Fix false positive unused_lifetime when referenced in generic bounds.

clippy_lints/src/lifetimes.rs

@@ -103,6 +103,11 @@ fn check_fn_inner<'a, 'tcx>(
     let mut bounds_lts = Vec::new();
     for typ in generics.ty_params() {
         for bound in &typ.bounds {
+            let mut visitor = RefVisitor::new(cx);
+            walk_ty_param_bound(&mut visitor, bound);
+            if visitor.lts.iter().any(|lt| matches!(lt, RefLt::Named(_))) {
+                return;
+            }
             if let TraitTyParamBound(ref trait_ref, _) = *bound {
                 let params = &trait_ref
                     .trait_ref

tests/ui/lifetimes.rs

@@ -53,6 +53,22 @@ fn fn_bound_2<'a, F, I>(_m: Lt<'a, I>, _f: F) -> Lt<'a, I>
     where for<'x> F: Fn(Lt<'x, I>) -> Lt<'x, I>
 { unreachable!() }
 
+fn fn_bound_3<'a, F: FnOnce(&'a i32)>(x: &'a i32, f: F) { // no error, see below
+    f(x);
+}
+
+fn fn_bound_3_cannot_elide() {
+    let x = 42;
+    let p = &x;
+    let mut q = &x;
+    fn_bound_3(p, |y| q = y); // this will fail if we elides lifetimes of `fn_bound_3`
+}
+
+// no error, multiple input refs
+fn fn_bound_4<'a, F: FnOnce() -> &'a ()>(cond: bool, x: &'a (), f: F) -> &'a () {
+    if cond { x } else { f() }
+}
+
 struct X {
     x: u8,
 }

tests/ui/lifetimes.stderr

@@ -45,45 +45,45 @@ error: explicit lifetimes given in parameter types where they could be elided
    | |__________________^
 
 error: explicit lifetimes given in parameter types where they could be elided
-  --> $DIR/lifetimes.rs:61:5
+  --> $DIR/lifetimes.rs:77:5
    |
-61 |     fn self_and_out<'s>(&'s self) -> &'s u8 { &self.x }
+77 |     fn self_and_out<'s>(&'s self) -> &'s u8 { &self.x }
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: explicit lifetimes given in parameter types where they could be elided
-  --> $DIR/lifetimes.rs:65:5
+  --> $DIR/lifetimes.rs:81:5
    |
-65 |     fn distinct_self_and_in<'s, 't>(&'s self, _x: &'t u8) { }
+81 |     fn distinct_self_and_in<'s, 't>(&'s self, _x: &'t u8) { }
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: explicit lifetimes given in parameter types where they could be elided
-  --> $DIR/lifetimes.rs:81:1
+  --> $DIR/lifetimes.rs:97:1
    |
-81 | fn struct_with_lt<'a>(_foo: Foo<'a>) -> &'a str { unimplemented!() }
+97 | fn struct_with_lt<'a>(_foo: Foo<'a>) -> &'a str { unimplemented!() }
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: explicit lifetimes given in parameter types where they could be elided
-   --> $DIR/lifetimes.rs:101:1
+   --> $DIR/lifetimes.rs:117:1
     |
-101 | fn trait_obj_elided2<'a>(_arg: &'a Drop) -> &'a str { unimplemented!() }
+117 | fn trait_obj_elided2<'a>(_arg: &'a Drop) -> &'a str { unimplemented!() }
     | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: explicit lifetimes given in parameter types where they could be elided
-   --> $DIR/lifetimes.rs:105:1
+   --> $DIR/lifetimes.rs:121:1
     |
-105 | fn alias_with_lt<'a>(_foo: FooAlias<'a>) -> &'a str { unimplemented!() }
+121 | fn alias_with_lt<'a>(_foo: FooAlias<'a>) -> &'a str { unimplemented!() }
     | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: explicit lifetimes given in parameter types where they could be elided
-   --> $DIR/lifetimes.rs:116:1
+   --> $DIR/lifetimes.rs:132:1
     |
-116 | fn named_input_elided_output<'a>(_arg: &'a str) -> &str { unimplemented!() }
+132 | fn named_input_elided_output<'a>(_arg: &'a str) -> &str { unimplemented!() }
     | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: explicit lifetimes given in parameter types where they could be elided
-   --> $DIR/lifetimes.rs:120:1
+   --> $DIR/lifetimes.rs:136:1
     |
-120 | fn trait_bound_ok<'a, T: WithLifetime<'static>>(_: &'a u8, _: T) { unimplemented!() }
+136 | fn trait_bound_ok<'a, T: WithLifetime<'static>>(_: &'a u8, _: T) { unimplemented!() }
     | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: aborting due to 14 previous errors