Auto merge of #21521 - defuz:interval-with-path, r=pnkfelix

Fixing #21475. Right now this code can not be parsed:

```rust
use m::{START, END};

fn main() {
    match 42u32 {
        m::START...m::END => {}, // error: expected one of `::`, `=>`, or `|`, found `...`
        _  => {},
    }
}

mod m {
  pub const START: u32 = 4;
  pub const END:   u32 = 14;
}
```

I fixed the parser and added test for this case, but now there are still problems with mixing literals and paths in interval:

```rust
    match 42u32 {
        0u32...m::END => {},       // mismatched types in range [E0031]
        m::START...59u32 => {},    // mismatched types in range [E0031]
        _  => {},
    }
}
```

I'll try fix this problem and need review.

Author: bors

src/libsyntax/parse/parser.rs

@@ -3538,6 +3538,19 @@ impl<'a> Parser<'a> {
                         self.bump();
                         pat = PatStruct(enum_path, fields, etc);
                     }
+                    token::DotDotDot => {
+                        let hi = self.last_span.hi;
+                        let start = self.mk_expr(lo, hi, ExprPath(None, enum_path));
+                        self.eat(&token::DotDotDot);
+                        let end = if self.token.is_ident() || self.token.is_path() {
+                            let path = self.parse_path(LifetimeAndTypesWithColons);
+                            let hi = self.span.hi;
+                            self.mk_expr(lo, hi, ExprPath(None, path))
+                        } else {
+                            self.parse_literal_maybe_minus()
+                        };
+                        pat = PatRange(start, end);
+                    }
                     _ => {
                         let mut args: Vec<P<Pat>> = Vec::new();
                         match self.token {

src/test/run-pass/issue-21475.rs

@@ -0,0 +1,25 @@
+// 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.
+//
+// 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.
+
+use m::{START, END};
+
+fn main() {
+    match 42u32 {
+        m::START...m::END => {},
+        0u32...m::END => {},
+        m::START...59u32 => {},
+        _  => {},
+    }
+}
+
+mod m {
+  pub const START: u32 = 4;
+  pub const END:   u32 = 14;
+}