Add E0790 as more specific variant of E0283

compiler/rustc_error_codes/src/error_codes/E0283.md

@@ -3,48 +3,31 @@ An implementation cannot be chosen unambiguously because of lack of information.
 Erroneous code example:
 
 ```compile_fail,E0283
-trait Generator {
-    fn create() -> u32;
-}
+struct Foo;
 
-struct Impl;
-
-impl Generator for Impl {
-    fn create() -> u32 { 1 }
-}
-
-struct AnotherImpl;
-
-impl Generator for AnotherImpl {
-    fn create() -> u32 { 2 }
+impl Into<u32> for Foo {
+    fn into(self) -> u32 { 1 }
 }
 
 fn main() {
-    let cont: u32 = Generator::create();
-    // error, impossible to choose one of Generator trait implementation
-    // Should it be Impl or AnotherImpl, maybe something else?
+    let foo = Foo;
+    let bar: u32 = foo.into() * 1u32;
 }
 ```
 
 This error can be solved by adding type annotations that provide the missing
-information to the compiler. In this case, the solution is to use a concrete
-type:
+information to the compiler. In this case, the solution is to specify the
+fully-qualified method:
 
 ```
-trait Generator {
-    fn create() -> u32;
-}
+struct Foo;
 
-struct AnotherImpl;
-
-impl Generator for AnotherImpl {
-    fn create() -> u32 { 2 }
+impl Into<u32> for Foo {
+    fn into(self) -> u32 { 1 }
 }
 
 fn main() {
-    let gen1 = AnotherImpl::create();
-
-    // if there are multiple methods with same name (different traits)
-    let gen2 = <AnotherImpl as Generator>::create();
+    let foo = Foo;
+    let bar: u32 = <Foo as Into<u32>>::into(foo) * 1u32;
 }
 ```

compiler/rustc_error_codes/src/error_codes/E0790.md

@@ -0,0 +1,51 @@
+You need to specify a specific implementation of the trait in order to call the
+method.
+
+Erroneous code example:
+
+```compile_fail,E0790
+trait Generator {
+    fn create() -> u32;
+}
+
+struct Impl;
+
+impl Generator for Impl {
+    fn create() -> u32 { 1 }
+}
+
+struct AnotherImpl;
+
+impl Generator for AnotherImpl {
+    fn create() -> u32 { 2 }
+}
+
+fn main() {
+    let cont: u32 = Generator::create();
+    // error, impossible to choose one of Generator trait implementation
+    // Should it be Impl or AnotherImpl, maybe something else?
+}
+```
+
+This error can be solved by adding type annotations that provide the missing
+information to the compiler. In this case, the solution is to use a concrete
+type:
+
+```
+trait Generator {
+    fn create() -> u32;
+}
+
+struct AnotherImpl;
+
+impl Generator for AnotherImpl {
+    fn create() -> u32 { 2 }
+}
+
+fn main() {
+    let gen1 = AnotherImpl::create();
+
+    // if there are multiple methods with same name (different traits)
+    let gen2 = <AnotherImpl as Generator>::create();
+}
+```