Fix trailing comma in lifetime suggestion for empty angle brackets

[fru1tworld]

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]

r? @mati865

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[Kivooeo]

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]

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]

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

[fru1tworld]

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