Add a SmallVec data structure that saves a list on the stack

Cargo.lock

@@ -3193,6 +3193,7 @@ dependencies = [
  "hashbrown 0.13.2",
  "im",
  "im-rc",
+ "smallvec",
  "wyhash",
 ]
 

Cargo.toml

@@ -146,6 +146,7 @@ serde-xml-rs = "0.6.0"
 serde_json = "1.0.94" # update roc_std/Cargo.toml on change
 serial_test = "1.0.0"
 signal-hook = "0.3.15"
+smallvec = { version = "1.10.0", features = ["const_generics", "const_new"] }
 snafu = { version = "0.7.4", features = ["backtraces"] }
 static_assertions = "1.1.0" # update roc_std/Cargo.toml on change
 strip-ansi-escapes = "0.1.1"

crates/compiler/collections/Cargo.toml

@@ -15,3 +15,4 @@ hashbrown.workspace = true
 im-rc.workspace = true
 im.workspace = true
 wyhash.workspace = true
+smallvec.workspace = true

crates/compiler/collections/src/lib.rs

@@ -6,6 +6,7 @@
 pub mod all;
 mod reference_matrix;
 mod small_string_interner;
+mod small_vec;
 pub mod soa;
 mod vec_map;
 mod vec_set;
@@ -13,5 +14,6 @@ mod vec_set;
 pub use all::{default_hasher, BumpMap, ImEntry, ImMap, ImSet, MutMap, MutSet, SendMap};
 pub use reference_matrix::{ReferenceMatrix, Sccs, TopologicalSort};
 pub use small_string_interner::SmallStringInterner;
+pub use small_vec::SmallVec;
 pub use vec_map::VecMap;
 pub use vec_set::VecSet;

crates/compiler/collections/src/small_vec.rs

@@ -0,0 +1,61 @@
+use std::fmt::Debug;
+
+use smallvec::SmallVec as Vec;
+
+#[derive(Default)]
+pub struct SmallVec<T, const N: usize>(Vec<[T; N]>);
+
+impl<T, const N: usize> SmallVec<T, N> {
+    pub const fn new() -> Self {
+        Self(Vec::new_const())
+    }
+
+    pub fn push(&mut self, value: T) {
+        self.0.push(value)
+    }
+
+    pub fn pop(&mut self) -> Option<T> {
+        self.0.pop()
+    }
+}
+
+impl<T, const N: usize> std::ops::Deref for SmallVec<T, N> {
+    type Target = [T];
+
+    fn deref(&self) -> &Self::Target {
+        self.0.deref()
+    }
+}
+
+impl<T, const N: usize> Debug for SmallVec<T, N>
+where
+    T: Debug,
+{
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        self.0.fmt(f)
+    }
+}
+
+impl<T, const N: usize> Clone for SmallVec<T, N>
+where
+    T: Clone,
+{
+    fn clone(&self) -> Self {
+        Self(self.0.clone())
+    }
+}
+
+impl<T, const N: usize> IntoIterator for SmallVec<T, N> {
+    type Item = T;
+    type IntoIter = <Vec<[T; N]> as IntoIterator>::IntoIter;
+
+    fn into_iter(self) -> Self::IntoIter {
+        self.0.into_iter()
+    }
+}
+
+impl<T, const N: usize> FromIterator<T> for SmallVec<T, N> {
+    fn from_iter<I: IntoIterator<Item = T>>(iter: I) -> Self {
+        Self(Vec::from_iter(iter))
+    }
+}