librustc: Make `Copy` opt-in.

This change makes the compiler no longer infer whether types (structures
and enumerations) implement the `Copy` trait (and thus are implicitly
copyable). Rather, you must implement `Copy` yourself via `impl Copy for
MyType {}`.

A new warning has been added, `missing_copy_implementations`, to warn
you if a non-generic public type has been added that could have
implemented `Copy` but didn't.

For convenience, you may *temporarily* opt out of this behavior by using
`#![feature(opt_out_copy)]`. Note though that this feature gate will never be
accepted and will be removed by the time that 1.0 is released, so you should
transition your code away from using it.

This breaks code like:

    #[deriving(Show)]
    struct Point2D {
        x: int,
        y: int,
    }

    fn main() {
        let mypoint = Point2D {
            x: 1,
            y: 1,
        };
        let otherpoint = mypoint;
        println!("{}{}", mypoint, otherpoint);
    }

Change this code to:

    #[deriving(Show)]
    struct Point2D {
        x: int,
        y: int,
    }

    impl Copy for Point2D {}

    fn main() {
        let mypoint = Point2D {
            x: 1,
            y: 1,
        };
        let otherpoint = mypoint;
        println!("{}{}", mypoint, otherpoint);
    }

This is the backwards-incompatible part of #13231.

Part of RFC #3.

[breaking-change]

2 files changed, 10 insertions, 0 deletions

diff --git a/src/libsyntax/ast_map/blocks.rs b/src/libsyntax/ast_map/blocks.rs
index 8db12fbd835..639a33a8063 100644
--- a/src/libsyntax/ast_map/blocks.rs
+++ b/src/libsyntax/ast_map/blocks.rs
@@ -43,6 +43,8 @@ use visit;
 /// To construct one, use the `Code::from_node` function.
 pub struct FnLikeNode<'a> { node: ast_map::Node<'a> }
 
+impl<'a> Copy for FnLikeNode<'a> {}
+
 /// MaybeFnLike wraps a method that indicates if an object
 /// corresponds to some FnLikeNode.
 pub trait MaybeFnLike { fn is_fn_like(&self) -> bool; }
@@ -85,6 +87,8 @@ pub enum Code<'a> {
     BlockCode(&'a Block),
 }
 
+impl<'a> Copy for Code<'a> {}
+
 impl<'a> Code<'a> {
     pub fn id(&self) -> ast::NodeId {
         match *self {
diff --git a/src/libsyntax/ast_map/mod.rs b/src/libsyntax/ast_map/mod.rs
index ce2fe6e7220..2c985f403f8 100644
--- a/src/libsyntax/ast_map/mod.rs
+++ b/src/libsyntax/ast_map/mod.rs
@@ -38,6 +38,8 @@ pub enum PathElem {
     PathName(Name)
 }
 
+impl Copy for PathElem {}
+
 impl PathElem {
     pub fn name(&self) -> Name {
         match *self {
@@ -120,6 +122,8 @@ pub enum Node<'ast> {
     NodeLifetime(&'ast Lifetime),
 }
 
+impl<'ast> Copy for Node<'ast> {}
+
 /// Represents an entry and its parent Node ID
 /// The odd layout is to bring down the total size.
 #[deriving(Show)]
@@ -147,6 +151,8 @@ enum MapEntry<'ast> {
     RootInlinedParent(&'ast InlinedParent)
 }
 
+impl<'ast> Copy for MapEntry<'ast> {}
+
 impl<'ast> Clone for MapEntry<'ast> {
     fn clone(&self) -> MapEntry<'ast> {
         *self