Auto merge of #23606 - quantheory:associated_const, r=nikomatsakis

Closes #17841.

The majority of the work should be done, e.g. trait and inherent impls, different forms of UFCS syntax, defaults, and cross-crate usage. It's probably enough to replace the constants in `f32`, `i8`, and so on, or close to good enough.

There is still some significant functionality missing from this commit:

 - ~~Associated consts can't be used in match patterns at all. This is simply because I haven't updated the relevant bits in the parser or `resolve`, but it's *probably* not hard to get working.~~
 - Since you can't select an impl for trait-associated consts until partway through type-checking, there are some problems with code that assumes that you can check constants earlier. Associated consts that are not in inherent impls cause ICEs if you try to use them in array sizes or match ranges. For similar reasons, `check_static_recursion` doesn't check them properly, so the stack goes ka-blooey if you use an associated constant that's recursively defined. That's a bit trickier to solve; I'm not entirely sure what the best approach is yet.
 - Dealing with consts associated with type parameters will raise some new issues (e.g. if you have a `T: Int` type parameter and want to use `<T>::ZERO`). See rust-lang/rfcs#865.
 - ~~Unused associated consts don't seem to trigger the `dead_code` lint when they should. Probably easy to fix.~~

Also, this is the first time I've been spelunking in rustc to such a large extent, so I've probably done some silly things in a couple of places.

9 files changed, 58 insertions, 6 deletions

diff --git a/src/test/parse-fail/brace-after-qualified-path-in-match.rs b/src/test/parse-fail/brace-after-qualified-path-in-match.rs
new file mode 100644
index 00000000000..66f462df05a
--- /dev/null
+++ b/src/test/parse-fail/brace-after-qualified-path-in-match.rs
@@ -0,0 +1,17 @@
+// Copyright 2015 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.
+
+fn foo() {
+    match x {
+        <T as Trait>::Type{key: value} => (),
+        //~^ ERROR unexpected `{` after qualified path
+        _ => (),
+    }
+}
diff --git a/src/test/parse-fail/issue-20711-2.rs b/src/test/parse-fail/issue-20711-2.rs
index 2c993b7654e..be6bd516d6f 100644
--- a/src/test/parse-fail/issue-20711-2.rs
+++ b/src/test/parse-fail/issue-20711-2.rs
@@ -16,6 +16,6 @@ impl Foo {
     fn foo() {}
 
     #[stable(feature = "rust1", since = "1.0.0")]
-} //~ ERROR expected one of `extern`, `fn`, `pub`, `type`, or `unsafe`, found `}`
+} //~ ERROR expected one of `const`, `extern`, `fn`, `pub`, `type`, or `unsafe`, found `}`
 
 fn main() {}
diff --git a/src/test/parse-fail/issue-20711.rs b/src/test/parse-fail/issue-20711.rs
index 8462bd8fd01..d1d8d3acf91 100644
--- a/src/test/parse-fail/issue-20711.rs
+++ b/src/test/parse-fail/issue-20711.rs
@@ -14,6 +14,6 @@ struct Foo;
 
 impl Foo {
     #[stable(feature = "rust1", since = "1.0.0")]
-} //~ ERROR expected one of `extern`, `fn`, `pub`, `type`, or `unsafe`, found `}`
+} //~ ERROR expected one of `const`, `extern`, `fn`, `pub`, `type`, or `unsafe`, found `}`
 
 fn main() {}
diff --git a/src/test/parse-fail/issue-21153.rs b/src/test/parse-fail/issue-21153.rs
index 44d979ba979..76a4687f544 100644
--- a/src/test/parse-fail/issue-21153.rs
+++ b/src/test/parse-fail/issue-21153.rs
@@ -11,5 +11,5 @@
 // compile-flags: -Z parse-only
 
 trait MyTrait<T>: Iterator {
-    Item = T; //~ ERROR expected one of `extern`, `fn`, `type`, or `unsafe`, found `Item`
+    Item = T; //~ ERROR expected one of `const`, `extern`, `fn`, `type`, or `unsafe`, found `Item`
 }
diff --git a/src/test/parse-fail/paren-after-qualified-path-in-match.rs b/src/test/parse-fail/paren-after-qualified-path-in-match.rs
new file mode 100644
index 00000000000..d06fd2bb4e7
--- /dev/null
+++ b/src/test/parse-fail/paren-after-qualified-path-in-match.rs
@@ -0,0 +1,17 @@
+// Copyright 2015 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.
+
+fn foo() {
+    match x {
+        <T as Trait>::Type(2) => (),
+        //~^ ERROR unexpected `(` after qualified path
+        _ => (),
+    }
+}
diff --git a/src/test/parse-fail/removed-syntax-static-fn.rs b/src/test/parse-fail/removed-syntax-static-fn.rs
index be1e89dd717..7b6caad86b6 100644
--- a/src/test/parse-fail/removed-syntax-static-fn.rs
+++ b/src/test/parse-fail/removed-syntax-static-fn.rs
@@ -14,5 +14,5 @@ struct S;
 
 impl S {
     static fn f() {}
-    //~^ ERROR expected one of `extern`, `fn`, `pub`, `type`, `unsafe`, or `}`, found `static`
 }
+//~^^ ERROR expected one of `const`, `extern`, `fn`, `pub`, `type`, `unsafe`, or `}`, found `static`
diff --git a/src/test/parse-fail/trait-pub-assoc-const.rs b/src/test/parse-fail/trait-pub-assoc-const.rs
new file mode 100644
index 00000000000..adce0d7bbf4
--- /dev/null
+++ b/src/test/parse-fail/trait-pub-assoc-const.rs
@@ -0,0 +1,16 @@
+// Copyright 2015 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.
+
+trait Foo {
+    pub const Foo: u32;
+     //~^ ERROR expected one of `const`, `extern`, `fn`, `type`, or `unsafe`, found `pub`
+}
+
+fn main() {}
diff --git a/src/test/parse-fail/trait-pub-assoc-ty.rs b/src/test/parse-fail/trait-pub-assoc-ty.rs
index 02d76234d4e..dab6c433aba 100644
--- a/src/test/parse-fail/trait-pub-assoc-ty.rs
+++ b/src/test/parse-fail/trait-pub-assoc-ty.rs
@@ -9,7 +9,8 @@
 // except according to those terms.
 
 trait Foo {
-    pub type Foo; //~ ERROR expected one of `extern`, `fn`, `type`, or `unsafe`, found `pub`
+    pub type Foo;
+    //~^ ERROR expected one of `const`, `extern`, `fn`, `type`, or `unsafe`, found `pub`
 }
 
 fn main() {}
diff --git a/src/test/parse-fail/trait-pub-method.rs b/src/test/parse-fail/trait-pub-method.rs
index e76802d2ea0..7cb9363830c 100644
--- a/src/test/parse-fail/trait-pub-method.rs
+++ b/src/test/parse-fail/trait-pub-method.rs
@@ -9,7 +9,8 @@
 // except according to those terms.
 
 trait Foo {
-    pub fn foo(); //~ ERROR expected one of `extern`, `fn`, `type`, or `unsafe`, found `pub`
+    pub fn foo();
+    //~^ ERROR expected one of `const`, `extern`, `fn`, `type`, or `unsafe`, found `pub`
 }
 
 fn main() {}