The recommended option is Markdown diagnostics, and we have already migrated everything to emit them. The empty help link we're currently emitting everywhere is a bug.
WMO stands for whole module optimization. It's a compilation mode where
all sources of a module are compiled together, e.g.
```
swift-frontend -emit-module A.swift B.swift -o Module.swiftmodule
```
This is opposed to incremental mode, where one would do something like
```
swift-frontend -emit-module -primary-file A.swift B.swift -module-name Module -o Module~A.swiftmodule
swift-frontend -emit-module A.swift -primary-file B.swift -module-name Module -o Module~B.swiftmodule
swift-frontend -merge-modules Module~A.swiftmodule Module~B.swiftmodule -o Module.swiftmodule
```
In WMO mode we were skipping extraction of all files after the first
one, because we were filtering in only files with an associated output,
and internally swift only assigns the output to the first input file in
WMO mode (which is just an implementation detail).
This patch refines that filter, by getting all input source files in
case there are no primary inputs.
File hashing is now done internally in `SwiftFileInterception` (and
exported as a `getHashOfRealFile` function for future use in linkage
awareness), and using a per-process in-memory cache. The persistent
caching of paths is removed, so the solution is now robust against input
file changes during the build.
For the same reason, the hash to artifact mapping have the symlinks
reversed now. The artifacts themselves are stored using the hash as
filenames, and the original paths of the artifacts are reacreated in the
scratch dir with symlinks mostly for debugging purposes (to understand
what artifact each hash corresponds to, and to follow what was built by
the extractor).
The hash map mechanism that was already in use for reading swiftmodule
files on macOS is now in use also on Linux. The output replacing
mechanism has been also reworked so that:
* frontend module emission modes have the remapping done directly in
the internal frontend options instead of painstakingly modifying input
flags (this requires a patch on the swift headers though)
* object emission mode is silenced to be just a type checking pass,
thus producing no output files
* all other passes but some debugging and version related ones become
noops
The open file read redirection uses a global weak pointer instance to
maximize robustness in the face of possibly multi-threaded calls to open
happening while `main` is exiting. Possibly overkill, but better safe
than sorry.
