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, 13 insertions, 0 deletions

diff --git a/src/libcore/fmt/rt.rs b/src/libcore/fmt/rt.rs
index 145e78dc668..748bd0bc4bd 100644
--- a/src/libcore/fmt/rt.rs
+++ b/src/libcore/fmt/rt.rs
@@ -20,6 +20,7 @@ pub use self::Alignment::*;
 pub use self::Count::*;
 pub use self::Position::*;
 pub use self::Flag::*;
+use kinds::Copy;
 
 #[doc(hidden)]
 pub struct Argument<'a> {
@@ -27,6 +28,8 @@ pub struct Argument<'a> {
     pub format: FormatSpec,
 }
 
+impl<'a> Copy for Argument<'a> {}
+
 #[doc(hidden)]
 pub struct FormatSpec {
     pub fill: char,
@@ -36,6 +39,8 @@ pub struct FormatSpec {
     pub width: Count,
 }
 
+impl Copy for FormatSpec {}
+
 /// Possible alignments that can be requested as part of a formatting directive.
 #[deriving(PartialEq)]
 pub enum Alignment {
@@ -49,16 +54,22 @@ pub enum Alignment {
     AlignUnknown,
 }
 
+impl Copy for Alignment {}
+
 #[doc(hidden)]
 pub enum Count {
     CountIs(uint), CountIsParam(uint), CountIsNextParam, CountImplied,
 }
 
+impl Copy for Count {}
+
 #[doc(hidden)]
 pub enum Position {
     ArgumentNext, ArgumentIs(uint)
 }
 
+impl Copy for Position {}
+
 /// Flags which can be passed to formatting via a directive.
 ///
 /// These flags are discovered through the `flags` field of the `Formatter`
@@ -78,3 +89,5 @@ pub enum Flag {
     /// being aware of the sign to be printed.
     FlagSignAwareZeroPad,
 }
+
+impl Copy for Flag {}