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Add `rustc_fluent_macro` to decouple fluent from `rustc_macros` Fluent, with all the icu4x it brings in, takes quite some time to compile. `fluent_messages!` is only needed in further downstream rustc crates, but is blocking more upstream crates like `rustc_index`. By splitting it out, we allow `rustc_macros` to be compiled earlier, which speeds up `x check compiler` by about 5 seconds (and even more after the needless dependency on `serde_json` is removed from `rustc_data_structures`).
For high-level intro to how type checking works in rustc, see the type checking chapter of the rustc dev guide.