rust: box some futures in callers#1945
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NickeZ
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Apr 21, 2026
Reduces binary size by ~1360b. async fn lowers to a concrete state-machine type. In a function like signtx::_process() in src/rust/bitbox02-rust/src/hww/api/bitcoin/signtx.rs, every directly-awaited child future becomes part of that parent future’s stored state. If the child is large, the parent future gets larger too, and its generated poll() logic has to handle that larger layout. Box::pin(child()).await changes the shape of the parent future: - The child future is still a concrete type. - But the parent no longer stores that full child state inline. - It stores a boxed pointer instead. - So the parent future becomes smaller and simpler.
The top-level HWW API dispatch function builds one large async state machine that has to cover every request variant. That keeps all of the branch-specific future layouts live in a single monomorphized type and costs a noticeable amount of ROM. Change the top-level dispatch helpers to return boxed futures and box each branch individually. This moves the size cost from code generation into a small runtime allocation at dispatch time, while leaving the API surface and request handling behavior unchanged. Saves 1832 bytes in firmware.bin.
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Reduces binary size by ~1360b.
async fn lowers to a concrete state-machine type. In a function like signtx::_process() in src/rust/bitbox02-rust/src/hww/api/bitcoin/signtx.rs, every directly-awaited child future becomes part of that parent
future’s stored state. If the child is large, the parent future gets larger too, and its generated poll() logic has to handle that larger layout.
Box::pin(child()).await changes the shape of the parent future: