Add a section about rustc_clean

Author: JonathanBrouwer

src/tests/compiletest.md

@@ -187,6 +187,22 @@ still pass.
 cause an Internal Compiler Error (ICE). This is a highly specialized directive
 to check that the incremental cache continues to work after an ICE.
 
+Incremental tests may use the attribute `#[rustc_clean(...)]` attribute. This attribute compares
+the fingerprint from the current compilation session with the previous one. 
+The first revision should never have an active `rustc_clean` attribute, since it will always be dirty.
+
+In the default mode, it checks that the fingerprints must be the same. 
+The attribute takes the following arguments:
+
+* `cfg="<cond>"` — checks the cfg condition `<cond>`, and only runs the check if the config condition evaluates to true.
+  This can be used to only run the `rustc_clean` attribute in a specific revision.
+* `except="<query1>,<query2>,..."` — asserts that the query results for the listed queries must be different, 
+  rather than the same.
+* `loaded_from_disk="<query1>,<query2>,..."` — asserts that the query results for the listed queries 
+  were actually loaded from disk (not just marked green). 
+  This can be useful to ensure that a test is actually exercising the deserialization
+  logic for a particular query result. This can be combined with `except`.
+
 [`tests/incremental`]: https://github.com/rust-lang/rust/tree/HEAD/tests/incremental