auto merge of #17853 : alexcrichton/rust/issue-17718, r=pcwalton

This change is an implementation of [RFC 69][rfc] which adds a third kind of
global to the language, `const`. This global is most similar to what the old
`static` was, and if you're unsure about what to use then you should use a
`const`.

The semantics of these three kinds of globals are:

* A `const` does not represent a memory location, but only a value. Constants
  are translated as rvalues, which means that their values are directly inlined
  at usage location (similar to a #define in C/C++). Constant values are, well,
  constant, and can not be modified. Any "modification" is actually a
  modification to a local value on the stack rather than the actual constant
  itself.

  Almost all values are allowed inside constants, whether they have interior
  mutability or not. There are a few minor restrictions listed in the RFC, but
  they should in general not come up too often.

* A `static` now always represents a memory location (unconditionally). Any
  references to the same `static` are actually a reference to the same memory
  location. Only values whose types ascribe to `Sync` are allowed in a `static`.
  This restriction is in place because many threads may access a `static`
  concurrently. Lifting this restriction (and allowing unsafe access) is a
  future extension not implemented at this time.

* A `static mut` continues to always represent a memory location. All references
  to a `static mut` continue to be `unsafe`.

This is a large breaking change, and many programs will need to be updated
accordingly. A summary of the breaking changes is:

* Statics may no longer be used in patterns. Statics now always represent a
  memory location, which can sometimes be modified. To fix code, repurpose the
  matched-on-`static` to a `const`.

      static FOO: uint = 4;
      match n {
          FOO => { /* ... */ }
          _ => { /* ... */ }
      }

  change this code to:

      const FOO: uint = 4;
      match n {
          FOO => { /* ... */ }
          _ => { /* ... */ }
      }

* Statics may no longer refer to other statics by value. Due to statics being
  able to change at runtime, allowing them to reference one another could
  possibly lead to confusing semantics. If you are in this situation, use a
  constant initializer instead. Note, however, that statics may reference other
  statics by address, however.

* Statics may no longer be used in constant expressions, such as array lengths.
  This is due to the same restrictions as listed above. Use a `const` instead.

[breaking-change]
Closes #17718 

[rfc]: rust-lang/rfcs#246

Author: bors

lib.rs

@@ -155,23 +155,23 @@ pub mod color {
     /// Number for a terminal color
     pub type Color = u16;
 
-    pub static BLACK:   Color = 0u16;
-    pub static RED:     Color = 1u16;
-    pub static GREEN:   Color = 2u16;
-    pub static YELLOW:  Color = 3u16;
-    pub static BLUE:    Color = 4u16;
-    pub static MAGENTA: Color = 5u16;
-    pub static CYAN:    Color = 6u16;
-    pub static WHITE:   Color = 7u16;
-
-    pub static BRIGHT_BLACK:   Color = 8u16;
-    pub static BRIGHT_RED:     Color = 9u16;
-    pub static BRIGHT_GREEN:   Color = 10u16;
-    pub static BRIGHT_YELLOW:  Color = 11u16;
-    pub static BRIGHT_BLUE:    Color = 12u16;
-    pub static BRIGHT_MAGENTA: Color = 13u16;
-    pub static BRIGHT_CYAN:    Color = 14u16;
-    pub static BRIGHT_WHITE:   Color = 15u16;
+    pub const BLACK:   Color = 0u16;
+    pub const RED:     Color = 1u16;
+    pub const GREEN:   Color = 2u16;
+    pub const YELLOW:  Color = 3u16;
+    pub const BLUE:    Color = 4u16;
+    pub const MAGENTA: Color = 5u16;
+    pub const CYAN:    Color = 6u16;
+    pub const WHITE:   Color = 7u16;
+
+    pub const BRIGHT_BLACK:   Color = 8u16;
+    pub const BRIGHT_RED:     Color = 9u16;
+    pub const BRIGHT_GREEN:   Color = 10u16;
+    pub const BRIGHT_YELLOW:  Color = 11u16;
+    pub const BRIGHT_BLUE:    Color = 12u16;
+    pub const BRIGHT_MAGENTA: Color = 13u16;
+    pub const BRIGHT_CYAN:    Color = 14u16;
+    pub const BRIGHT_WHITE:   Color = 15u16;
 }
 
 /// Terminal attributes