account for lifetime bounds in single_use_lifetimes

compiler/rustc_resolve/src/late/diagnostics.rs

@@ -3630,6 +3630,9 @@ pub(crate) fn maybe_report_lifetime_uses(
             };
             match use_set {
                 Some(LifetimeUseSet::Many) => {}
+                // A lifetime bound is a real use of that lifetime parameter, even
+                // though visiting a bound like `'b: 'a` only records a use of `'a`.
+                Some(LifetimeUseSet::One { .. }) if !param.bounds.is_empty() => {}
                 Some(LifetimeUseSet::One { use_span, use_ctxt }) => {
                     let param_ident = param.ident;
                     let deletion_span =

tests/ui/single-use-lifetime/issue-104440.rs

@@ -1,3 +1,5 @@
+//@ check-pass
+
 #![feature(decl_macro, rustc_attrs)]
 #![deny(single_use_lifetimes)]
 
@@ -35,7 +37,7 @@ fn k<T: Clone>(t1: $T, t2: T) -> (T, $T) {
 
 mod lifetime_params {
     macro m($a:lifetime) {
-        fn f<'b, 'c, $a: 'b, 'a: 'c>(t1: &$a(), t2: &'a ()) -> (&'b (), &'c ()) { //~ ERROR lifetime parameter `'a` only used once
+        fn f<'b, 'c, $a: 'b, 'a: 'c>(t1: &$a(), t2: &'a ()) -> (&'b (), &'c ()) {
             (t1, t2)
         }
     }
@@ -60,7 +62,7 @@ fn k<'a>(t1: &$a(), t2: &'a ()) -> (&'a (), &$a ()) {
         }
     }
 
-    m!('a); //~ ERROR lifetime parameter `'a` only used once
+    m!('a);
     n!('a);
     p!('a);
 }

tests/ui/single-use-lifetime/issue-104440.stderr

@@ -1,28 +0,0 @@
-error: lifetime parameter `'a` only used once
-  --> $DIR/issue-104440.rs:63:8
-   |
-LL |     m!('a);
-   |        ^^
-   |        |
-   |        this lifetime...
-   |        ...is used only here
-   |
-note: the lint level is defined here
-  --> $DIR/issue-104440.rs:2:9
-   |
-LL | #![deny(single_use_lifetimes)]
-   |         ^^^^^^^^^^^^^^^^^^^^
-
-error: lifetime parameter `'a` only used once
-  --> $DIR/issue-104440.rs:38:30
-   |
-LL |         fn f<'b, 'c, $a: 'b, 'a: 'c>(t1: &$a(), t2: &'a ()) -> (&'b (), &'c ()) {
-   |                              ^^ this lifetime...     -- ...is used only here
-...
-LL |     m!('a);
-   |     ------ in this macro invocation
-   |
-   = note: this error originates in the macro `m` (in Nightly builds, run with -Z macro-backtrace for more info)
-
-error: aborting due to 2 previous errors
-

tests/ui/single-use-lifetime/issue-117965.rs

@@ -1,3 +1,5 @@
+//@ check-pass
+
 #![deny(single_use_lifetimes)]
 
 pub enum Data<'a> {
@@ -7,7 +9,6 @@ pub enum Data<'a> {
 
 impl<'a> Data<'a> {
     pub fn get<'b: 'a>(&'b self) -> &'a str {
-        //~^ ERROR lifetime parameter `'b` only used once
         match &self {
             Self::Borrowed(val) => val,
             Self::Owned(val) => &val,

tests/ui/single-use-lifetime/issue-117965.stderr

@@ -1,16 +0,0 @@
-error: lifetime parameter `'b` only used once
-  --> $DIR/issue-117965.rs:9:16
-   |
-LL |     pub fn get<'b: 'a>(&'b self) -> &'a str {
-   |                ^^       -- ...is used only here
-   |                |
-   |                this lifetime...
-   |
-note: the lint level is defined here
-  --> $DIR/issue-117965.rs:1:9
-   |
-LL | #![deny(single_use_lifetimes)]
-   |         ^^^^^^^^^^^^^^^^^^^^
-
-error: aborting due to 1 previous error
-

tests/ui/single-use-lifetime/lifetime-bounds.rs

@@ -0,0 +1,34 @@
+// Regression test for https://github.com/rust-lang/rust/issues/153836.
+#![deny(single_use_lifetimes)]
+#![allow(dead_code)]
+#![allow(unused_variables)]
+
+struct Foo<'a>(&'a i32);
+struct Bar<'b>(&'b i32);
+
+fn function<'a, 'b: 'a>(_: Foo<'a>, _: Bar<'b>) {}
+
+type FnPtr = for<'a, 'b: 'a> fn(Foo<'a>, Bar<'b>);
+//~^ ERROR bounds cannot be used in this context
+
+trait WhereBound<'a, 'b> {}
+
+fn where_bound<T>()
+where
+    T: for<'a, 'b: 'a> WhereBound<'a, 'b>,
+    //~^ ERROR bounds cannot be used in this context
+{
+}
+
+trait ImplTrait<'a> {
+    fn foo(self, foo: Foo<'a>);
+}
+
+impl<'a, 'b: 'a> ImplTrait<'a> for Bar<'b> {
+    fn foo(self, foo: Foo<'a>) {
+        let _: &'a i32 = self.0;
+        let _: Foo<'a> = foo;
+    }
+}
+
+fn main() {}

tests/ui/single-use-lifetime/lifetime-bounds.stderr

@@ -0,0 +1,14 @@
+error: bounds cannot be used in this context
+  --> $DIR/lifetime-bounds.rs:11:26
+   |
+LL | type FnPtr = for<'a, 'b: 'a> fn(Foo<'a>, Bar<'b>);
+   |                          ^^
+
+error: bounds cannot be used in this context
+  --> $DIR/lifetime-bounds.rs:18:20
+   |
+LL |     T: for<'a, 'b: 'a> WhereBound<'a, 'b>,
+   |                    ^^
+
+error: aborting due to 2 previous errors
+

tests/ui/single-use-lifetime/two-uses-in-trait-impl.rs

@@ -19,18 +19,4 @@ fn next(&mut self) -> Option<Self::Item> {
     }
 }
 
-trait BoundTrait<'a> {
-    fn foo(self, handler: &Handler<'a>);
-}
-
-struct Handler<'a>(fn(&'a u32));
-struct Bar<'b>(&'b u32);
-
-// https://github.com/rust-lang/rust/issues/153836
-impl<'a, 'b: 'a> BoundTrait<'a> for Bar<'b> {
-    fn foo(self, handler: &Handler<'a>) {
-        (handler.0)(self.0);
-    }
-}
-
 fn main() {}