Test that we allow non-static lifetime transmutes

Author: devonhollowood

tests/ui/transmute.rs

@@ -140,6 +140,21 @@ fn bytes_to_str(b: &[u8], mb: &mut [u8]) {
     let _: &mut str = unsafe { std::mem::transmute(mb) };
 }
 
+// Make sure we can modify lifetimes, which is one of the recommended uses
+// of transmute
+
+// Make sure we can do static lifetime transmutes
+#[warn(transmute_ptr_to_ptr)]
+unsafe fn transmute_lifetime_to_static<'a, T>(t: &'a T) -> &'static T {
+    std::mem::transmute::<&'a T, &'static T>(t)
+}
+
+// Make sure we can do non-static lifetime transmutes
+#[warn(transmute_ptr_to_ptr)]
+unsafe fn transmute_lifetime<'a, 'b, T>(t: &'a T, u: &'b T) -> &'b T {
+    std::mem::transmute::<&'a T, &'b T>(t)
+}
+
 #[warn(transmute_ptr_to_ptr)]
 fn transmute_ptr_to_ptr() {
     let ptr = &1u32 as *const u32;
@@ -159,10 +174,6 @@ fn transmute_ptr_to_ptr() {
     let _ = mut_ptr as *mut f32;
     let _ = unsafe { &*(&1u32 as *const u32 as *const f32) };
     let _ = unsafe { &mut *(&mut 1u32 as *mut u32 as *mut f32) };
-    // This is just modifying the lifetime, and is one of the recommended uses
-    // of transmute
-    let n = 1u32;
-    let _ = unsafe { std::mem::transmute::<&'_ u32, &'static u32>(&n) };
 }
 
 fn main() { }

tests/ui/transmute.stderr

@@ -205,29 +205,29 @@ error: transmute from a `&mut [u8]` to a `&mut str`
     |                                ^^^^^^^^^^^^^^^^^^^^^^^ help: consider using: `std::str::from_utf8_mut(mb).unwrap()`
 
 error: transmute from a pointer to a pointer
-   --> $DIR/transmute.rs:149:29
+   --> $DIR/transmute.rs:164:29
     |
-149 |         let _: *const f32 = std::mem::transmute(ptr);
+164 |         let _: *const f32 = std::mem::transmute(ptr);
     |                             ^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `ptr as *const f32`
     |
     = note: `-D transmute-ptr-to-ptr` implied by `-D warnings`
 
 error: transmute from a pointer to a pointer
-   --> $DIR/transmute.rs:150:27
+   --> $DIR/transmute.rs:165:27
     |
-150 |         let _: *mut f32 = std::mem::transmute(mut_ptr);
+165 |         let _: *mut f32 = std::mem::transmute(mut_ptr);
     |                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `mut_ptr as *mut f32`
 
 error: transmute from a reference to a reference
-   --> $DIR/transmute.rs:152:23
+   --> $DIR/transmute.rs:167:23
     |
-152 |         let _: &f32 = std::mem::transmute(&1u32);
+167 |         let _: &f32 = std::mem::transmute(&1u32);
     |                       ^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `&*(&1u32 as *const u32 as *const f32)`
 
 error: transmute from a reference to a reference
-   --> $DIR/transmute.rs:153:27
+   --> $DIR/transmute.rs:168:27
     |
-153 |         let _: &mut f32 = std::mem::transmute(&mut 1u32);
+168 |         let _: &mut f32 = std::mem::transmute(&mut 1u32);
     |                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `&mut *(&mut 1u32 as *mut u32 as *mut f32)`
 
 error: aborting due to 36 previous errors