consortium-log

A tiny logging facade that forwards to defmt, tracing, both, or neither — chosen by Cargo features, resolved at compile time, zero cost when off.

The problem it solves

Consortium code spans two worlds that log differently and cannot log each other's way:

World	Logs with	Why not the other
`no_std` firmware (M-core, R-core)	`defmt`	`tracing` needs `alloc` and formatting machinery a Cortex-M image should not carry.
`std` host tooling (codegen, CLI)	`tracing`	`defmt` needs a decoder on the other end of a wire.

But plenty of code is shared between them — the codec families, the IPC trait stack, the shared-memory protocol. Without a facade, every such crate needs #[cfg] around every log line, twice.

This crate makes one call site work in both:

consortium_log::trace!("channel {} lowered", name);

defmt on → routes to the matching defmt macro.
tracing on → routes to the matching tracing macro.
both on → the event goes to both backends.
neither on → the macro expands to nothing.

Format syntax: the portable subset

Write a format string plus positional arguments, or Rust-style implicit captures:

trace!("channel {} lowered", name);      // positional
trace!("channel {name} lowered");        // implicit capture

Implicit captures are normalized to indexed arguments by consortium-log-macros before being forwarded, because defmt does not parse them itself.

Each argument must satisfy whatever the enabled backends require — defmt::Format for defmt, Display/Debug for tracing. Backend-specific extensions are only safe when that backend is the sole one enabled:

Extension	Belongs to	Portable?
`{=u8}`, `{=[u8]}` type hints	`defmt`	no
`field = value` structured fields	`tracing`	no
`{}`, `{:?}`, `{name}`	both	yes

Arguments must be side-effect free. With no backend enabled the macros expand to nothing and the arguments are never evaluated, so trace!("n={}", counter.next()) silently changes behaviour between builds.

Features

Feature	Effect
`defmt`	Forward to the `defmt` macros.
`tracing`	Forward to the `tracing` macros. Implies `alloc`.
`alloc`	Marker capability required by `tracing`; alone it pulls no backend.

Default is neither — a crate that only wants to be loggable pays nothing until a consumer picks a backend.

The `defmt` dependency requirement

defmt's proc-macros expand to literal defmt::… paths resolved at the call site, not $crate-hygienic ones. So:

A crate that enables consortium-log/defmt must also depend on defmt directly. The facade cannot supply the name.

This is the one sharp edge in the design, and it interacts badly with Cargo feature unification: building several workspace crates together can enable consortium-log/defmt from one crate's feature graph while a different crate in the same build has no defmt dependency of its own, giving failed to resolve: use of unresolved module or unlinked crate defmt at that crate's log call sites. If you hit that, the fix is a direct defmt dependency in the crate that failed — matching what the firmware crates in this workspace already do.

tracing's macros are $crate-hygienic, so the tracing backend needs no such companion dependency.

How the dispatch works

Worth knowing before editing this crate: #[cfg] inside an exported macro's body is resolved against the calling crate's features, not this crate's. So feature selection cannot live in the public macros. Instead:

Each backend's macros are re-exported by name under hidden aliases (__defmt_trace, __tracing_trace, …). Re-exporting individual macros rather than the whole crate is what makes them resolvable through $crate::… from a downstream crate.
Two hidden dispatch macros, __defmt! and __tracing!, are each defined twice — once gated on the backend feature, once as a no-op arm. That gating happens on the definition, where this crate's features are visible.
The public level macros carry no #[cfg] at all; they call both dispatchers and let the definitions decide.

License

Apache-2.0. See LICENSE.

consortium-log 0.5.0