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diff --git a/compiler/rustc_error_codes/src/error_codes/E0643.md b/compiler/rustc_error_codes/src/error_codes/E0643.md
new file mode 100644
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+++ b/compiler/rustc_error_codes/src/error_codes/E0643.md
@@ -0,0 +1,12 @@
+This error indicates that there is a mismatch between generic parameters and
+impl Trait parameters in a trait declaration versus its impl.
+
+```compile_fail,E0643
+trait Foo {
+    fn foo(&self, _: &impl Iterator);
+}
+impl Foo for () {
+    fn foo<U: Iterator>(&self, _: &U) { } // error method `foo` has incompatible
+                                          // signature for trait
+}
+```