Auto merge of #156839 - qaijuang:fix-single-use-lifetimes-impl-bounds, r=mu001999

[single_use_lifetimes]: Account for lifetime bounds

This PR makes `single_use_lifetimes` account for bounds on lifetime parameters, so a lifetime such as `'b` in `<'a, 'b: 'a>` is not treated as single-use just because its bound is stored separately from ordinary lifetime uses.

Fixes #153836

Author: bors

compiler/rustc_resolve/src/late/diagnostics.rs

@@ -3630,6 +3630,9 @@ impl<'ast, 'ra, 'tcx> LateResolutionVisitor<'_, 'ast, 'ra, 'tcx> {
             };
             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 @@ mod type_params {
 
 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 @@ mod lifetime_params {
         }
     }
 
-    m!('a); //~ ERROR lifetime parameter `'a` only used once
+    m!('a);
     n!('a);
     p!('a);
 }

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

@@ -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.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/issue-117965.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
+