auto merge of #16419 : huonw/rust/pretty-expanded-hygiene, r=pnkfelix

Different Identifiers and Names can have identical textual representations, but different internal representations, due to the macro hygiene machinery (syntax contexts and gensyms). This provides a way to see these internals by compiling with `--pretty expanded,hygiene`.

This is useful for debugging & hacking on macros (e.g. diagnosing https://github.com/rust-lang/rust/issues/15750/https://github.com/rust-lang/rust/issues/15962 likely would've been faster with this functionality).

E.g. 

```rust
#![feature(macro_rules)]
// minimal junk
#![no_std]

macro_rules! foo {
    ($x: ident) => { y + $x }
}

fn bar() {
    foo!(x)
}
```
```rust
#![feature(macro_rules)]
// minimal junk
#![no_std]


fn bar /* 61#0 */() { y /* 60#2 */ + x /* 58#3 */ }
```

src/test/run-make/pretty-expanded-hygiene/Makefile

@@ -0,0 +1,20 @@
+-include ../tools.mk
+
+REPLACEMENT := s/[0-9][0-9]*\#[0-9][0-9]*/$(shell date)/g
+
+all:
+	$(RUSTC) -o $(TMPDIR)/input.out --pretty expanded,hygiene input.rs
+
+	# the name/ctxt numbers are very internals-dependent and thus
+	# change relatively frequently, and testing for their exact values
+	# them will fail annoyingly, so we just check their positions
+	# (using a non-constant replacement like this will make it less
+	# likely the compiler matches whatever other dummy value we
+	# choose).
+	#
+	# (These need to be out-of-place because OSX/BSD & GNU sed
+	# differ.)
+	sed "$(REPLACEMENT)" input.pp.rs > $(TMPDIR)/input.pp.rs
+	sed "$(REPLACEMENT)" $(TMPDIR)/input.out > $(TMPDIR)/input.out.replaced
+
+	diff -u $(TMPDIR)/input.out.replaced $(TMPDIR)/input.pp.rs

src/test/run-make/pretty-expanded-hygiene/input.pp.rs

@@ -0,0 +1,16 @@
+// 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.
+
+#![feature(macro_rules)]
+// minimal junk
+#![no_std]
+
+
+fn bar /* 62#0 */() { let x /* 59#2 */ = 1; y /* 61#4 */ + x /* 59#5 */ }

src/test/run-make/pretty-expanded-hygiene/input.rs

@@ -0,0 +1,22 @@
+// 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.
+
+#![feature(macro_rules)]
+// minimal junk
+#![no_std]
+
+macro_rules! foo {
+    ($x: ident) => { y + $x }
+}
+
+fn bar() {
+    let x = 1;
+    foo!(x)
+}