Add an OOM-handling BTreeMap implementation (#12712)

* Add an OOM-handling `BTreeMap` implementation

* review feedback

* fix docs links

Author: fitzgen

Cargo.lock

@@ -508,4 +508,14 @@ impl<T> Slab<T> {
                 Entry::Free { .. } => None,
             })
     }
+
+    /// Clear all of the values from inside this slab.
+    ///
+    /// Does not deallocate internal storage.
+    #[inline]
+    pub fn clear(&mut self) {
+        self.len = 0;
+        self.free = None;
+        self.entries.clear();
+    }
 }

crates/core/src/slab.rs

@@ -23,6 +23,7 @@ postcard = { workspace = true }
 cpp_demangle = { version = "0.4.3", optional = true }
 cranelift-entity = { workspace = true, features = ['enable-serde'] }
 cranelift-bitset = { workspace = true, features = ['enable-serde'] }
+cranelift-bforest = { workspace = true }
 hashbrown = { workspace = true, features = ["default-hasher"] }
 wasmparser = { workspace = true, features = ['validate', 'serde', 'features'] }
 indexmap = { workspace = true, features = ["serde"] }
@@ -57,6 +58,10 @@ arbitrary = { workspace = true, features = ["derive"] }
 name = "factc"
 required-features = ['component-model', 'compile']
 
+[[example]]
+name = "btrees"
+required-features = ["std"]
+
 [features]
 anyhow = ["wasmtime-core/anyhow"]
 backtrace = ["std", "wasmtime-core/backtrace"]

crates/environ/Cargo.toml

@@ -0,0 +1,50 @@
+use std::{collections::BTreeMap, str::FromStr};
+use wasmtime_environ::{
+    collections,
+    error::{Context as _, Result, bail, format_err},
+};
+
+type Key = [u128; 2];
+type Value = [u8; 16];
+
+trait Map {
+    fn insert(&mut self, key: Key, value: Value) -> Result<()>;
+}
+
+impl Map for BTreeMap<Key, Value> {
+    fn insert(&mut self, key: Key, value: Value) -> Result<()> {
+        BTreeMap::insert(self, key, value);
+        Ok(())
+    }
+}
+
+impl Map for collections::BTreeMap<Key, Value> {
+    fn insert(&mut self, key: Key, value: Value) -> Result<()> {
+        collections::BTreeMap::insert(self, key, value)?;
+        Ok(())
+    }
+}
+
+fn main() -> Result<()> {
+    let kind = std::env::args()
+        .nth(1)
+        .ok_or_else(|| format_err!("must provide first argument: 'std' or 'bforest'"))?;
+
+    let mut map: Box<dyn Map> = match kind.as_str() {
+        "std" => Box::new(BTreeMap::new()),
+        "bforest" => Box::new(collections::BTreeMap::new()),
+        _ => bail!("first argument must be either 'std' or 'bforest', got: '{kind}'"),
+    };
+
+    let n = std::env::args().nth(2);
+    let n = n.as_deref().unwrap_or("1000");
+    let n = u128::from_str(n).context("failed to parse second argument as `u32` integer")?;
+
+    println!("Inserting {n} entries into `{kind}`-based `BTreeMap`...");
+
+    for i in 0..n {
+        map.insert([i, i], [0; 16])?;
+    }
+
+    Ok(())
+}

crates/environ/examples/btrees.rs

@@ -1,12 +1,14 @@
 //! Fallible, OOM-handling collections.
 
+pub mod btree_map;
 mod entity_set;
 mod hash_map;
 mod hash_set;
 mod index_map;
 mod primary_map;
 mod secondary_map;
 
+pub use btree_map::BTreeMap;
 pub use entity_set::EntitySet;
 pub use hash_map::HashMap;
 pub use hash_set::HashSet;