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7585: More information for mbe in architecture.md r=edwin0cheng a=edwin0cheng

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Co-authored-by: Edwin Cheng <edwin0cheng@gmail.com>
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@ -258,8 +258,13 @@ They are independent from the rest of the code.
`mbe` crate contains tools for transforming between syntax trees and token tree.
And it also handles the actual parsing and expansion of declarative macro (a-la "Macros By Example" or mbe).
For proc macros, we pass the token trees by loading the corresponding dynamic library (which built by `cargo`).
That's why the client (`proc_macro_api`) and server (`proc_macro_srv`) model are used to run proc-macro in separate process in background.
For proc macros, the client-server model are used.
We pass an argument `--proc-macro` to `rust-analyzer` binary to start a separate process (`proc_macro_srv`).
And the client (`proc_macro_api`) provides an interface to talk to that server separately.
And then token trees are passed from client, and the server will load the corresponding dynamic library (which built by `cargo`).
And due to the fact the api for getting result from proc macro are always unstable in `rustc`,
we maintain our own copy (and paste) of that part of code to allow us to build the whole thing in stable rust.
**Architecture Invariant:**
Bad proc macros may panic or segfault accidentally. So we run it in another process and recover it from fatal error.