3540f9396a
Due to macro expansion, its possible to end up with two distinct `ExpnId`s that have the same `ExpnData` contents. This violates the contract of `HashStable`, since two unequal `ExpnId`s will end up with equal `Fingerprint`s. This commit adds a `disambiguator` field to `ExpnData`, which is used to force two otherwise-equivalent `ExpnData`s to be distinct.
For more information about how rustc works, see the rustc dev guide.