auto merge of #16053 : pcwalton/rust/at-pattern-bindings, r=pnkfelix

This is an alternative to upgrading the way rvalues are handled in the
borrow check. Making rvalues handled more like lvalues in the borrow
check caused numerous problems related to double mutable borrows and
rvalue scopes. Rather than come up with more borrow check rules to try
to solve these problems, I decided to just forbid pattern bindings after
`@`. This affected fewer than 10 lines of code in the compiler and
libraries.

This breaks code like:

    match x {
        y @ z => { ... }
    }

    match a {
        b @ Some(c) => { ... }
    }

Change this code to use nested `match` or `let` expressions. For
example:

    match x {
        y => {
            let z = y;
            ...
        }
    }

    match a {
        Some(c) => {
            let b = Some(c);
            ...
        }
    }

Closes #14587.

[breaking-change]

May need discussion at the meeting, but r? @nikomatsakis anyway

5 files changed, 16 insertions, 97 deletions

diff --git a/src/test/compile-fail/bind-by-move-neither-can-live-while-the-other-survives-1.rs b/src/test/compile-fail/bind-by-move-neither-can-live-while-the-other-survives-1.rs
deleted file mode 100644
index a60348c4a3a..00000000000
--- a/src/test/compile-fail/bind-by-move-neither-can-live-while-the-other-survives-1.rs
+++ /dev/null
@@ -1,25 +0,0 @@
-// Copyright 2012 The Rust Project Developers. See the COPYRIGHT
-// file at the top-level directory of this distribution and at
-// http://rust-lang.org/COPYRIGHT.
-//
-// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
-// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
-// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
-// option. This file may not be copied, modified, or distributed
-// except according to those terms.
-
-struct X { x: () }
-
-impl Drop for X {
-    fn drop(&mut self) {
-        println!("destructor runs");
-    }
-}
-
-fn main() {
-    let x = Some(X { x: () });
-    match x {
-        Some(ref _y @ _z) => { }, //~ ERROR cannot bind by-move and by-ref in the same pattern
-        None => fail!()
-    }
-}
diff --git a/src/test/compile-fail/bind-by-move-no-sub-bindings-fun-args.rs b/src/test/compile-fail/bind-by-move-no-sub-bindings-fun-args.rs
deleted file mode 100644
index 0e5b659f125..00000000000
--- a/src/test/compile-fail/bind-by-move-no-sub-bindings-fun-args.rs
+++ /dev/null
@@ -1,21 +0,0 @@
-// Copyright 2012 The Rust Project Developers. See the COPYRIGHT
-// file at the top-level directory of this distribution and at
-// http://rust-lang.org/COPYRIGHT.
-//
-// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
-// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
-// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
-// option. This file may not be copied, modified, or distributed
-// except according to those terms.
-
-// Issue #12534.
-
-struct A(Box<uint>);
-
-fn f(a @ A(u): A) -> Box<uint> {    //~ ERROR cannot bind by-move with sub-bindings
-    drop(a);
-    u
-}
-
-fn main() {}
-
diff --git a/src/test/compile-fail/bind-by-move-no-sub-bindings.rs b/src/test/compile-fail/pattern-bindings-after-at.rs
index 34b83af1d2e..9cd2d0d28b1 100644
--- a/src/test/compile-fail/bind-by-move-no-sub-bindings.rs
+++ b/src/test/compile-fail/pattern-bindings-after-at.rs
@@ -1,4 +1,4 @@
-// Copyright 2012 The Rust Project Developers. See the COPYRIGHT
+// Copyright 2014 The Rust Project Developers. See the COPYRIGHT
 // file at the top-level directory of this distribution and at
 // http://rust-lang.org/COPYRIGHT.
 //
@@ -8,18 +8,19 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-struct X { x: (), }
-
-impl Drop for X {
-    fn drop(&mut self) {
-        println!("destructor runs");
-    }
+enum Option<T> {
+    None,
+    Some(T),
 }
 
 fn main() {
-    let x = Some(X { x: () });
-    match x {
-        Some(_y @ ref _z) => { }, //~ ERROR cannot bind by-move with sub-bindings
-        None => fail!()
+    match &mut Some(1i) {
+        ref mut z @ &Some(ref a) => {
+        //~^ ERROR pattern bindings are not allowed after an `@`
+            **z = None;
+            println!("{}", *a);
+        }
+        _ => ()
     }
 }
+
diff --git a/src/test/run-pass/match-pattern-bindings.rs b/src/test/run-pass/match-pattern-bindings.rs
index 61e905e5b17..e6ce94ec5d4 100644
--- a/src/test/run-pass/match-pattern-bindings.rs
+++ b/src/test/run-pass/match-pattern-bindings.rs
@@ -15,24 +15,15 @@ fn main() {
         ref b @ None => b
     }, &Some(1i));
     assert_eq!(match value {
-        ref a @ ref _c @ Some(_) => a,
-        ref b @ None => b
-    }, &Some(1i));
-    assert_eq!(match value {
-        _a @ ref c @ Some(_) => c,
+        ref c @ Some(_) => c,
         ref b @ None => b
     }, &Some(1i));
     assert_eq!(match "foobarbaz" {
-        _a @ b @ _ => b
+        b @ _ => b
     }, "foobarbaz");
-
-    let a @ b @ c = "foobarbaz";
+    let a @ _ = "foobarbaz";
     assert_eq!(a, "foobarbaz");
-    assert_eq!(b, "foobarbaz");
-    assert_eq!(c, "foobarbaz");
     let value = Some(true);
-    let ref a @ b @ ref c = value;
+    let ref a @ _ = value;
     assert_eq!(a, &Some(true));
-    assert_eq!(b, Some(true));
-    assert_eq!(c, &Some(true));
 }
diff --git a/src/test/run-pass/nested-patterns.rs b/src/test/run-pass/nested-patterns.rs
deleted file mode 100644
index 08816d345b4..00000000000
--- a/src/test/run-pass/nested-patterns.rs
+++ /dev/null
@@ -1,27 +0,0 @@
-// Copyright 2012 The Rust Project Developers. See the COPYRIGHT
-// file at the top-level directory of this distribution and at
-// http://rust-lang.org/COPYRIGHT.
-//
-// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
-// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
-// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
-// option. This file may not be copied, modified, or distributed
-// except according to those terms.
-
-struct A { a: int, b: int }
-struct B { a: int, b: C }
-struct D { a: int, d: C }
-struct C { c: int }
-
-pub fn main() {
-    match (A {a: 10, b: 20}) {
-        x@A {a, b: 20} => { assert!(x.a == 10); assert!(a == 10); }
-        A {b: _b, ..} => { fail!(); }
-    }
-    let mut x@B {b, ..} = B {a: 10, b: C {c: 20}};
-    x.b.c = 30;
-    assert_eq!(b.c, 20);
-    let mut y@D {d, ..} = D {a: 10, d: C {c: 20}};
-    y.d.c = 30;
-    assert_eq!(d.c, 20);
-}