Fix trailing comma in lifetime suggestion for empty angle brackets by fru1tworld · Pull Request #154703 · rust-lang/rust

fru1tworld · 2026-04-02T06:21:31Z

When suggesting a lifetime parameter (e.g., 'a) for a type like Foo<> (empty angle brackets), the compiler was incorrectly producing Foo<'a, > with a trailing comma.

Root Cause

The has_existing_params check used segment.args to determine if generic parameters exist. However, for empty angle brackets (<>), the parser doesn't create any args, so has_existing_params was false — even though the angle brackets themselves exist.

This caused the suggestion logic to skip the comma+space suffix ("'a, "), but the insert position was still inside the angle brackets, leading to Foo<'a, > instead of Foo<'a>.

Fix

Replace the segment.args-based check with a source text inspection using span_to_snippet. The new logic:

Finds the < character in the source text
Extracts the content between < and >
Checks if that content is non-empty (after trimming whitespace)

This correctly identifies <> as having no existing parameters, avoiding the trailing comma.

Test

Added tests/ui/lifetimes/E0106-trailing-comma-in-lifetime-suggestion.rs covering the specific case of empty angle brackets with lifetime suggestions.

rustbot · 2026-04-02T08:32:29Z

r? @mati865

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Kivooeo · 2026-04-02T10:36:56Z

i personally think, that having trailing comma is fine, because
a) this code compiles
b) this is not misleading in any way

i mean, sure that ideally we want to not have this trailing comma, but looking at fix it looks to me like using a tank against a fly

so, if I were to weigh up the pros and cons, i think i'd be against it

fru1tworld · 2026-04-02T10:46:55Z

i mean, sure that ideally we want to not have this trailing comma,

Thanks for the feedback
that's a fair point. I agree the current approach is heavier than it needs to be for a cosmetic issue.

Would a more minimal fix be acceptable? Instead of adding fields to Segment/MissingLifetime, I could just adjust the suggestion string in the MissingLifetimeKind::Comma branch — no struct changes, no new field propagation.

That would keep the diff to a few lines in diagnostics.rs only.

If you still feel it's not worth it even at that scope, I'm fine closing this.

Kivooeo · 2026-04-02T10:53:18Z

if you know more easier approach why was this chosen originally?

fru1tworld · 2026-04-02T11:07:45Z

I wasn't aware of span_look_ahead when I wrote the original version
This revision drops all struct changes and uses it instead

chenyukang · 2026-04-03T01:41:13Z

compiler/rustc_resolve/src/late/diagnostics.rs

+                let sugg: String = std::iter::repeat_n(existing_name.as_str(), lt.count)
+                    .intersperse(", ")
                    .collect();
+                let is_empty_brackets = source_map.span_look_ahead(lt.span, ">", Some(1)).is_some();


nit: span_look_ahead can ignore white space, if you set Some(1) here, " >" will not be checked for your code, so maybe Some(50) for limit is better.

TIL, thanks! fixed!

chenyukang · 2026-04-03T01:43:07Z

nit: please don't use AI to generate PR description, it just too verbose and noisy, keep it empty for trivial fix and only add some extra important notes for a PR.

fru1tworld · 2026-04-03T02:25:32Z

Got it, I'll keep it concise. Thanks for the feedback

mati865 · 2026-04-03T08:24:12Z

r? @Kivooeo

As they already started reviewing, feel free to reroll.

chenyukang · 2026-04-04T00:13:05Z

Thanks!
@bors r=chenyukang

rust-bors · 2026-04-04T00:13:08Z

📌 Commit 46b0527 has been approved by chenyukang

It is now in the queue for this repository.

…ma, r=chenyukang Fix trailing comma in lifetime suggestion for empty angle brackets Fixes rust-lang#154600 When suggesting a lifetime parameter (e.g., `'a`) for a type like `Foo<>` (empty angle brackets), the compiler was incorrectly producing `Foo<'a, >` with a trailing comma. ## Root Cause The `has_existing_params` check used `segment.args` to determine if generic parameters exist. However, for empty angle brackets (`<>`), the parser doesn't create any `args`, so `has_existing_params` was `false` — even though the angle brackets themselves exist. This caused the suggestion logic to skip the comma+space suffix (`"'a, "`), but the insert position was still inside the angle brackets, leading to `Foo<'a, >` instead of `Foo<'a>`. ## Fix Replace the `segment.args`-based check with a source text inspection using `span_to_snippet`. The new logic: 1. Finds the `<` character in the source text 2. Extracts the content between `<` and `>` 3. Checks if that content is non-empty (after trimming whitespace) This correctly identifies `<>` as having no existing parameters, avoiding the trailing comma. ## Test Added `tests/ui/lifetimes/E0106-trailing-comma-in-lifetime-suggestion.rs` covering the specific case of empty angle brackets with lifetime suggestions.

Rollup of 10 pull requests Successful merges: - #154376 (Remove more BuiltinLintDiag variants - part 4) - #127534 (feat(core): impl Step for NonZero<u*>) - #153286 (various fixes for scalable vectors) - #153592 (Add `min_adt_const_params` gate) - #154675 (Improve shadowed private field diagnostics) - #154703 (Fix trailing comma in lifetime suggestion for empty angle brackets) - #154653 (Remove rustc_on_unimplemented's append_const_msg) - #154743 (Remove an unused `StableHash` impl.) - #154752 (Add comment to borrow-checker) - #154764 (Add tests for three ICEs that have already been fixed)

Rollup of 5 pull requests Successful merges: - #154376 (Remove more BuiltinLintDiag variants - part 4) - #154731 (llvm: Fix array ABI test to not check equality implementation) - #127534 (feat(core): impl Step for NonZero<u*>) - #154703 (Fix trailing comma in lifetime suggestion for empty angle brackets) - #154776 (Fix ICE in read_discriminant for enums with non-contiguous discriminants)

Rollup merge of #154703 - fru1tworld:fix-154600-trailing-comma, r=chenyukang Fix trailing comma in lifetime suggestion for empty angle brackets Fixes #154600 When suggesting a lifetime parameter (e.g., `'a`) for a type like `Foo<>` (empty angle brackets), the compiler was incorrectly producing `Foo<'a, >` with a trailing comma. ## Root Cause The `has_existing_params` check used `segment.args` to determine if generic parameters exist. However, for empty angle brackets (`<>`), the parser doesn't create any `args`, so `has_existing_params` was `false` — even though the angle brackets themselves exist. This caused the suggestion logic to skip the comma+space suffix (`"'a, "`), but the insert position was still inside the angle brackets, leading to `Foo<'a, >` instead of `Foo<'a>`. ## Fix Replace the `segment.args`-based check with a source text inspection using `span_to_snippet`. The new logic: 1. Finds the `<` character in the source text 2. Extracts the content between `<` and `>` 3. Checks if that content is non-empty (after trimming whitespace) This correctly identifies `<>` as having no existing parameters, avoiding the trailing comma. ## Test Added `tests/ui/lifetimes/E0106-trailing-comma-in-lifetime-suggestion.rs` covering the specific case of empty angle brackets with lifetime suggestions.

rustbot added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Apr 2, 2026

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fru1tworld force-pushed the fix-154600-trailing-comma branch from 37c3b75 to 0f7bc29 Compare April 2, 2026 07:00

fru1tworld marked this pull request as ready for review April 2, 2026 08:32

rustbot added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Apr 2, 2026

rustbot assigned mati865 Apr 2, 2026

rustbot removed the S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. label Apr 2, 2026

fru1tworld force-pushed the fix-154600-trailing-comma branch from f7e0899 to adf06b7 Compare April 2, 2026 11:06

fru1tworld force-pushed the fix-154600-trailing-comma branch from adf06b7 to e0fc2da Compare April 2, 2026 11:15

chenyukang reviewed Apr 3, 2026

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fru1tworld force-pushed the fix-154600-trailing-comma branch from e0fc2da to 59d3092 Compare April 3, 2026 02:20

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Increase span_look_ahead limit to handle whitespace

46b0527

fru1tworld force-pushed the fix-154600-trailing-comma branch from 59d3092 to 46b0527 Compare April 3, 2026 02:28

chenyukang mentioned this pull request Apr 3, 2026

Replace span_look_ahead with span_followed_by #154745

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rustbot assigned Kivooeo and unassigned mati865 Apr 3, 2026

fru1tworld requested a review from chenyukang April 3, 2026 10:33

rust-bors bot added the S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. label Apr 4, 2026

rust-bors bot removed the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Apr 4, 2026

jhpratt mentioned this pull request Apr 4, 2026

Rollup of 10 pull requests #154793

Closed

matthiaskrgr mentioned this pull request Apr 4, 2026

Rollup of 5 pull requests #154802

Merged

rust-bors bot merged commit 66457ac into rust-lang:main Apr 4, 2026
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rustbot added this to the 1.96.0 milestone Apr 4, 2026

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