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, 8 insertions, 0 deletions

diff --git a/src/librustc/lint/mod.rs b/src/librustc/lint/mod.rs
index d6b83752cc5..4b4ba2ab94c 100644
--- a/src/librustc/lint/mod.rs
+++ b/src/librustc/lint/mod.rs
@@ -64,6 +64,8 @@ pub struct Lint {
     pub desc: &'static str,
 }
 
+impl Copy for Lint {}
+
 impl Lint {
     /// Get the lint's name, with ASCII letters converted to lowercase.
     pub fn name_lower(&self) -> String {
@@ -179,6 +181,8 @@ pub struct LintId {
     lint: &'static Lint,
 }
 
+impl Copy for LintId {}
+
 impl PartialEq for LintId {
     fn eq(&self, other: &LintId) -> bool {
         (self.lint as *const Lint) == (other.lint as *const Lint)
@@ -214,6 +218,8 @@ pub enum Level {
     Allow, Warn, Deny, Forbid
 }
 
+impl Copy for Level {}
+
 impl Level {
     /// Convert a level to a lower-case string.
     pub fn as_str(self) -> &'static str {
@@ -251,6 +257,8 @@ pub enum LintSource {
     CommandLine,
 }
 
+impl Copy for LintSource {}
+
 pub type LevelSource = (Level, LintSource);
 
 pub mod builtin;