From 46b05279dd1c6c82ab1e850220fffb04531b9030 Mon Sep 17 00:00:00 2001
From: fru1tworld <fruitworld.planet@gmail.com>
Date: Thu, 2 Apr 2026 20:15:33 +0900
Subject: [PATCH] Increase span_look_ahead limit to handle whitespace

---
 .../rustc_resolve/src/late/diagnostics.rs     |  8 ++++--
 tests/ui/generics/wrong-number-of-args.stderr | 12 ++++-----
 ...6-trailing-comma-in-lifetime-suggestion.rs | 18 +++++++++++++
 ...ailing-comma-in-lifetime-suggestion.stderr | 25 +++++++++++++++++++
 4 files changed, 55 insertions(+), 8 deletions(-)
 create mode 100644 tests/ui/lifetimes/E0106-trailing-comma-in-lifetime-suggestion.rs
 create mode 100644 tests/ui/lifetimes/E0106-trailing-comma-in-lifetime-suggestion.stderr

diff --git a/compiler/rustc_resolve/src/late/diagnostics.rs b/compiler/rustc_resolve/src/late/diagnostics.rs
index cf048231bd607..3be653f272d9c 100644
--- a/compiler/rustc_resolve/src/late/diagnostics.rs
+++ b/compiler/rustc_resolve/src/late/diagnostics.rs
@@ -4064,6 +4064,7 @@ impl<'ast, 'ra, 'tcx> LateResolutionVisitor<'_, 'ast, 'ra, 'tcx> {
         };
 
         let mut spans_suggs: Vec<_> = Vec::new();
+        let source_map = self.r.tcx.sess.source_map();
         let build_sugg = |lt: MissingLifetime| match lt.kind {
             MissingLifetimeKind::Underscore => {
                 debug_assert_eq!(lt.count, 1);
@@ -4074,9 +4075,12 @@ impl<'ast, 'ra, 'tcx> LateResolutionVisitor<'_, 'ast, 'ra, 'tcx> {
                 (lt.span.shrink_to_hi(), format!("{existing_name} "))
             }
             MissingLifetimeKind::Comma => {
-                let sugg: String = std::iter::repeat_n([existing_name.as_str(), ", "], lt.count)
-                    .flatten()
+                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(50)).is_some();
+                let sugg = if is_empty_brackets { sugg } else { format!("{sugg}, ") };
                 (lt.span.shrink_to_hi(), sugg)
             }
             MissingLifetimeKind::Brackets => {
diff --git a/tests/ui/generics/wrong-number-of-args.stderr b/tests/ui/generics/wrong-number-of-args.stderr
index 554d017d67e33..953cd273eb764 100644
--- a/tests/ui/generics/wrong-number-of-args.stderr
+++ b/tests/ui/generics/wrong-number-of-args.stderr
@@ -28,8 +28,8 @@ LL |     type E = Ty<>;
    |
 help: consider introducing a named lifetime parameter
    |
-LL |     type E<'a> = Ty<'a, >;
-   |           ++++      +++
+LL |     type E<'a> = Ty<'a>;
+   |           ++++      ++
 
 error[E0106]: missing lifetime specifier
   --> $DIR/wrong-number-of-args.rs:120:22
@@ -55,12 +55,12 @@ LL |     type F = Box<dyn GenericLifetime<>>;
    |
 help: consider making the bound lifetime-generic with a new `'a` lifetime
    |
-LL |     type F = Box<dyn for<'a> GenericLifetime<'a, >>;
-   |                      +++++++                 +++
+LL |     type F = Box<dyn for<'a> GenericLifetime<'a>>;
+   |                      +++++++                 ++
 help: consider introducing a named lifetime parameter
    |
-LL |     type F<'a> = Box<dyn GenericLifetime<'a, >>;
-   |           ++++                           +++
+LL |     type F<'a> = Box<dyn GenericLifetime<'a>>;
+   |           ++++                           ++
 
 error[E0106]: missing lifetime specifier
   --> $DIR/wrong-number-of-args.rs:163:43
diff --git a/tests/ui/lifetimes/E0106-trailing-comma-in-lifetime-suggestion.rs b/tests/ui/lifetimes/E0106-trailing-comma-in-lifetime-suggestion.rs
new file mode 100644
index 0000000000000..acf93ae51cc4d
--- /dev/null
+++ b/tests/ui/lifetimes/E0106-trailing-comma-in-lifetime-suggestion.rs
@@ -0,0 +1,18 @@
+// Regression test for <https://github.com/rust-lang/rust/issues/154600>.
+//
+// When suggesting lifetime parameters for empty angle brackets like `Foo<>`,
+// the suggestion should not include a trailing comma (e.g., `Foo<'a>` not `Foo<'a, >`).
+// When there are other generic arguments like `Foo<T>`, the trailing comma is needed
+// (e.g., `Foo<'a, T>`).
+
+#![crate_type = "lib"]
+
+struct Foo<'a>(&'a ());
+
+type A = Foo<>;
+//~^ ERROR missing lifetime specifier [E0106]
+
+struct Bar<'a, T>(&'a T);
+
+type B = Bar<u8>;
+//~^ ERROR missing lifetime specifier [E0106]
diff --git a/tests/ui/lifetimes/E0106-trailing-comma-in-lifetime-suggestion.stderr b/tests/ui/lifetimes/E0106-trailing-comma-in-lifetime-suggestion.stderr
new file mode 100644
index 0000000000000..f189c259d65f9
--- /dev/null
+++ b/tests/ui/lifetimes/E0106-trailing-comma-in-lifetime-suggestion.stderr
@@ -0,0 +1,25 @@
+error[E0106]: missing lifetime specifier
+  --> $DIR/E0106-trailing-comma-in-lifetime-suggestion.rs:12:13
+   |
+LL | type A = Foo<>;
+   |             ^ expected named lifetime parameter
+   |
+help: consider introducing a named lifetime parameter
+   |
+LL | type A<'a> = Foo<'a>;
+   |       ++++       ++
+
+error[E0106]: missing lifetime specifier
+  --> $DIR/E0106-trailing-comma-in-lifetime-suggestion.rs:17:13
+   |
+LL | type B = Bar<u8>;
+   |             ^ expected named lifetime parameter
+   |
+help: consider introducing a named lifetime parameter
+   |
+LL | type B<'a> = Bar<'a, u8>;
+   |       ++++       +++
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0106`.