fix(stylo_taffy): macro no-tracing arm consumes args to silence unused_variables

[nixpt]

Summary

The log_fallback!() macro in convert.rs has a #[cfg(not(feature = "tracing"))] arm that expands to empty {}. Empty expansion leaves caller bindings unconsumed, producing a unused_variables warning at every match-arm call site. Fix: the no-tracing arm now does let _ = (&$value, &$to); which consumes the args at the call site (zero runtime cost, just a binding consumption).

PR-D of 4 in the s190 cascade arc. Backport of the same fix from DioxusLabs/blitz#443 (upstream-of-upstream — same macro, same problem, same fix).

Changes

packages/stylo_taffy/src/convert.rs:75-78:

 #[cfg(not(feature = "tracing"))]
 macro_rules! log_fallback {
-    ($value:expr, $to:expr) => {};
+    ($value:expr, $to:expr) => {
+        let _ = (&$value, &$to);
+    };
 }

Net delta: 3 LOC.

Why

The warning surfaces on every match-arm call site like:

unsupported => {
    log_fallback!(&format!("display:{:?}", unsupported), "DEFAULT");
    taffy::Display::DEFAULT
}

The unsupported binding gets bound by the match pattern, then passed to the macro. The compiler can't see through macro expansion — from its perspective the macro expanded to {} and unsupported was never used. Result: warning: unused variable: unsupported at compile time in no-tracing builds (which is the default).

The fix is the canonical "macro arm that wants to consume args without emitting runtime code": bind both args to _. The & prevents move-out so the caller's bindings stay usable downstream (currently not needed but cheap insurance).

Test plan

• CARGO_TARGET_DIR=/build/target-foreman-bliss-main-merge cargo check -p stylo_taffy → clean. The previously-warned packages/stylo_taffy/src/convert.rs:260:9 unused_variables warning is gone.
• No behavioural change — the no-tracing arm still produces zero runtime code (let _ = (&_, &_) is a no-op after dead-code elimination).
• Tracing arm untouched.

Downstream context

This is the same warning DioxusLabs#443 fixes upstream-of-upstream. Bliss-engine carries its own stylo_taffy fork (PR #1 landed the panic-elimination + tracing-feature work), so it doesn't auto-inherit the upstream fix. Backporting it here keeps bliss-engine no-tracing builds warning-clean and aligns the macro shape with blitz.

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