Run cargo clean on CI when the version of rustc changes

.travis.yml

@@ -11,7 +11,8 @@ cache:
   cargo: true
   directories:
     - $HOME/.npm
-  timeout: 600
+  # Set timeout to 6 minutes (double the default of 3 minutes)
+  timeout: 360
 
 env:
   global:
@@ -26,6 +27,7 @@ env:
     - PERCY_PROJECT=crates-io/crates.io
 
 install:
+  - script/cargo-clean-on-new-rustc-version.sh
   - cargo install --force diesel_cli --vers `cat .diesel_version` --no-default-features --features postgres && export PATH=$HOME/.cargo/bin:$PATH
 
 before_script:
@@ -68,9 +70,6 @@ matrix:
       script:
         - cargo build
         - cargo test
-      # This portion of the cache is quickly invalidated anyway
-      before_cache:
-        - cargo clean
 
 notifications:
   email:

script/cargo-clean-on-new-rustc-version.sh

@@ -0,0 +1,22 @@
+#!/bin/sh
+
+set -e
+
+stamp_file=target/rustc_version_stamp
+current_version=$(rustc --version)
+
+if [ -f $stamp_file ]; then
+    # Compare the current version against the previous version
+    if echo "$current_version" | cmp -s $stamp_file -; then
+        echo "Version of rustc hasn't changed, keeping the cache intact"
+    else
+        echo "The version of rustc has changed, running cargo clean"
+        cargo clean
+    fi
+else
+    echo "There is no existing version stamp, keeping the cache intact"
+fi
+
+# Save the version stamp for next time
+mkdir -p target/
+echo $current_version > $stamp_file