Add DynStorageAlloc using FnOnce

Our DynStorage trait uses a &mut dyn FnMut as a callback for
implementing mount_and_then (instead of a FnOnce like Filesystem).  This
is because there is no no-std way to pass a FnOnce in an dyn compatible
trait method.

If alloc is available, this limitation can be avoided by using a
Box<dyn FnOnce>.  This patch adds an alloc feature to the crate and a
DynStorageAlloc trait behind that feature that provides mount_and_then
functions using a boxed FnOnce instead of a reference to an FnMut.

Author: robin-nitrokey

CHANGELOG.md

@@ -7,7 +7,9 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 
 ## Unreleased
 
--
+### Added
+
+- Added `DynStorageAlloc` trait behind the `alloc` feature.
 
 ## [v0.6.0](https://github.com/trussed-dev/littlefs2/releases/tag/0.6.0) - 2025-02-28
 

Cargo.toml

@@ -33,6 +33,7 @@ serde = { version = "1.0", default-features = false, features = ["derive"] }
 
 [features]
 default = ["serde"]
+alloc = []
 serde = ["littlefs2-core/serde"]
 # enable assertions in backend C code
 ll-assertions = ["littlefs2-sys/assertions"]

src/lib.rs

@@ -125,6 +125,9 @@ assert_eq!(&buf, b"black smoke");
 ```
 */
 
+#[cfg(feature = "alloc")]
+extern crate alloc;
+
 /// Low-level bindings
 pub use littlefs2_sys as ll;
 

src/object_safe.rs

@@ -15,6 +15,9 @@ pub use littlefs2_core::{DirEntriesCallback, DynFile, DynFilesystem, FileCallbac
 const _: Option<&dyn DynStorage> = None;
 
 pub type FilesystemCallback<'a, R = ()> = &'a mut dyn FnMut(&dyn DynFilesystem) -> Result<R>;
+#[cfg(feature = "alloc")]
+pub type BoxedFilesystemCallback<'a, R = ()> =
+    alloc::boxed::Box<dyn FnOnce(&dyn DynFilesystem) -> Result<R> + 'a>;
 
 impl<S: Storage> DynFile for File<'_, '_, S> {
     fn sync(&self) -> Result<()> {
@@ -238,3 +241,27 @@ impl dyn DynStorage + '_ {
         result
     }
 }
+
+#[cfg(feature = "alloc")]
+pub trait DynStorageAlloc: DynStorage {
+    fn mount_and_then_unit_boxed(&mut self, f: BoxedFilesystemCallback<'_>) -> Result<()>;
+}
+
+#[cfg(feature = "alloc")]
+impl<T: Storage> DynStorageAlloc for T {
+    fn mount_and_then_unit_boxed(&mut self, f: BoxedFilesystemCallback<'_>) -> Result<()> {
+        Filesystem::mount_and_then(self, |fs| f(fs))
+    }
+}
+
+#[cfg(feature = "alloc")]
+impl dyn DynStorageAlloc + '_ {
+    pub fn mount_and_then_boxed<R>(&mut self, f: BoxedFilesystemCallback<'_, R>) -> Result<R> {
+        let mut result = Err(Error::IO);
+        self.mount_and_then_unit_boxed(alloc::boxed::Box::new(|fs| {
+            result = Ok(f(fs)?);
+            Ok(())
+        }))?;
+        result
+    }
+}