Motivation

tracing-appender 0.2.5 added the latest_symlink builder option (#3447), which
depends on the symlink crate. That crate
only defines symlink_file and remove_symlink_file on unix, windows, and
redox, so on other targets — notably wasm32-unknown-unknown — the appender no
longer compiles:

error[E0425]: cannot find function `remove_symlink_file` in crate `symlink`
   --> tracing-appender/src/rolling.rs:804:26
    |
804 |         let _ = symlink::remove_symlink_file(&symlink_path);
    |                          ^^^^^^^^^^^^^^^^^^^ not found in `symlink`
note: found an item that was configured out
    |
133 | #[cfg(any(target_os = "redox", unix, windows))]

tracing-appender 0.2.4 built fine on wasm32; 0.2.5 regressed.

Fixes: #3536

Solution

Gate the symlink dependency to the platforms that support symbolic links, and
gate the code that uses it the same way:

• In Cargo.toml, move symlink into
  [target.'cfg(any(unix, windows, target_os = "redox"))'.dependencies].
• In rolling.rs, extract the symlink logic from create_writer into an
  update_latest_symlink helper with two cfg-gated definitions: the real one
  for symlink-capable targets, and a no-op for the rest. test_latest_symlink
  is gated the same way.

On platforms without symbolic-link support, latest_symlink is now a no-op (the
log file is still written; no symlink is created), and the crate compiles again.
The existing test-build-wasm CI job builds tracing-appender for wasm32.