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]

1 files changed, 4 insertions, 0 deletions

diff --git a/src/liblog/lib.rs b/src/liblog/lib.rs
index 5642ec91ba3..8b79078eac6 100644
--- a/src/liblog/lib.rs
+++ b/src/liblog/lib.rs
@@ -234,6 +234,8 @@ struct DefaultLogger {
 #[deriving(PartialEq, PartialOrd)]
 pub struct LogLevel(pub u32);
 
+impl Copy for LogLevel {}
+
 impl fmt::Show for LogLevel {
     fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
         let LogLevel(level) = *self;
@@ -344,6 +346,8 @@ pub struct LogLocation {
     pub line: uint,
 }
 
+impl Copy for LogLocation {}
+
 /// Tests whether a given module's name is enabled for a particular level of
 /// logging. This is the second layer of defense about determining whether a
 /// module's log statement should be emitted or not.