Implement Witty traits for log::Level (#1877)

* Add a `log` feature to `linera-witty`

Prepare to implement the Witty traits for a type from the `log` crate,
but only if requested by the user.

* Implement Witty traits for `log::Level`

Place it behind a feature gate, so that the `log` dependency is only
included if requested.

* Test WIT roundtrip of `log::Level`

Ensure all variants can be stored in memory and loaded from memory
following the WIT specification, as well as lowered into and lifted from
its flat layout.

linera-witty/Cargo.toml

@@ -12,6 +12,7 @@ edition = "2021"
 
 [features]
 default = ["macros"]
+log = ["dep:log"]
 macros = ["linera-witty-macros"]
 test = ["linera-witty-macros?/test"]
 wasmer = ["dep:wasmer", "linera-witty-macros?/wasmer", "wasmer-vm"]
@@ -22,6 +23,7 @@ anyhow.workspace = true
 either.workspace = true
 frunk.workspace = true
 linera-witty-macros = { workspace = true, optional = true }
+log = { workspace = true, optional = true }
 thiserror.workspace = true
 wasmer = { workspace = true, optional = true }
 wasmer-vm = { workspace = true, optional = true }

linera-witty/build.rs

@@ -3,6 +3,7 @@
 
 fn main() {
     cfg_aliases::cfg_aliases! {
+        with_log: { feature = "log" },
         with_testing: { any(test, feature = "test") },
         with_wasmer: { feature = "wasmer" },
         with_wasmtime: { feature = "wasmtime" },

linera-witty/src/type_traits/implementations/log.rs

@@ -0,0 +1,117 @@
+// Copyright (c) Zefchain Labs, Inc.
+// SPDX-License-Identifier: Apache-2.0
+
+//! Implementations of the custom traits for types from the [`log`] crate.
+
+use std::borrow::Cow;
+
+use frunk::{hlist_pat, HList};
+use log::Level;
+
+use crate::{
+    GuestPointer, InstanceWithMemory, Layout, Memory, Runtime, RuntimeError, RuntimeMemory,
+    WitLoad, WitStore, WitType,
+};
+
+impl WitType for Level {
+    const SIZE: u32 = 1;
+
+    type Layout = HList![i8];
+    type Dependencies = HList![];
+
+    fn wit_type_name() -> Cow<'static, str> {
+        "log-level".into()
+    }
+
+    fn wit_type_declaration() -> Cow<'static, str> {
+        concat!(
+            "    enum log-level {\n",
+            "        error,\n",
+            "        warn,\n",
+            "        info,\n",
+            "        debug,\n",
+            "        trace,\n",
+            "    }\n",
+        )
+        .into()
+    }
+}
+
+impl WitLoad for Level {
+    fn load<Instance>(
+        memory: &Memory<'_, Instance>,
+        location: GuestPointer,
+    ) -> Result<Self, RuntimeError>
+    where
+        Instance: InstanceWithMemory,
+        <Instance::Runtime as Runtime>::Memory: RuntimeMemory<Instance>,
+    {
+        match u8::load(memory, location)? {
+            0 => Ok(Level::Error),
+            1 => Ok(Level::Warn),
+            2 => Ok(Level::Info),
+            3 => Ok(Level::Debug),
+            4 => Ok(Level::Trace),
+            _ => unreachable!("Invalid log level"),
+        }
+    }
+
+    fn lift_from<Instance>(
+        hlist_pat![discriminant]: <Self::Layout as Layout>::Flat,
+        _memory: &Memory<'_, Instance>,
+    ) -> Result<Self, RuntimeError>
+    where
+        Instance: InstanceWithMemory,
+        <Instance::Runtime as Runtime>::Memory: RuntimeMemory<Instance>,
+    {
+        match discriminant {
+            0 => Ok(Level::Error),
+            1 => Ok(Level::Warn),
+            2 => Ok(Level::Info),
+            3 => Ok(Level::Debug),
+            4 => Ok(Level::Trace),
+            _ => unreachable!("Invalid log level"),
+        }
+    }
+}
+
+impl WitStore for Level {
+    fn store<Instance>(
+        &self,
+        memory: &mut Memory<'_, Instance>,
+        location: GuestPointer,
+    ) -> Result<(), RuntimeError>
+    where
+        Instance: InstanceWithMemory,
+        <Instance::Runtime as Runtime>::Memory: RuntimeMemory<Instance>,
+    {
+        let discriminant: i8 = match self {
+            Level::Error => 0,
+            Level::Warn => 1,
+            Level::Info => 2,
+            Level::Debug => 3,
+            Level::Trace => 4,
+        };
+
+        discriminant.store(memory, location)
+    }
+
+    fn lower<Instance>(
+        &self,
+        memory: &mut Memory<'_, Instance>,
+    ) -> Result<<Self::Layout as Layout>::Flat, RuntimeError>
+    where
+        Instance: InstanceWithMemory,
+        <Instance::Runtime as Runtime>::Memory: RuntimeMemory<Instance>,
+    {
+        let discriminant: i8 = match self {
+            Level::Error => 0,
+            Level::Warn => 1,
+            Level::Info => 2,
+            Level::Debug => 3,
+            Level::Trace => 4,
+        };
+
+        discriminant.lower(memory)
+    }
+}

linera-witty/src/type_traits/implementations/mod.rs

@@ -5,6 +5,8 @@
 
 mod custom_types;
 mod frunk;
+#[cfg(with_log)]
+mod log;
 mod std;
 #[cfg(test)]
 mod tests;

linera-witty/src/type_traits/implementations/tests.rs

@@ -202,6 +202,17 @@ fn btree_map() {
     );
 }
 
+/// Test roundtrip of [`log::Level`].
+#[cfg(with_log)]
+#[test]
+fn log_level() {
+    use log::Level::*;
+    for (index, level) in [Error, Warn, Info, Debug, Trace].into_iter().enumerate() {
+        test_memory_roundtrip(&level, &[index as u8], &[]);
+        test_flattening_roundtrip(&level, hlist![index as i32], &[]);
+    }
+}
+
 /// Test storing an instance of `T` to memory, checking that the `layout_data` bytes followed by
 /// the `heap_data` bytes are correctly written, and check that the instance can be loaded from
 /// those bytes.