Rollup merge of rust-lang#25398 - nham:E0066_E0069, r=huonw

Adds explanations for E0053, E0066, E0069, E0251, E0252, E0255, E0256, E0368.

cc rust-lang#24407

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 bar::foo; // error, do `use bar::foo as baz` instead
+
+fn foo() {}
+
+mod bar {
+     pub fn foo() {}
+}
+
+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
+
+type Bar = u32;
+
+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/check/mod.rs

@@ -3082,8 +3082,8 @@ fn check_expr_with_unifier<'a, 'tcx, F>(fcx: &FnCtxt<'a, 'tcx>,
           let mut checked = false;
           opt_place.as_ref().map(|place| match place.node {
               ast::ExprPath(None, ref path) => {
-                  // FIXME(pcwalton): For now we hardcode the two permissible
-                  // places: the exchange heap and the managed heap.
+                  // FIXME(pcwalton): For now we hardcode the only permissible
+                  // place: the exchange heap.
                   let definition = lookup_full_def(tcx, path.span, place.id);
                   let def_id = definition.def_id();
                   let referent_ty = fcx.expr_ty(&**subexpr);
@@ -3097,7 +3097,7 @@ fn check_expr_with_unifier<'a, 'tcx, F>(fcx: &FnCtxt<'a, 'tcx>,
 
           if !checked {
               span_err!(tcx.sess, expr.span, E0066,
-                  "only the managed heap and exchange heap are currently supported");
+                  "only the exchange heap is currently supported");
               fcx.write_ty(id, tcx.types.err);
           }
       }
@@ -3317,7 +3317,8 @@ fn check_expr_with_unifier<'a, 'tcx, F>(fcx: &FnCtxt<'a, 'tcx>,
                         if let Err(_) = fcx.mk_eqty(false, infer::Misc(expr.span),
                                                     result_type, ty::mk_nil(fcx.tcx())) {
                             span_err!(tcx.sess, expr.span, E0069,
-                                "`return;` in function returning non-nil");
+                                "`return;` in a function whose return type is \
+                                 not `()`");
                         },
                     Some(ref e) => {
                         check_expr_coercable_to_type(fcx, &**e, result_type);

src/librustc_typeck/diagnostics.rs

@@ -64,6 +64,43 @@ impl Foo for Bar {
 ```
 "##,
 
+E0053: r##"
+For any given method of a trait, the mutabilities of the parameters must match
+between the trait definition and 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:
@@ -91,6 +128,16 @@ enum variant, one of the fields was not provided. Each field should be specified
 exactly once.
 "##,
 
+E0066: r##"
+Box placement expressions (like C++'s "placement new") do not yet support any
+place expression except the exchange heap (i.e. `std::boxed::HEAP`).
+Furthermore, the syntax is changing to use `in` instead of `box`. See [RFC 470]
+and [RFC 809] for more details.
+
+[RFC 470]: https://github.com/rust-lang/rfcs/pull/470
+[RFC 809]: https://github.com/rust-lang/rfcs/pull/809
+"##,
+
 E0067: r##"
 The left-hand side of an assignment operator must be an lvalue expression. An
 lvalue expression represents a memory location and includes item paths (ie,
@@ -108,6 +155,21 @@ LinkedList::new() += 1;
 ```
 "##,
 
+E0069: r##"
+The compiler found a function whose body contains a `return;` statement but
+whose return type is not `()`. An example of this is:
+
+```
+// error
+fn foo() -> u8 {
+    return;
+}
+```
+
+Since `return;` is just like `return ();`, there is a mismatch between the
+function's return type and the value being returned.
+"##,
+
 E0081: r##"
 Enum discriminants are used to differentiate enum variants stored in memory.
 This error indicates that the same value was used for two or more variants,
@@ -458,6 +520,48 @@ 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
 "##
 
 }
@@ -478,15 +582,12 @@ 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,
     E0060,
     E0061,
-    E0066,
     E0068,
-    E0069,
     E0070,
     E0071,
     E0072,
@@ -606,7 +707,6 @@ 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

src/test/compile-fail/issue-14084.rs

@@ -12,5 +12,5 @@
 
 fn main() {
     box ( () ) 0;
-    //~^ ERROR: only the managed heap and exchange heap are currently supported
+    //~^ ERROR: only the exchange heap is currently supported
 }

src/test/compile-fail/ret-non-nil.rs

@@ -8,7 +8,7 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-// error-pattern: `return;` in function returning non-nil
+// error-pattern: `return;` in a function whose return type is not `()`
 
 fn f() { return; }