Add error explanations for E0053, E0251, E0252, E0255, E0256, E0368.

Author: Nick Hamann

src/librustc_resolve/diagnostics.rs

@@ -49,6 +49,88 @@ about what constitutes an Item declaration and what does not:
 http://doc.rust-lang.org/reference.html#statements
 "##,
 
+E0251: r##"
+Two items of the same name cannot be imported without rebinding one of the
+items under a new local name.
+
+An example of this error:
+
+```
+use foo::baz;
+use bar::*; // error, do `use foo::baz as quux` instead on the previous line
+
+fn main() {}
+
+mod foo {
+    pub struct baz;
+}
+
+mod bar {
+    pub mod baz {}
+}
+```
+"##,
+
+E0252: r##"
+Two items of the same name cannot be imported without rebinding one of the
+items under a new local name.
+
+An example of this error:
+
+```
+use foo::baz;
+use bar::baz; // error, do `use bar::baz as quux` instead
+
+fn main() {}
+
+mod foo {
+    pub struct baz;
+}
+
+mod bar {
+    pub mod baz {}
+}
+```
+"##,
+
+E0255: r##"
+You can't import a value whose name is the same as another value defined in the
+module.
+
+An example of this error:
+
+```
+use foo::FOO; // error, do `use foo::FOO as BAR` instead
+
+fn FOO() {}
+
+mod foo {
+    pub const FOO: bool = true;
+}
+
+fn main() {}
+```
+"##,
+
+E0256: r##"
+You can't import a type or module when the name of the item being imported is
+the same as another type or submodule defined in the module.
+
+An example of this error:
+
+```
+use foo::Bar; // error
+
+struct Bar;
+
+mod foo {
+    pub mod Bar { }
+}
+
+fn main() {}
+```
+"##,
+
 E0259: r##"
 The name chosen for an external crate conflicts with another external crate that
 has been imported into the current module.
@@ -122,14 +204,10 @@ http://doc.rust-lang.org/reference.html#types
 register_diagnostics! {
     E0157,
     E0153,
-    E0251, // a named type or value has already been imported in this module
-    E0252, // a named type or value has already been imported in this module
     E0253, // not directly importable
     E0254, // import conflicts with imported crate in this module
-    E0255, // import conflicts with value in this module
-    E0256, // import conflicts with type in this module
-    E0257, // inherent implementations are only allowed on types defined in the current module
-    E0258, // import conflicts with existing submodule
+    E0257,
+    E0258,
     E0364, // item is private
     E0365  // item is private
 }

src/librustc_typeck/diagnostics.rs

@@ -64,6 +64,44 @@ impl Foo for Bar {
 ```
 "##,
 
+E0053: r##"
+In a trait method implementation, the function signature must match exactly.
+This error indicates a mutability mismatch between a trait method signature
+and the signature of the implementation.
+
+Here's an example where the mutability of the `self` parameter is wrong:
+
+```
+trait Foo { fn foo(&self); }
+
+struct Bar;
+
+impl Foo for Bar {
+    // error, the signature should be `fn foo(&self)` instead
+    fn foo(&mut self) { }
+}
+
+fn main() {}
+```
+
+Here's another example, this time for a non-`self` parameter:
+
+```
+trait Foo { fn foo(x: &mut bool) -> bool; }
+
+struct Bar;
+
+impl Foo for Bar {
+    // error, the type of `x` should be `&mut bool` instead
+    fn foo(x: &bool) -> bool { *x }
+}
+
+fn main() {}
+```
+
+
+"##,
+
 E0054: r##"
 It is not allowed to cast to a bool. If you are trying to cast a numeric type
 to a bool, you can compare it with zero instead:
@@ -483,6 +521,48 @@ The `Sized` trait is a special trait built-in to the compiler for types with a
 constant size known at compile-time. This trait is automatically implemented
 for types as needed by the compiler, and it is currently disallowed to
 explicitly implement it for a type.
+"##,
+
+E0368: r##"
+This error indicates that a binary assignment operator like `+=` or `^=` was
+applied to the wrong types.
+
+A couple examples of this are as follows:
+
+```
+let mut x: u16 = 5;
+x ^= true; // error, `^=` cannot be applied to types `u16` and `bool`
+x += ();   // error, `+=` cannot be applied to types `u16` and `()`
+```
+
+Another problem you might be facing is this: suppose you've overloaded the `+`
+operator for some type `Foo` by implementing the `std::ops::Add` trait for
+`Foo`, but you find that using `+=` does not work, as in this example:
+
+```
+use std::ops::Add;
+
+struct Foo(u32);
+
+impl Add for Foo {
+    type Output = Foo;
+
+    fn add(self, rhs: Foo) -> Foo {
+        Foo(self.0 + rhs.0)
+    }
+}
+
+fn main() {
+    let mut x: Foo = Foo(5);
+    x += Foo(7); // error, `+= cannot be applied to types `Foo` and `Foo`
+}
+```
+
+This is because the binary assignment operators currently do not work off of
+traits, so it is not possible to overload them. See [RFC 953] for a proposal
+to change this.
+
+[RFC 953]: https://github.com/rust-lang/rfcs/pull/953
 "##
 
 }
@@ -503,7 +583,6 @@ register_diagnostics! {
     E0040, // explicit use of destructor method
     E0044, // foreign items may not have type parameters
     E0045, // variadic function must have C calling convention
-    E0053,
     E0055, // method has an incompatible type for trait
     E0057, // method has an incompatible type for trait
     E0059,
@@ -629,7 +708,6 @@ register_diagnostics! {
     E0328, // cannot implement Unsize explicitly
     E0366, // dropck forbid specialization to concrete type or region
     E0367, // dropck forbid specialization to predicate not in struct/enum
-    E0368, // binary operation `<op>=` cannot be applied to types
     E0369, // binary operation `<op>` cannot be applied to types
     E0371, // impl Trait for Trait is illegal
     E0372, // impl Trait for Trait where Trait is not object safe