Auto merge of #32811 - alexcrichton:check-lints, r=nrc

[rust.git] / src / librustc_typeck / diagnostics.rs

diff --git a/src/librustc_typeck/diagnostics.rs b/src/librustc_typeck/diagnostics.rs
index 4a4bcd96ad06c0fb58dd0ea79bf58d9f9e32b5c4..1cfbfb37d34bd3fb2131429eb165717d26159fc9 100644 (file)
--- a/src/librustc_typeck/diagnostics.rs
+++ b/src/librustc_typeck/diagnostics.rs
@@ -3644,6 +3644,68 @@ fn main() {
 ```
 "##,
 
+E0520: r##"
+A non-default implementation was already made on this type so it cannot be
+specialized further. Erroneous code example:
+
+```compile_fail
+#![feature(specialization)]
+
+trait SpaceLlama {
+    fn fly(&self);
+}
+
+// applies to all T
+impl<T> SpaceLlama for T {
+    default fn fly(&self) {}
+}
+
+// non-default impl
+// applies to all `Clone` T and overrides the previous impl
+impl<T: Clone> SpaceLlama for T {
+    fn fly(&self) {}
+}
+
+// since `i32` is clone, this conflicts with the previous implementation
+impl SpaceLlama for i32 {
+    default fn fly(&self) {}
+    // error: item `fly` is provided by an `impl` that specializes
+    //        another, but the item in the parent `impl` is not marked
+    //        `default` and so it cannot be specialized.
+}
+```
+
+Specialization only allows you to override `default` functions in
+implementations.
+
+To fix this error, you need to mark all the parent implementations as default.
+Example:
+
+```
+#![feature(specialization)]
+
+trait SpaceLlama {
+    fn fly(&self);
+}
+
+// applies to all T
+impl<T> SpaceLlama for T {
+    default fn fly(&self) {} // This is a parent implementation.
+}
+
+// applies to all `Clone` T; overrides the previous impl
+impl<T: Clone> SpaceLlama for T {
+    default fn fly(&self) {} // This is a parent implementation but was
+                             // previously not a default one, causing the error
+}
+
+// applies to i32, overrides the previous two impls
+impl SpaceLlama for i32 {
+    fn fly(&self) {} // And now that's ok!
+}
+```
+"##,
+
 }
 
 register_diagnostics! {
@@ -3720,6 +3782,5 @@ fn main() {
            // type `{}` was overridden
     E0436, // functional record update requires a struct
     E0513, // no type for local variable ..
-    E0520, // cannot specialize non-default item
     E0521  // redundant default implementations of trait
 }