community/rust: fix build error with rustdoc

Building rust fails with the error:

error[E0523]: found two different crates with name `bitflags` that are not
distinguished by differing `-C metadata`. This will result in symbol conflicts
between the two.
  --> src/librustdoc/html/markdown.rs:36:5
     |
     36 | use pulldown_cmark::{html, CowStr, Event, Options, Parser, Tag};
        |     ^^^^^^^^^^^^^^

Upstream fixed it with the included patch.

1 files changed, 35 insertions, 0 deletions

diff --git a/community/rust/rustdoc-fix-conflicting-symbols.patch b/community/rust/rustdoc-fix-conflicting-symbols.patch
new file mode 100644
index 0000000000..b399b16bf9
--- /dev/null
+++ b/community/rust/rustdoc-fix-conflicting-symbols.patch
@@ -0,0 +1,35 @@
+From 73369f32621f6a844a80a8513ae3ded901e4a406 Mon Sep 17 00:00:00 2001
+From: Mark Rousskov <mark.simulacrum@gmail.com>
+Date: Tue, 5 Nov 2019 11:16:46 -0500
+Subject: [PATCH] Hopefully fix rustdoc build
+
+It's super unclear why this broke when we switched to beta but not
+previously -- but at least it's hopefully fixed now.
+---
+ src/bootstrap/builder.rs | 13 ++++++++++++-
+ 1 file changed, 12 insertions(+), 1 deletion(-)
+
+diff --git a/src/bootstrap/builder.rs b/src/bootstrap/builder.rs
+index 2748903f2d47..2edcef203ad2 100644
+--- a/src/bootstrap/builder.rs
++++ b/src/bootstrap/builder.rs
+@@ -886,7 +886,18 @@ impl<'a> Builder<'a> {
+         // things still build right, please do!
+         match mode {
+             Mode::Std => metadata.push_str("std"),
+-            _ => {},
++            // When we're building rustc tools, they're built with a search path
++            // that contains things built during the rustc build. For example,
++            // bitflags is built during the rustc build, and is a dependency of
++            // rustdoc as well. We're building rustdoc in a different target
++            // directory, though, which means that Cargo will rebuild the
++            // dependency. When we go on to build rustdoc, we'll look for
++            // bitflags, and find two different copies: one built during the
++            // rustc step and one that we just built. This isn't always a
++            // problem, somehow -- not really clear why -- but we know that this
++            // fixes things.
++            Mode::ToolRustc => metadata.push_str("tool-rustc"),
++            _ => {}
+         }
+         cargo.env("__CARGO_DEFAULT_LIB_METADATA", &metadata);
+