--- a/servo/.travis.yml
+++ b/servo/.travis.yml
@@ -15,31 +15,33 @@ matrix:
       before_install:
         - sudo add-apt-repository ppa:ubuntu-toolchain-r/test -y
         - sudo add-apt-repository 'deb http://apt.llvm.org/precise/ llvm-toolchain-precise-3.9 main' -y
         - sudo apt-get update -q
         - sudo apt-get install clang-3.9 llvm-3.9 llvm-3.9-runtime -y
         - export LLVM_CONFIG=/usr/lib/llvm-3.9/bin/llvm-config
         - export CC=gcc-5 CXX=g++-5
       script:
-         - RUSTFLAGS='-D warnings' ./mach build -d --verbose
-         - RUSTFLAGS='-D warnings' ./mach test-unit
+         - ./mach build -d --verbose
+         - ./mach test-unit
          - ./mach clean
-         - RUSTFLAGS='-D warnings' ./mach build-geckolib
-         - RUSTFLAGS='-D warnings' ./mach test-stylo
+         - ./mach build-geckolib
+         - ./mach test-stylo
          - bash etc/ci/lockfile_changed.sh
       cache:
         directories:
           - .cargo
           - .servo
           - $HOME/.ccache
       before_cache:
         - ./mach clean-nightlies --keep 2 --force
         - ./mach clean-cargo-cache --keep 2 --force
-      env: CCACHE=/usr/bin/ccache
+      env:
+        CCACHE=/usr/bin/ccache
+        RUSTFLAGS=-Dwarnings
       addons:
         apt:
           sources:
             - ubuntu-toolchain-r-test
             - sourceline: 'ppa:jonathonf/ffmpeg-3'
           packages:
             - cmake
             - dbus-x11

--- a/servo/appveyor.yml
+++ b/servo/appveyor.yml
@@ -1,13 +1,14 @@
 version: 1.0.{build}
 
 environment:
   CCACHE_DIR: "%APPVEYOR_BUILD_FOLDER%\\.ccache"
   RUST_BACKTRACE: 1
+  RUSTFLAGS: -Dwarnings
   # The appveyor image we use has a pretty huge set of things installed... we make the
   # initial PATH something sane so we know what to expect
   PATH: "C:\\windows\\system32;\
     C:\\windows;\
     C:\\windows\\System32\\Wbem;\
     C:\\windows\\System32\\WindowsPowerShell\\v1.0;\
     C:\\ProgramData\\chocolatey\\bin;\
     C:\\Python27;\
@@ -43,13 +44,12 @@ cache:
 # Uncomment these lines to expose RDP access information to the build machine in the build log.
 #init:
 #  - ps: iex ((new-object net.webclient).DownloadString('https://raw.githubusercontent.com/appveyor/ci/master/scripts/enable-rdp.ps1'))
 #
 #on_finish:
 #  - ps: $blockRdp = $true; iex ((new-object net.webclient).DownloadString('https://raw.githubusercontent.com/appveyor/ci/master/scripts/enable-rdp.ps1'))
 
 build_script:
-  - set RUSTFLAGS=-D warnings
   - mach build -d -v
   - mach test-unit
 
 test: off

--- a/servo/etc/ci/buildbot_steps.yml
+++ b/servo/etc/ci/buildbot_steps.yml
@@ -1,8 +1,11 @@
+env:
+  RUSTFLAGS: -Dwarnings
+
 mac-rel-wpt1:
   - ./mach clean-nightlies --keep 3 --force
   - env PKG_CONFIG_PATH=/usr/local/opt/zlib/lib/pkgconfig ./mach build --release
   - ./mach run -r -o output.png
   - ./mach test-wpt-failure
   - ./mach test-wpt --release --processes 4 --total-chunks 6 --this-chunk 1 --log-raw test-wpt.log --log-errorsummary wpt-errorsummary.log --always-succeed
   - ./mach filter-intermittents wpt-errorsummary.log --log-intermittents intermittents.log --log-filteredsummary filtered-wpt-errorsummary.log --tracker-api default --reporter-api default
   - ./mach test-wpt --release --binary-arg=--multiprocess --processes 8 --log-raw test-wpt-mp.log --log-errorsummary wpt-mp-errorsummary.log eventsource
@@ -27,21 +30,21 @@ mac-rel-wpt4:
   - env PKG_CONFIG_PATH=/usr/local/opt/zlib/lib/pkgconfig ./mach build --release
   - ./mach test-wpt --release --processes 4 --total-chunks 6 --this-chunk 4 --log-raw test-wpt.log --log-errorsummary wpt-errorsummary.log --always-succeed
   - ./mach filter-intermittents wpt-errorsummary.log --log-intermittents intermittents.log --log-filteredsummary filtered-wpt-errorsummary.log --tracker-api default --reporter-api default
   - ./mach test-wpt --release --pref dom.servoparser.async_html_tokenizer.enabled --processes=8 --log-raw test-async-parsing.log --log-errorsummary async-parsing-errorsummary.log --always-succeed domparsing html/syntax html/dom/documents html/dom/dynamic-markup-insertion
   - ./mach filter-intermittents async-parsing-errorsummary.log --log-intermittents async-parsing-intermittents.log --log-filteredsummary filtered-async-parsing-errorsummary.log --tracker-api default --reporter-api default
 
 mac-dev-unit:
   - ./mach clean-nightlies --keep 3 --force
-  - env PKG_CONFIG_PATH=/usr/local/opt/zlib/lib/pkgconfig RUSTFLAGS=-Dwarnings ./mach build --dev
-  - env PKG_CONFIG_PATH=/usr/local/opt/zlib/lib/pkgconfig RUSTFLAGS=-Dwarnings ./mach test-unit
-  - env ./mach package --dev
-  - env PKG_CONFIG_PATH=/usr/local/opt/zlib/lib/pkgconfig RUSTFLAGS=-Dwarnings ./mach build-cef
-  - env RUSTFLAGS=-Dwarnings ./mach build-geckolib
+  - env PKG_CONFIG_PATH=/usr/local/opt/zlib/lib/pkgconfig ./mach build --dev
+  - env PKG_CONFIG_PATH=/usr/local/opt/zlib/lib/pkgconfig ./mach test-unit
+  - ./mach package --dev
+  - env PKG_CONFIG_PATH=/usr/local/opt/zlib/lib/pkgconfig ./mach build-cef
+  - ./mach build-geckolib
   - bash ./etc/ci/lockfile_changed.sh
   - bash ./etc/ci/manifest_changed.sh
 
 mac-rel-css1:
   - ./mach clean-nightlies --keep 3 --force
   - env PKG_CONFIG_PATH=/usr/local/opt/zlib/lib/pkgconfig ./mach build --release
   - ./mach test-wpt --release --processes 4 --total-chunks 6 --this-chunk 5 --log-raw test-wpt.log --log-errorsummary wpt-errorsummary.log --always-succeed
   - ./mach filter-intermittents wpt-errorsummary.log --log-intermittents intermittents.log --log-filteredsummary filtered-wpt-errorsummary.log --tracker-api default --reporter-api default
@@ -75,23 +78,23 @@ mac-rel-intermittent:
   - ./mach clean-nightlies --keep 3 --force
   - env PKG_CONFIG_PATH=/usr/local/opt/zlib/lib/pkgconfig ./mach build --release
   - ./etc/ci/check_intermittents.sh --log-raw intermittents.log
 
 linux-dev:
   - ./mach clean-nightlies --keep 3 --force
   - ./mach test-tidy --no-progress --all
   - ./mach test-tidy --no-progress --self-test
-  - env CC=gcc-5 CXX=g++-5 RUSTFLAGS=-Dwarnings ./mach build --dev
-  - env CC=gcc-5 CXX=g++-5 RUSTFLAGS=-Dwarnings ./mach test-unit
-  - env ./mach package --dev
-  - env CC=gcc-5 CXX=g++-5 RUSTFLAGS=-Dwarnings ./mach build-cef
-  - env CC=gcc-5 CXX=g++-5 RUSTFLAGS=-Dwarnings ./mach build --dev --no-default-features --features default-except-unstable
-  - env RUSTFLAGS=-Dwarnings ./mach build-geckolib
-  - env RUSTFLAGS=-Dwarnings ./mach test-stylo
+  - env CC=gcc-5 CXX=g++-5 ./mach build --dev
+  - env CC=gcc-5 CXX=g++-5 ./mach test-unit
+  - ./mach package --dev
+  - env CC=gcc-5 CXX=g++-5 ./mach build-cef
+  - env CC=gcc-5 CXX=g++-5 ./mach build --dev --no-default-features --features default-except-unstable
+  - ./mach build-geckolib
+  - ./mach test-stylo
   - bash ./etc/ci/lockfile_changed.sh
   - bash ./etc/ci/manifest_changed.sh
   - bash ./etc/ci/check_no_panic.sh
 
 linux-rel-wpt:
   - ./mach clean-nightlies --keep 3 --force
   - env CC=gcc-5 CXX=g++-5 ./mach build --release --with-debug-assertions
   - ./mach test-wpt-failure
@@ -115,43 +118,42 @@ linux-nightly:
   - ./mach package --release
   - ./mach upload-nightly linux
   - ./mach test-perf
   - python3 ./etc/ci/performance/download_buildbot_timings.py --verbose
   - aws s3 sync --size-only --acl public-read ./etc/ci/performance/output s3://servo-perf
 
 android:
   - ./mach clean-nightlies --keep 3 --force
-  - env ANDROID_SDK=/home/servo/android/sdk/r25.2.3 RUSTFLAGS=-Dwarnings ./mach build --android --dev
+  - env ANDROID_SDK=/home/servo/android/sdk/r25.2.3 ./mach build --android --dev
   - env ANDROID_SDK=/home/servo/android/sdk/r25.2.3 ./mach package --android --dev
   - bash ./etc/ci/lockfile_changed.sh
   - bash ./etc/ci/manifest_changed.sh
   - python ./etc/ci/check_dynamic_symbols.py
 
 android-nightly:
   - ./mach clean-nightlies --keep 3 --force
   - env ANDROID_SDK=/home/servo/android/sdk/r25.2.3 ./mach build --android --release
   - env ANDROID_SDK=/home/servo/android/sdk/r25.2.3 ./mach package --android --release
   - ./mach upload-nightly android
 
 arm32:
   - ./mach clean-nightlies --keep 3 --force
-  - env RUSTFLAGS=-Dwarnings ./mach build --rel --target=arm-unknown-linux-gnueabihf
+  - ./mach build --rel --target=arm-unknown-linux-gnueabihf
   - bash ./etc/ci/lockfile_changed.sh
   - bash ./etc/ci/manifest_changed.sh
 
 arm64:
   - ./mach clean-nightlies --keep 3 --force
-  - env RUSTFLAGS=-Dwarnings ./mach build --rel --target=aarch64-unknown-linux-gnu
+  - ./mach build --rel --target=aarch64-unknown-linux-gnu
   - bash ./etc/ci/lockfile_changed.sh
   - bash ./etc/ci/manifest_changed.sh
 
 windows-msvc-dev:
   - mach.bat clean-nightlies --keep 3 --force
-  - set RUSTFLAGS=-D warnings
   - mach.bat build --dev
   - mach.bat test-unit
   - mach.bat package --dev
   - mach.bat build-geckolib
   - mach.bat test-stylo
 
 windows-msvc-nightly:
   - mach.bat clean-nightlies --keep 3 --force

--- a/servo/python/requirements.txt
+++ b/servo/python/requirements.txt
@@ -10,16 +10,17 @@ setuptools == 18.5
 toml == 0.9.2
 
 # For Python linting
 flake8 == 2.4.1
 pep8 == 1.5.7
 pyflakes == 0.8.1
 
 # For buildbot checking
+voluptuous == 0.10.5
 PyYAML == 3.12
 
 # For test-webidl
 ply == 3.8
 
 # For Cross-platform colored terminal text
 colorama == 0.3.7
 

--- a/servo/python/tidy/servo_tidy/tidy.py
+++ b/servo/python/tidy/servo_tidy/tidy.py
@@ -12,18 +12,20 @@ import fnmatch
 import imp
 import itertools
 import json
 import os
 import re
 import StringIO
 import subprocess
 import sys
+
 import colorama
 import toml
+import voluptuous
 import yaml
 
 from licenseck import MPL, APACHE, COPYRIGHT, licenses_toml, licenses_dep_toml
 
 CONFIG_FILE_PATH = os.path.join(".", "servo-tidy.toml")
 
 # Default configs
 config = {
@@ -770,25 +772,34 @@ def duplicate_key_yaml_constructor(loade
         if key in mapping:
             raise KeyError(key)
         value = loader.construct_object(value_node, deep=deep)
         mapping[key] = value
     return loader.construct_mapping(node, deep)
 
 
 def lint_buildbot_steps_yaml(mapping):
-    # Check for well-formedness of contents
-    # A well-formed buildbot_steps.yml should be a map to list of strings
-    for k in mapping.keys():
-        if not isinstance(mapping[k], list):
-            raise ValueError("Key '{}' maps to type '{}', but list expected".format(k, type(mapping[k]).__name__))
+    from voluptuous import Any, Extra, Required, Schema
 
-        # check if value is a list of strings
-        for item in itertools.ifilter(lambda i: not isinstance(i, str), mapping[k]):
-            raise ValueError("List mapped to '{}' contains non-string element".format(k))
+    # Note: dictionary keys are optional by default in voluptuous
+    env = Schema({Extra: str})
+    commands = Schema([str])
+    schema = Schema({
+        'env': env,
+        Extra: Any(
+            commands,
+            {
+                'env': env,
+                Required('commands'): commands,
+            },
+        ),
+    })
+
+    # Signals errors via exception throwing
+    schema(mapping)
 
 
 class SafeYamlLoader(yaml.SafeLoader):
     """Subclass of yaml.SafeLoader to avoid mutating the global SafeLoader."""
     pass
 
 
 def check_yaml(file_name, contents):
@@ -806,18 +817,18 @@ def check_yaml(file_name, contents):
     try:
         contents = yaml.load(contents, Loader=SafeYamlLoader)
         lint_buildbot_steps_yaml(contents)
     except yaml.YAMLError as e:
         line = e.problem_mark.line + 1 if hasattr(e, 'problem_mark') else None
         yield (line, e)
     except KeyError as e:
         yield (None, "Duplicated Key ({})".format(e.message))
-    except ValueError as e:
-        yield (None, e.message)
+    except voluptuous.MultipleInvalid as e:
+        yield (None, str(e))
 
 
 def check_for_possible_duplicate_json_keys(key_value_pairs):
     keys = [x[0] for x in key_value_pairs]
     seen_keys = set()
     for key in keys:
         if key in seen_keys:
             raise KeyError("Duplicated Key (%s)" % key)

--- a/servo/python/tidy/servo_tidy_tests/test_tidy.py
+++ b/servo/python/tidy/servo_tidy_tests/test_tidy.py
@@ -205,22 +205,22 @@ class CheckTidiness(unittest.TestCase):
 
     def test_yaml_with_duplicate_key(self):
         errors = tidy.collect_errors_for_files(iterFile('duplicate_keys_buildbot_steps.yml'), [tidy.check_yaml], [], print_text=False)
         self.assertEqual('Duplicated Key (duplicate_yaml_key)', errors.next()[2])
         self.assertNoMoreErrors(errors)
 
     def test_non_list_mapped_buildbot_steps(self):
         errors = tidy.collect_errors_for_files(iterFile('non_list_mapping_buildbot_steps.yml'), [tidy.check_yaml], [], print_text=False)
-        self.assertEqual("Key 'non-list-key' maps to type 'str', but list expected", errors.next()[2])
+        self.assertEqual("expected a list for dictionary value @ data['non-list-key']", errors.next()[2])
         self.assertNoMoreErrors(errors)
 
     def test_non_string_list_mapping_buildbot_steps(self):
         errors = tidy.collect_errors_for_files(iterFile('non_string_list_buildbot_steps.yml'), [tidy.check_yaml], [], print_text=False)
-        self.assertEqual("List mapped to 'mapping_key' contains non-string element", errors.next()[2])
+        self.assertEqual("expected str @ data['mapping_key'][0]", errors.next()[2])
         self.assertNoMoreErrors(errors)
 
     def test_lock(self):
         errors = tidy.collect_errors_for_files(iterFile('duplicated_package.lock'), [tidy.check_lock], [], print_text=False)
         msg = """duplicate versions for package `test`
 \t\x1b[93mThe following packages depend on version 0.4.9 from 'crates.io':\x1b[0m
 \t\ttest2
 \t\x1b[93mThe following packages depend on version 0.5.1 from 'crates.io':\x1b[0m"""