document rust caveats

Author: luser

README.md

@@ -5,6 +5,8 @@ sccache - Shared Compilation Cache
 
 Sccache is a [ccache](https://ccache.samba.org/)-like tool. It is used as a compiler wrapper and avoids compilation when possible, storing a cache in a remote storage using the S3 API.
 
+Sccache now includes [experimental Rust support](docs/Rust.md).
+
 It works as a client-server. The client spawns a server if one is not running already, and sends the wrapped command line as a request to the server, which then does the work and returns stdout/stderr for the job.  The client-server model allows the server to be more efficient in its handling of the remote storage.
 
 Sccache can also be used with local storage instead of remote.

docs/Rust.md

@@ -0,0 +1,10 @@
+sccache now includes experimental support for caching Rust compilation. This includes many caveats, and is primarily focused on caching rustc invocations as produced by cargo. A (possibly-incomplete) list follows:
+* `--emit` is required.
+* Only `link` and `dep-info` are supported as `--emit` values, and `link` must be present.
+* `--out-dir` is required.
+* `-o file` is not supported.
+* Compilation from stdin is not supported, a source file must be provided.
+* Values from `env!` will not be tracked in caching.
+* Procedural macros that read files from the filesystem may not be cached properly
+* The system linker is not factored in as a cache input, so changing the linker may produce incorrect cached results.
+* Target specs aren't hashed (e.g. custom target specs)