Use associated constants

Fixes bitflags#20.
Closes bitflags#21.

Author: tamird

.travis.yml

@@ -8,9 +8,9 @@ script:
   - cargo build --verbose
   - cargo test --verbose
   - cargo test --verbose --features no_std
-  - if [ "$TRAVIS_RUST_VERSION" = "nightly" ]; then
-       cargo test --verbose --features assignment_operators;
-       cargo test --verbose --features "no_std assignment_operators";
+  - if [ "$TRAVIS_RUST_VERSION" = 'nightly' ]; then
+       cargo test --verbose --features 'assignment_operators';
+       cargo test --verbose --features 'no_std assignment_operators';
     fi
   - cargo doc
 after_success: |

Cargo.toml

@@ -12,5 +12,8 @@ description = """
 A macro to generate structures which behave like bitflags.
 """
 
+[dev-dependencies]
+compiletest_rs = "~0.1.1"
+
 [features]
 assignment_operators = []