Right now, if you have a chain of dependencies (e.g. A depends on B depends on C) and rebuild C without rebuilding B, when you rebuild A, you will get an error like:

task 'rustc' failed at 'lookup_item: id not found: NUMBER at servo/src/compiler/rust/src/librustc/metadata/decoder.rs:92
Internal Compiler Error

If this is the only case where that error message occurs, it would be nice if it said something like "rebuild B", where possible.