Add setup-toolchain.sh script to configure the master version of rustc,

and update CONTRIBUTING.md accordingly.

Author: jsgf

.travis.yml

@@ -90,10 +90,7 @@ matrix:
 script:
   - |
       rm rust-toolchain
-      cargo install rustup-toolchain-install-master --debug || echo "rustup-toolchain-install-master already installed"
-      RUSTC_HASH=$(git ls-remote https://github.com/rust-lang/rust.git master | awk '{print $1}')
-      travis_retry rustup-toolchain-install-master -f -n master $RUSTC_HASH
-      rustup default master
+      ./setup-toolchain.sh
       export LD_LIBRARY_PATH=$(rustc --print sysroot)/lib
   - |
     if [ -z ${INTEGRATION} ]; then

CONTRIBUTING.md

@@ -73,6 +73,10 @@ an AST expression). `match_def_path()` in Clippy's `utils` module can also be us
 
 ## Writing code
 
+Clippy depends on the current git master version of rustc, which can change rapidly. Make sure you're
+working near rust-clippy's master, and use the `setup-toolchain.sh` script to configure the appropriate
+toolchain for this directory.
+
 [Llogiq's blog post on lints](https://llogiq.github.io/2015/06/04/workflows.html) is a nice primer
 to lint-writing, though it does get into advanced stuff. Most lints consist of an implementation of
 `LintPass` with one or more of its default methods overridden. See the existing lints for examples

setup-toolchain.sh

@@ -0,0 +1,9 @@
+#!/bin/bash
+# Set up the appropriate rustc toolchain
+
+cd $(dirname $0)
+
+cargo install rustup-toolchain-install-master --debug || echo "rustup-toolchain-install-master already installed"
+RUSTC_HASH=$(git ls-remote https://github.com/rust-lang/rust.git master | awk '{print $1}')
+rustup-toolchain-install-master -f -n master $RUSTC_HASH
+rustup override set master