank compiler: suggest `.collect()` when `String` is expected and `Iterator` is found This commit adds a diagnostic suggestion to help users who forget to call `.collect()` when they have an iterator and the function or variable expects a `String`. The logic checks if the expected type is `std::string::String` and if the found type implements the `Iterator` trait, if so the compiler provides a suggestion to add `.collect()` Includes also a UI test to verify if the suggestion appears correctly

…uwer Rollup of 14 pull requests Successful merges: - #151742 (Remove redundant information in `rustc_abi::Variants`) - #151362 (Add interior-mutability suggestion to `static_mut_refs`) - #156121 (compiler: suggest `.collect()` when `String` is expected and `Iterator` is found) - #156208 (Emit retags in codegen to support BorrowSanitizer (part 1)) - #156596 (Split `LintExpectationId`s) - #156607 (ci: Update FreeBSD version to FreeBSD 14) - #156376 (suggest hex escapes for C-style escapes) - #156577 (Test EII UI tests with prefer-dynamic) - #156585 (explicit tail calls: ignore some tests on unsupported LLVM targets) - #156598 (Avoid rustfix suggestions for macro-expanded unused imports) - #156616 (rustdoc: add test case for `-Drustdoc::` and `--cap-lints`) - #156633 (Add regression test for issue #41261) - #156635 (rename unexpected_try_recover function) - #156636 (minor `rustc_mir_transform` cleanup)

Rollup merge of #156121 - thiago-fealves:suggest-collect-string, r=estebank compiler: suggest `.collect()` when `String` is expected and `Iterator` is found This commit adds a diagnostic suggestion to help users who forget to call `.collect()` when they have an iterator and the function or variable expects a `String`. The logic checks if the expected type is `std::string::String` and if the found type implements the `Iterator` trait, if so the compiler provides a suggestion to add `.collect()` Includes also a UI test to verify if the suggestion appears correctly

…uwer Rollup of 14 pull requests Successful merges: - rust-lang/rust#151742 (Remove redundant information in `rustc_abi::Variants`) - rust-lang/rust#151362 (Add interior-mutability suggestion to `static_mut_refs`) - rust-lang/rust#156121 (compiler: suggest `.collect()` when `String` is expected and `Iterator` is found) - rust-lang/rust#156208 (Emit retags in codegen to support BorrowSanitizer (part 1)) - rust-lang/rust#156596 (Split `LintExpectationId`s) - rust-lang/rust#156607 (ci: Update FreeBSD version to FreeBSD 14) - rust-lang/rust#156376 (suggest hex escapes for C-style escapes) - rust-lang/rust#156577 (Test EII UI tests with prefer-dynamic) - rust-lang/rust#156585 (explicit tail calls: ignore some tests on unsupported LLVM targets) - rust-lang/rust#156598 (Avoid rustfix suggestions for macro-expanded unused imports) - rust-lang/rust#156616 (rustdoc: add test case for `-Drustdoc::` and `--cap-lints`) - rust-lang/rust#156633 (Add regression test for issue rust-lang/rust#41261) - rust-lang/rust#156635 (rename unexpected_try_recover function) - rust-lang/rust#156636 (minor `rustc_mir_transform` cleanup)

rustbot assigned JohnTitor May 3, 2026

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels May 3, 2026

thiago-fealves force-pushed the suggest-collect-string branch from 5513a24 to 41aa2be Compare May 3, 2026 23:25

teor2345 reviewed May 3, 2026

View reviewed changes

Comment thread tests/ui/suggestions/suggest-collect.rs

This comment has been minimized.

Sign in to view

thiago-fealves force-pushed the suggest-collect-string branch from 41aa2be to a4bd5f5 Compare May 4, 2026 13:55

qaijuang reviewed May 4, 2026

View reviewed changes

Comment thread tests/ui/suggestions/suggest-collect.rs

qaijuang reviewed May 4, 2026

View reviewed changes

Comment thread compiler/rustc_hir_typeck/src/fn_ctxt/suggestions.rs Outdated

Update compiler/rustc_hir_typeck/src/fn_ctxt/suggestions.rs

e1941ff

Co-authored-by: Qai Juang <qaijuang@gmail.com>

teor2345 reviewed May 4, 2026

View reviewed changes

estebank changed the title ~~compiler: suggest .collect() when String is expected and `Iterato…~~ compiler: suggest .collect() when String is expected and Iterator is found May 15, 2026

rust-bors Bot 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 May 15, 2026

This comment has been minimized.

Sign in to view

rust-bors Bot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. labels May 16, 2026

rust-bors Bot 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 May 16, 2026

GuillaumeGomez mentioned this pull request May 16, 2026

Rollup of 6 pull requests #156652

Closed

This comment has been minimized.

Sign in to view

rust-bors Bot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. labels May 17, 2026

rust-bors Bot 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 May 17, 2026

JonathanBrouwer mentioned this pull request May 17, 2026

Rollup of 11 pull requests #156658

Closed

This was referenced May 17, 2026

Rollup of 9 pull requests #156663

Closed

Rollup of 14 pull requests #156670

Merged

rust-bors Bot merged commit fe32c70 into rust-lang:main May 18, 2026
11 of 12 checks passed

rustbot added this to the 1.97.0 milestone May 18, 2026

thiago-fealves deleted the suggest-collect-string branch May 18, 2026 03:02

Uh oh!

Conversation

thiago-fealves commented May 3, 2026 • edited by rustbot Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

rustbot commented May 3, 2026

Uh oh!

Uh oh!

This comment has been minimized.

teor2345 commented May 4, 2026

Uh oh!

thiago-fealves commented May 4, 2026

Uh oh!

teor2345 commented May 4, 2026

Uh oh!

thiago-fealves commented May 4, 2026

Uh oh!

thiago-fealves commented May 4, 2026

Uh oh!

thiago-fealves commented May 4, 2026

Uh oh!

Uh oh!

Uh oh!

teor2345 left a comment • edited by rustbot Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Choose a reason for hiding this comment

Uh oh!

estebank commented May 15, 2026

Uh oh!

rust-bors Bot commented May 15, 2026

Uh oh!

This comment has been minimized.

rust-bors Bot commented May 16, 2026

Uh oh!

rust-log-analyzer commented May 16, 2026

Uh oh!

Kobzol commented May 16, 2026

Uh oh!

This comment has been minimized.

rust-log-analyzer commented May 17, 2026

Uh oh!

rust-bors Bot commented May 17, 2026

Uh oh!

JonathanBrouwer commented May 17, 2026

Uh oh!

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

9 participants

thiago-fealves commented May 3, 2026 •

edited by rustbot

Loading

teor2345 left a comment •

edited by rustbot

Loading