1 files changed, 86 insertions, 10 deletions

diff --git a/src/tools/clippy/tests/ui/needless_lifetimes.rs b/src/tools/clippy/tests/ui/needless_lifetimes.rs
index fc686b1da..2efc93675 100644
--- a/src/tools/clippy/tests/ui/needless_lifetimes.rs
+++ b/src/tools/clippy/tests/ui/needless_lifetimes.rs
@@ -29,11 +29,20 @@ fn multiple_in_and_out_1<'a>(x: &'a u8, _y: &'a u8) -> &'a u8 {
     x
 }
 
-// No error; multiple input refs.
-fn multiple_in_and_out_2<'a, 'b>(x: &'a u8, _y: &'b u8) -> &'a u8 {
+// Error; multiple input refs, but the output lifetime is not elided, i.e., the following is valid:
+//   fn multiple_in_and_out_2a<'a>(x: &'a u8, _y: &u8) -> &'a u8
+//                                                ^^^
+fn multiple_in_and_out_2a<'a, 'b>(x: &'a u8, _y: &'b u8) -> &'a u8 {
     x
 }
 
+// Error; multiple input refs, but the output lifetime is not elided, i.e., the following is valid:
+//   fn multiple_in_and_out_2b<'b>(_x: &u8, y: &'b u8) -> &'b u8
+//                                     ^^^
+fn multiple_in_and_out_2b<'a, 'b>(_x: &'a u8, y: &'b u8) -> &'b u8 {
+    y
+}
+
 // No error; multiple input refs
 async fn func<'a>(args: &[&'a str]) -> Option<&'a str> {
     args.get(0).cloned()
@@ -44,11 +53,20 @@ fn in_static_and_out<'a>(x: &'a u8, _y: &'static u8) -> &'a u8 {
     x
 }
 
-// No error.
-fn deep_reference_1<'a, 'b>(x: &'a u8, _y: &'b u8) -> Result<&'a u8, ()> {
+// Error; multiple input refs, but the output lifetime is not elided, i.e., the following is valid:
+//   fn deep_reference_1a<'a>(x: &'a u8, _y: &u8) -> Result<&'a u8, ()>
+//                                           ^^^
+fn deep_reference_1a<'a, 'b>(x: &'a u8, _y: &'b u8) -> Result<&'a u8, ()> {
     Ok(x)
 }
 
+// Error; multiple input refs, but the output lifetime is not elided, i.e., the following is valid:
+//   fn deep_reference_1b<'b>(_x: &u8, y: &'b u8) -> Result<&'b u8, ()>
+//                                ^^^
+fn deep_reference_1b<'a, 'b>(_x: &'a u8, y: &'b u8) -> Result<&'b u8, ()> {
+    Ok(y)
+}
+
 // No error; two input refs.
 fn deep_reference_2<'a>(x: Result<&'a u8, &'a u8>) -> &'a u8 {
     x.unwrap()
@@ -129,11 +147,20 @@ impl X {
         &self.x
     }
 
-    // No error; multiple input refs.
-    fn self_and_in_out<'s, 't>(&'s self, _x: &'t u8) -> &'s u8 {
+    // Error; multiple input refs, but the output lifetime is not elided, i.e., the following is valid:
+    //   fn self_and_in_out_1<'s>(&'s self, _x: &u8) -> &'s u8
+    //                                          ^^^
+    fn self_and_in_out_1<'s, 't>(&'s self, _x: &'t u8) -> &'s u8 {
         &self.x
     }
 
+    // Error; multiple input refs, but the output lifetime is not elided, i.e., the following is valid:
+    //   fn self_and_in_out_2<'t>(&self, x: &'t u8) -> &'t u8
+    //                            ^^^^^
+    fn self_and_in_out_2<'s, 't>(&'s self, x: &'t u8) -> &'t u8 {
+        x
+    }
+
     fn distinct_self_and_in<'s, 't>(&'s self, _x: &'t u8) {}
 
     // No error; same lifetimes on two params.
@@ -167,8 +194,19 @@ fn struct_with_lt3<'a>(_foo: &Foo<'a>) -> &'a str {
     unimplemented!()
 }
 
-// No warning; two input lifetimes.
-fn struct_with_lt4<'a, 'b>(_foo: &'a Foo<'b>) -> &'a str {
+// Warning; two input lifetimes, but the output lifetime is not elided, i.e., the following is
+// valid:
+//   fn struct_with_lt4a<'a>(_foo: &'a Foo<'_>) -> &'a str
+//                                         ^^
+fn struct_with_lt4a<'a, 'b>(_foo: &'a Foo<'b>) -> &'a str {
+    unimplemented!()
+}
+
+// Warning; two input lifetimes, but the output lifetime is not elided, i.e., the following is
+// valid:
+//   fn struct_with_lt4b<'b>(_foo: &Foo<'b>) -> &'b str
+//                                 ^^^^
+fn struct_with_lt4b<'a, 'b>(_foo: &'a Foo<'b>) -> &'b str {
     unimplemented!()
 }
 
@@ -203,8 +241,19 @@ fn alias_with_lt3<'a>(_foo: &FooAlias<'a>) -> &'a str {
     unimplemented!()
 }
 
-// No warning; two input lifetimes.
-fn alias_with_lt4<'a, 'b>(_foo: &'a FooAlias<'b>) -> &'a str {
+// Warning; two input lifetimes, but the output lifetime is not elided, i.e., the following is
+// valid:
+//   fn alias_with_lt4a<'a>(_foo: &'a FooAlias<'_>) -> &'a str
+//                                             ^^
+fn alias_with_lt4a<'a, 'b>(_foo: &'a FooAlias<'b>) -> &'a str {
+    unimplemented!()
+}
+
+// Warning; two input lifetimes, but the output lifetime is not elided, i.e., the following is
+// valid:
+//   fn alias_with_lt4b<'b>(_foo: &FooAlias<'b>) -> &'b str
+//                                ^^^^^^^^^
+fn alias_with_lt4b<'a, 'b>(_foo: &'a FooAlias<'b>) -> &'b str {
     unimplemented!()
 }
 
@@ -419,4 +468,31 @@ mod issue7296 {
     }
 }
 
+mod pr_9743_false_negative_fix {
+    #![allow(unused)]
+
+    fn foo<'a>(x: &'a u8, y: &'_ u8) {}
+
+    fn bar<'a>(x: &'a u8, y: &'_ u8, z: &'_ u8) {}
+}
+
+mod pr_9743_output_lifetime_checks {
+    #![allow(unused)]
+
+    // lint: only one input
+    fn one_input<'a>(x: &'a u8) -> &'a u8 {
+        unimplemented!()
+    }
+
+    // lint: multiple inputs, output would not be elided
+    fn multiple_inputs_output_not_elided<'a, 'b>(x: &'a u8, y: &'b u8, z: &'b u8) -> &'b u8 {
+        unimplemented!()
+    }
+
+    // don't lint: multiple inputs, output would be elided (which would create an ambiguity)
+    fn multiple_inputs_output_would_be_elided<'a, 'b>(x: &'a u8, y: &'b u8, z: &'b u8) -> &'a u8 {
+        unimplemented!()
+    }
+}
+
 fn main() {}