started work on #136827

Author: progressive-galib

compiler/rustc_error_codes/src/error_codes/E0495.md

@@ -1,40 +1,36 @@
-#### Note: this error code is no longer emitted by the compiler.
+A trait implementation returns a reference without an explicit lifetime linking it to `self`.
 
-A lifetime cannot be determined in the given situation.
+```compile_fail,E0495
+trait Tr<X> {
+    fn f(&self) -> X;
+}
 
-Erroneous code example:
+struct Wr<'b> {
+    ri: &'b i32,
+    f: f32,
+}
 
-```compile_fail
-fn transmute_lifetime<'a, 'b, T>(t: &'a (T,)) -> &'b T {
-    match (&t,) { // error!
-        ((u,),) => u,
+impl<'b> Tr<&f32> for Wr<'b> {
+    fn f(&self) -> &f32 { // error: missing lifetime linking &f32 to self
+        &self.f
     }
 }
-
-let y = Box::new((42,));
-let x = transmute_lifetime(&y);
 ```
+Specify an explicit lifetime in the trait to ensure that the reference returned is valid for at least as long as `self`:
 
-In this code, you have two ways to solve this issue:
- 1. Enforce that `'a` lives at least as long as `'b`.
- 2. Use the same lifetime requirement for both input and output values.
-
-So for the first solution, you can do it by replacing `'a` with `'a: 'b`:
-
-```
-fn transmute_lifetime<'a: 'b, 'b, T>(t: &'a (T,)) -> &'b T {
-    match (&t,) { // ok!
-        ((u,),) => u,
-    }
+```rust
+trait Tr<'a, X> {
+    fn f(&'a self) -> X;
 }
-```
 
-In the second you can do it by simply removing `'b` so they both use `'a`:
+struct Wr<'b> {
+    ri: &'b i32,
+    f: f32,
+}
 
-```
-fn transmute_lifetime<'a, T>(t: &'a (T,)) -> &'a T {
-    match (&t,) { // ok!
-        ((u,),) => u,
+impl<'a, 'b: 'a> Tr<'a, &'a f32> for Wr<'b> {
+    fn f(&'a self) -> &'a f32 { // ok!
+        &self.f
     }
 }
 ```