fix: Alloc new errorcode E0803 for E0495

Signed-off-by: xizheyin <xizheyin@smail.nju.edu.cn>

compiler/rustc_error_codes/src/error_codes/E0803.md

@@ -0,0 +1,46 @@
+A trait implementation returns a reference without an
+explicit lifetime linking it to `self`.
+It commonly arises in generic trait implementations
+requiring explicit lifetime bounds.
+
+Erroneous code example:
+
+```compile_fail,E0803
+trait DataAccess<T> {
+    fn get_ref(&self) -> T;
+}
+
+struct Container<'a> {
+    value: &'a f64,
+}
+
+// Attempting to implement reference return
+impl<'a> DataAccess<&f64> for Container<'a> {
+    fn get_ref(&self) -> &f64 { // Error: Lifetime mismatch
+        self.value
+    }
+}
+```
+
+The trait method returns &f64 requiring an independent lifetime
+The struct Container<'a> carries lifetime parameter 'a
+The compiler cannot verify if the returned reference satisfies 'a constraints
+Solution
+Explicitly bind lifetimes to clarify constraints:
+```
+// Modified trait with explicit lifetime binding
+trait DataAccess<'a, T> {
+    fn get_ref(&'a self) -> T;
+}
+
+struct Container<'a> {
+    value: &'a f64,
+}
+
+// Correct implementation (bound lifetimes)
+impl<'a> DataAccess<'a, &'a f64> for Container<'a> {
+    fn get_ref(&'a self) -> &'a f64 {
+        self.value
+    }
+}
+```

compiler/rustc_error_codes/src/lib.rs

@@ -546,6 +546,7 @@ E0799: 0799,
 E0800: 0800,
 E0801: 0801,
 E0802: 0802,
+E0803: 0803,
         );
     )
 }