fix(checkpoint): file-based persistence is native-only, and now says so (#22)

`cargo check --target wasm32-unknown-unknown` fails on the **default** feature
set with a dozen "`?` couldn't convert the error to `CheckpointError`".

The cause is a gate that drifted out of step with the error type it feeds.
`CheckpointError::Io` is `#[cfg(not(target_arch = "wasm32"))]` and its doc reads
"native only", with `Storage(String)` offered as the wasm alternative -- so the
error enum already encodes that file I/O is not a wasm concern. But
`checkpoint::recovery` and `InteractiveSession::{save,load}` were gated on
`feature = "checkpoint"` alone, which is on by default and says nothing about
the target. On wasm32 the file-I/O code was therefore still compiled while the
variant its `?` operators desugar into was not.

Both are now gated on `all(feature = "checkpoint", not(target_arch = "wasm32"))`,
matching the intent the error type had already declared. Gating rather than
adding a wasm `From<io::Error>` is deliberate: `wasm32-unknown-unknown` has no
filesystem, so `File::create` there could only ever be a compile-time promise of
a runtime failure.

Not a `parallel` problem, which is worth recording because it is the natural
guess: `--features parallel --no-default-features` already checks clean on
wasm32. `checkpoint` was the only blocker, so a downstream crate that reached
for `default-features = false` to get a wasm build no longer needs to.

wasm32 now checks clean with default features -- zero errors, zero warnings.
Native untouched: 681 tests pass, fmt and clippy clean.


Claude-Session: https://claude.ai/code/session_01SHNw2k18vFYcaTukhmvbAQ

Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: alexnodeland

Cargo.lock

@@ -4,21 +4,36 @@
 //! enabling long-running experiments to be paused and resumed.
 //!
 //! The `state` submodule (data structures) is always available.
-//! The `recovery` submodule (file I/O) requires the `checkpoint` feature.
-
-#[cfg(feature = "checkpoint")]
+//! The `recovery` submodule (file I/O) requires the `checkpoint` feature **and
+//! a target with a filesystem**.
+//!
+//! The second half of that condition is not a new restriction, it is the one
+//! [`CheckpointError`](crate::error::CheckpointError) already encodes: its
+//! `Io` variant is `#[cfg(not(target_arch = "wasm32"))]` and its docs read
+//! "native only", with `Storage(String)` offered as the wasm alternative. The
+//! module gate had not been kept in step, so on `wasm32` the file-I/O code was
+//! still compiled while the error variant its `?` operators desugar into was
+//! not — `cargo check --target wasm32-unknown-unknown` failed on the default
+//! feature set with a dozen "`?` couldn't convert the error to
+//! `CheckpointError`".
+//!
+//! Gating the module rather than adding a wasm `From<io::Error>` is the fix
+//! that matches the intent: `wasm32-unknown-unknown` has no filesystem, so
+//! `File::create` there could only ever be a compile-time promise of a
+//! runtime failure.
+#[cfg(all(feature = "checkpoint", not(target_arch = "wasm32")))]
 mod recovery;
 mod rng;
 mod state;
 
-#[cfg(feature = "checkpoint")]
+#[cfg(all(feature = "checkpoint", not(target_arch = "wasm32")))]
 pub use recovery::*;
 pub use rng::*;
 pub use state::*;
 
 /// Prelude for checkpoint module
 pub mod prelude {
-    #[cfg(feature = "checkpoint")]
+    #[cfg(all(feature = "checkpoint", not(target_arch = "wasm32")))]
     pub use super::recovery::*;
     pub use super::rng::*;
     pub use super::state::*;

src/checkpoint/mod.rs

@@ -6,11 +6,11 @@
 use serde::{Deserialize, Serialize};
 use std::collections::HashMap;
 
-#[cfg(feature = "checkpoint")]
+#[cfg(all(feature = "checkpoint", not(target_arch = "wasm32")))]
 use std::fs::File;
-#[cfg(feature = "checkpoint")]
+#[cfg(all(feature = "checkpoint", not(target_arch = "wasm32")))]
 use std::io::{BufReader, BufWriter};
-#[cfg(feature = "checkpoint")]
+#[cfg(all(feature = "checkpoint", not(target_arch = "wasm32")))]
 use std::path::Path;
 
 use super::aggregation::FitnessAggregator;
@@ -384,8 +384,10 @@ where
     }
 }
 
-/// File-based session persistence (requires `checkpoint` feature)
-#[cfg(feature = "checkpoint")]
+/// File-based session persistence (requires the `checkpoint` feature and a
+/// target with a filesystem — see [`crate::checkpoint`] for why the second
+/// half of that is not a new restriction).
+#[cfg(all(feature = "checkpoint", not(target_arch = "wasm32")))]
 impl<G> InteractiveSession<G>
 where
     G: EvolutionaryGenome + Serialize + for<'de> Deserialize<'de>,