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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. `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). 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`). For proc macros, the client-server model are used.
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. 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:** **Architecture Invariant:**
Bad proc macros may panic or segfault accidentally. So we run it in another process and recover it from fatal error. Bad proc macros may panic or segfault accidentally. So we run it in another process and recover it from fatal error.