cherry-pick over the tests I wrote for the reachability code

For the most part, the current code performs similarly, although it
differs in some particulars. I'll be nice to have these tests for
judging future changes, as well.

Author: nikomatsakis

src/test/ui/reachable/README.md

@@ -0,0 +1,7 @@
+A variety of tests around reachability. These tests in general check
+two things:
+
+- that we get unreachable code warnings in reasonable locations;
+- that we permit coercions **into** `!` from expressions which
+  diverge, where an expression "diverges" if it must execute some
+  subexpression of type `!`, or it has type `!` itself.

src/test/ui/reachable/expr_add.rs

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+// Copyright 2016 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.
+
+#![feature(never_type)]
+#![allow(unused_variables)]
+#![deny(unreachable_code)]
+
+use std::ops;
+
+struct Foo;
+
+impl ops::Add<!> for Foo {
+    type Output = !;
+    fn add(self, rhs: !) -> ! {
+        unimplemented!()
+    }
+}
+
+fn main() {
+    let x = Foo + return;
+}

src/test/ui/reachable/expr_add.stderr

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+error: unreachable expression
+  --> $DIR/expr_add.rs:27:13
+   |
+27 |     let x = Foo + return;
+   |             ^^^^^^^^^^^^
+   |
+note: lint level defined here
+  --> $DIR/expr_add.rs:13:9
+   |
+13 | #![deny(unreachable_code)]
+   |         ^^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+

src/test/ui/reachable/expr_again.rs

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+// Copyright 2016 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.
+
+#![feature(box_syntax)]
+#![allow(unused_variables)]
+#![deny(unreachable_code)]
+
+fn main() {
+    let x = loop {
+        continue;
+        println!("hi");
+    };
+}

src/test/ui/reachable/expr_again.stderr

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+error: unreachable statement
+  --> $DIR/expr_again.rs:18:9
+   |
+18 |         println!("hi");
+   |         ^^^^^^^^^^^^^^^
+   |
+note: lint level defined here
+  --> $DIR/expr_again.rs:13:9
+   |
+13 | #![deny(unreachable_code)]
+   |         ^^^^^^^^^^^^^^^^
+   = note: this error originates in a macro outside of the current crate
+
+error: aborting due to previous error
+

src/test/ui/reachable/expr_andand.rs

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+// Copyright 2016 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.
+
+#![allow(unused_variables)]
+#![allow(dead_code)]
+#![deny(unreachable_code)]
+
+fn foo() {
+    // No error here.
+    let x = false && (return);
+    println!("I am not dead.");
+}
+
+fn main() { }

src/test/ui/reachable/expr_andand.stderr

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+// Copyright 2016 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.
+
+#![allow(unused_variables)]
+#![allow(unused_assignments)]
+#![allow(dead_code)]
+#![deny(unreachable_code)]
+#![feature(never_type)]
+#![feature(type_ascription)]
+
+fn a() {
+    // the `22` is unreachable:
+    let x: [usize; 2] = [return, 22];
+}
+
+fn b() {
+    // the `array is unreachable:
+    let x: [usize; 2] = [22, return];
+}
+
+fn main() { }

src/test/ui/reachable/expr_array.rs

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+error: unreachable expression
+  --> $DIR/expr_array.rs:20:34
+   |
+20 |     let x: [usize; 2] = [return, 22];
+   |                                  ^^
+   |
+note: lint level defined here
+  --> $DIR/expr_array.rs:14:9
+   |
+14 | #![deny(unreachable_code)]
+   |         ^^^^^^^^^^^^^^^^
+
+error: unreachable expression
+  --> $DIR/expr_array.rs:25:25
+   |
+25 |     let x: [usize; 2] = [22, return];
+   |                         ^^^^^^^^^^^^
+
+error: aborting due to 2 previous errors
+

src/test/ui/reachable/expr_array.stderr

@@ -0,0 +1,39 @@
+// Copyright 2016 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.
+
+#![allow(unused_variables)]
+#![allow(unused_assignments)]
+#![allow(dead_code)]
+#![deny(unreachable_code)]
+#![feature(never_type)]
+
+fn foo() {
+    // No error here.
+    let x;
+    x = return;
+}
+
+fn bar() {
+    use std::ptr;
+    let p: *mut ! = ptr::null_mut::<!>();
+    unsafe {
+        // Here we consider the `return` unreachable because
+        // "evaluating" the `*p` has type `!`. This is somewhat
+        // dubious, I suppose.
+        *p = return;
+    }
+}
+
+fn baz() {
+    let mut i = 0;
+    *{return; &mut i} = 22;
+}
+
+fn main() { }