From c033c6e47c1a84e2902c75a1b5ec161362f34f18 Mon Sep 17 00:00:00 2001
From: Phlosioneer <mattmdrr2@gmail.com>
Date: Sun, 11 Mar 2018 10:03:23 -0400
Subject: [PATCH] Fix hygene issue when deriving Debug

The code for several of the core traits doesn't use hygenic macros.
This isn't a problem, except for the Debug trait, which is the only
one that uses a variable, named "builder".

Variables can't share names with unit structs, so attempting to
[derive(Debug)] on any type while a unit struct with the name
"builder" was in scope would result in an error.

This commit just changes the name of the variable to
"__debug_trait_builder", because I couldn't figure out how to get a
list of all unit structs in-scope from within the derive expansion
function. If someone wants to have a unit struct with
the exact name "__debug_trait_builder", they'll just have to do it
without a [derive(Debug)].
---
 src/libsyntax_ext/deriving/debug.rs |  2 +-
 src/test/run-pass/issue-42453.rs    | 17 +++++++++++++++++
 2 files changed, 18 insertions(+), 1 deletion(-)
 create mode 100644 src/test/run-pass/issue-42453.rs

diff --git a/src/libsyntax_ext/deriving/debug.rs b/src/libsyntax_ext/deriving/debug.rs
index 82fc09fca69a..7b23de582a79 100644
--- a/src/libsyntax_ext/deriving/debug.rs
+++ b/src/libsyntax_ext/deriving/debug.rs
@@ -70,7 +70,7 @@ fn show_substructure(cx: &mut ExtCtxt, span: Span, substr: &Substructure) -> P<E
     // We want to make sure we have the ctxt set so that we can use unstable methods
     let span = span.with_ctxt(cx.backtrace());
     let name = cx.expr_lit(span, ast::LitKind::Str(ident.name, ast::StrStyle::Cooked));
-    let builder = Ident::from_str("builder");
+    let builder = Ident::from_str("__debug_trait_builder");
     let builder_expr = cx.expr_ident(span, builder.clone());
 
     let fmt = substr.nonself_args[0].clone();
diff --git a/src/test/run-pass/issue-42453.rs b/src/test/run-pass/issue-42453.rs
new file mode 100644
index 000000000000..dfe3a21961e3
--- /dev/null
+++ b/src/test/run-pass/issue-42453.rs
@@ -0,0 +1,17 @@
+// 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.
+
+#[derive(Debug)]
+struct builder;
+
+fn main() {
+
+}
+