Merge pull request #56 from carols10cents/using-in-tests

Add instructions and example for using env_logger in tests

README.md

@@ -79,3 +79,82 @@ fn main() {
     // ...
 }
 ```
+
+## In tests
+
+Tests can use the `env_logger` crate to see log messages generated during that test:
+
+```toml
+[dependencies]
+log = "0.3"
+
+[dev-dependencies]
+env_logger = "0.3"
+```
+
+```rust
+#[macro_use]
+extern crate log;
+
+fn add_one(num: i32) -> i32 {
+    info!("add_one called with {}", num);
+    num + 1
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    extern crate env_logger;
+
+    #[test]
+    fn it_adds_one() {
+        let _ = env_logger::init();
+        info!("can log from the test too");
+        assert_eq!(3, add_one(2));
+    }
+
+    #[test]
+    fn it_handles_negative_numbers() {
+        let _ = env_logger::init();
+        info!("logging from another test");
+        assert_eq!(-7, add_one(-8));
+    }
+}
+```
+
+Assuming the module under test is called `my_lib`, running the tests with the
+`RUST_LOG` filtering to info messages from this module looks like:
+
+```bash
+$ RUST_LOG=my_lib=info cargo test
+     Running target/debug/my_lib-...
+
+running 2 tests
+INFO:my_lib::tests: logging from another test
+INFO:my_lib: add_one called with -8
+test tests::it_handles_negative_numbers ... ok
+INFO:my_lib::tests: can log from the test too
+INFO:my_lib: add_one called with 2
+test tests::it_adds_one ... ok
+
+test result: ok. 2 passed; 0 failed; 0 ignored; 0 measured
+```
+
+Note that `env_logger::init()` needs to be called in each test in which you
+want to enable logging. Additionally, the default behavior of tests to
+run in parallel means that logging output may be interleaved with test output.
+Either run tests in a single thread by specifying `RUST_TEST_THREADS=1` or by
+running one test by specifying its name as an argument to the test binaries as
+directed by the `cargo test` help docs:
+
+```bash
+$ RUST_LOG=my_lib=info cargo test it_adds_one
+     Running target/debug/my_lib-...
+
+running 1 test
+INFO:my_lib::tests: can log from the test too
+INFO:my_lib: add_one called with 2
+test tests::it_adds_one ... ok
+
+test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured
+```