Add E0327, E0395 and E0396 error explanations

src/librustc/diagnostics.rs

@@ -984,6 +984,57 @@ From [RFC 246]:
 [RFC 246]: https://github.com/rust-lang/rfcs/pull/246
 "##,
 
+E0395: r##"
+The value assigned to a constant expression must be known at compile time,
+which is not the case when comparing raw pointers. Erroneous code example:
+
+```
+static foo: i32 = 42;
+static bar: i32 = 43;
+
+static baz: bool = { (&foo as *const i32) == (&bar as *const i32) };
+// error: raw pointers cannot be compared in statics!
+```
+
+Please check that the result of the comparison can be determined at compile time
+or isn't assigned to a constant expression. Example:
+
+```
+static foo: i32 = 42;
+static bar: i32 = 43;
+
+let baz: bool = { (&foo as *const i32) == (&bar as *const i32) };
+// baz isn't a constant expression so it's ok
+```
+"##,
+
+E0396: r##"
+The value assigned to a constant expression must be known at compile time,
+which is not the case when dereferencing raw pointers. Erroneous code
+example:
+
+```
+const foo: i32 = 42;
+const baz: *const i32 = (&foo as *const i32);
+
+const deref: i32 = *baz;
+// error: raw pointers cannot be dereferenced in constants
+```
+
+To fix this error, please do not assign this value to a constant expression.
+Example:
+
+```
+const foo: i32 = 42;
+const baz: *const i32 = (&foo as *const i32);
+
+unsafe { let deref: i32 = *baz; }
+// baz isn't a constant expression so it's ok
+```
+
+You'll also note that this assignment must be done in an unsafe block!
+"##,
+
 E0397: r##"
 It is not allowed for a mutable static to allocate or have destructors. For
 example:
@@ -1039,7 +1090,5 @@ register_diagnostics! {
     E0314, // closure outlives stack frame
     E0315, // cannot invoke closure outside of its lifetime
     E0316, // nested quantification of lifetimes
-    E0370, // discriminant overflow
-    E0395, // pointer comparison in const-expr
-    E0396  // pointer dereference in const-expr
+    E0370  // discriminant overflow
 }

src/librustc_typeck/diagnostics.rs

@@ -1457,6 +1457,42 @@ impl Foo for Bar {
 ```
 "##,
 
+E0327: r##"
+You cannot use associated items other than constant items as patterns. This
+includes method items. Example of erroneous code:
+
+```
+enum B {}
+
+impl B {
+    fn bb() -> i32 { 0 }
+}
+
+fn main() {
+    match 0 {
+        B::bb => {} // error: associated items in match patterns must
+                    // be constants
+    }
+}
+```
+
+Please check that you're not using a method as a pattern. Example:
+
+```
+enum B {
+    ba,
+    bb
+}
+
+fn main() {
+    match B::ba {
+        B::bb => {} // ok!
+        _ => {}
+    }
+}
+```
+"##,
+
 E0368: r##"
 This error indicates that a binary assignment operator like `+=` or `^=` was
 applied to the wrong types. For example:
@@ -1639,7 +1675,6 @@ register_diagnostics! {
     E0323, // implemented an associated const when another trait item expected
     E0324, // implemented a method when another trait item expected
     E0325, // implemented an associated type when another trait item expected
-    E0327, // referred to method instead of constant in match pattern
     E0328, // cannot implement Unsize explicitly
     E0329, // associated const depends on type parameter or Self.
     E0366, // dropck forbid specialization to concrete type or region