Summary:
Introduce new rust library, taggederror, which contains utilities for attaching metadata to errors. The library provides two main methods for attaching metadata to an error, the TaggedError wrapper type, and the AnyhowExt trait methods. Provides a struct, CommonMetadata, which contains all the metadata types introduced by taggederror (fault, transience, category, and typename), which can also be attached individually (and the same pattern can be used to attach other metadata).
Introduce a new native rust command, debugthrowrustexception, which causes the command to return an error, with some attached metadata.
Modify hg rust native command dispatch error handling to use debug formatter to print anyhow::Error errors. This will print out the source chain, contexts, and backtrace if available, which will cause the metadata we attach as a wrapper error or context to be printed.
Reviewed By: DurhamG
Differential Revision: D22420941
fbshipit-source-id: d38c5a10b686d86b69a2c0a19f5bcbf4ca24dff6
Any native code (C/C++/Rust) that Mercurial (either core or extensions)
depends on should go here. Python code, or native code that depends on
Python code (e.g. #include <Python.h> or use cpython) is disallowed.
As we start to convert more of Mercurial into Rust, and write new paths
entrirely in native code, we'll want to limit our dependency on Python, which is
why this barrier exists.
See also hgext/extlib/README.md, mercurial/cext/README.mb.
How do I choose between lib and extlib (and cext)?
If your code is native and doesn't depend on Python (awesome!), it goes here.
Otherwise, put it in hgext/extlib (if it's only used by extensions) or
mercurial/cext (if it's used by extensions or core).
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