Add tests for "mutably borrowing copies into a temporary"

tests/ui/borrowck/return-borrow-mut-const.rs

@@ -0,0 +1,34 @@
+// See PR #104857 for details
+
+// don't want to tie this test to the lint, even though it's related
+#![allow(const_item_mutation)]
+
+fn main() {}
+
+const X: i32 = 42;
+
+fn borrow_const_immut() -> &'static i32 {
+    &X
+}
+
+fn borrow_const_immut_explicit_return() -> &'static i32 {
+    return &X;
+}
+
+fn borrow_const_immut_into_temp() -> &'static i32 {
+    let x_ref = &X;
+    x_ref
+}
+
+fn borrow_const_mut() -> &'static mut i32 {
+    return &mut X; //~ ERROR
+}
+
+fn borrow_const_mut_explicit_return() -> &'static mut i32 {
+    return &mut X; //~ ERROR
+}
+
+fn borrow_const_mut_into_temp() -> &'static mut i32 {
+    let x_ref = &mut X;
+    x_ref //~ ERROR
+}

tests/ui/borrowck/return-borrow-mut-const.stderr

@@ -0,0 +1,35 @@
+error[E0515]: cannot return reference to temporary value
+  --> $DIR/return-borrow-mut-const.rs:24:12
+   |
+LL |     return &mut X;
+   |            ^^^^^-
+   |            |    |
+   |            |    temporary value created here
+   |            returns a reference to data owned by the current function
+   |
+   = note: mutably borrowing a constant copies the value to a temporary
+
+error[E0515]: cannot return reference to temporary value
+  --> $DIR/return-borrow-mut-const.rs:28:12
+   |
+LL |     return &mut X;
+   |            ^^^^^-
+   |            |    |
+   |            |    temporary value created here
+   |            returns a reference to data owned by the current function
+   |
+   = note: mutably borrowing a constant copies the value to a temporary
+
+error[E0515]: cannot return value referencing temporary value
+  --> $DIR/return-borrow-mut-const.rs:33:5
+   |
+LL |     let x_ref = &mut X;
+   |                      - temporary value created here
+LL |     x_ref
+   |     ^^^^^ returns a value referencing data owned by the current function
+   |
+   = note: mutably borrowing a constant copies the value to a temporary
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0515`.

tests/ui/borrowck/return-borrow-mut-fn.rs

@@ -0,0 +1,31 @@
+// See PR #104857 for details
+
+fn main() {}
+
+fn do_nothing() {}
+
+fn borrow_fn_immut() -> &'static dyn Fn() {
+    &do_nothing
+}
+
+fn borrow_fn_immut_explicit_return() -> &'static dyn Fn() {
+    &do_nothing
+}
+
+fn borrow_fn_immut_into_temp() -> &'static dyn Fn() {
+    let f = &do_nothing;
+    f
+}
+
+fn borrow_fn_mut() -> &'static mut dyn FnMut() {
+    &mut do_nothing //~ ERROR
+}
+
+fn borrow_fn_mut_explicit_return() -> &'static mut dyn FnMut() {
+    &mut do_nothing //~ ERROR
+}
+
+fn borrow_fn_mut_into_temp() -> &'static mut dyn FnMut() {
+    let f = &mut do_nothing;
+    f //~ ERROR
+}

tests/ui/borrowck/return-borrow-mut-fn.stderr

@@ -0,0 +1,35 @@
+error[E0515]: cannot return reference to temporary value
+  --> $DIR/return-borrow-mut-fn.rs:21:5
+   |
+LL |     &mut do_nothing
+   |     ^^^^^----------
+   |     |    |
+   |     |    temporary value created here
+   |     returns a reference to data owned by the current function
+   |
+   = note: mutably borrowing a function copies the function pointer to a temporary
+
+error[E0515]: cannot return reference to temporary value
+  --> $DIR/return-borrow-mut-fn.rs:25:5
+   |
+LL |     &mut do_nothing
+   |     ^^^^^----------
+   |     |    |
+   |     |    temporary value created here
+   |     returns a reference to data owned by the current function
+   |
+   = note: mutably borrowing a function copies the function pointer to a temporary
+
+error[E0515]: cannot return value referencing temporary value
+  --> $DIR/return-borrow-mut-fn.rs:30:5
+   |
+LL |     let f = &mut do_nothing;
+   |                  ---------- temporary value created here
+LL |     f
+   |     ^ returns a value referencing data owned by the current function
+   |
+   = note: mutably borrowing a function copies the function pointer to a temporary
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0515`.