consortium-log-macros

Internal format-string normalization for consortium-log.

Implementation detail. Depend on consortium-log. This crate's API is not stable and its single macro is not meant to be called directly.

What it does

consortium-log forwards one call site to two backends that do not agree about format strings. Rust's implicit captures —

trace!("channel {name} lowered")

— work with tracing, which is built on core::fmt. They do not work with defmt, which parses format strings itself and has no notion of pulling name out of the enclosing scope.

Rather than banning the nicer syntax in shared code, expand_captures rewrites it into the indexed form both backends accept:

trace!("channel {name} lowered")
  →  __defmt!(trace, "channel {0} lowered", name)

Explicit positional arguments keep their indices; captures are appended after them, and a name used twice reuses one index.

Why the callback is an input

The macro takes three parenthesized groups — callback path, log level, and the original log arguments — and re-emits an invocation of that callback.

Threading the callback path through the macro is what preserves consortium-log's $crate hygiene. The facade passes $crate::__defmt, so the expansion names the facade's own dispatch macro regardless of which crate the log call was written in. A proc-macro cannot synthesize a $crate token of its own, so the path has to arrive as data.

What is deliberately left alone

Two inputs are forwarded byte-for-byte rather than rewritten:

Anything that fails to parse as a format string plus arguments. Emitting the original tokens lets the backend produce the real diagnostic, pointing at the user's call site instead of at this rewrite.
Named arguments (name = expression). Those already mean something to Rust and to tracing's structured fields, so {name} alongside them is not an implicit capture and must not be treated as one.

Testing

cargo test -p consortium-log-macros
cargo test -p consortium-log --features defmt      # end-to-end through the facade
cargo test -p consortium-log --features tracing

License