Implement custom string interner

boa_interner/Cargo.toml

@@ -14,3 +14,5 @@ license = "Unlicense/MIT"
 string-interner = "0.14.0"
 serde = { version = "1.0.137", features = ["derive"], optional = true }
 gc = { version = "0.4.1", features = ["derive"] }
+phf = { version = "0.10.1", features = ["macros"] }
+rustc-hash = "1.1.0"

boa_interner/src/interned_str.rs

@@ -0,0 +1,70 @@
+use std::{borrow::Borrow, ptr::NonNull};
+
+/// Wrapper for an interned str pointer, required to
+/// quickly check using a hash if a string is inside an [`Interner`].
+///
+/// # Safety
+///
+/// This struct could cause Undefined Behaviour on:
+/// - Use without ensuring the referenced memory is still allocated.
+/// - Construction of an [`InternedStr`] from an invalid [`NonNull<str>`].
+///
+/// In general, this should not be used outside of an [`Interner`].
+#[derive(Debug, Clone)]
+pub(super) struct InternedStr {
+    ptr: NonNull<str>,
+}
+
+impl InternedStr {
+    /// Create a new interned string from the given `str`.
+    ///
+    /// # Safety
+    ///
+    /// Not maintaining the invariants specified on the struct definition
+    /// could cause Undefined Behaviour.
+    #[inline]
+    pub(super) unsafe fn new(ptr: NonNull<str>) -> Self {
+        Self { ptr }
+    }
+
+    /// Returns a shared reference to the underlying string.
+    ///
+    /// # Safety
+    ///
+    /// Not maintaining the invariants specified on the struct definition
+    /// could cause Undefined Behaviour.
+    #[inline]
+    pub(super) unsafe fn as_str(&self) -> &str {
+        // SAFETY: The user must verify the invariants
+        // specified on the struct definition.
+        unsafe { self.ptr.as_ref() }
+    }
+}
+
+impl std::hash::Hash for InternedStr {
+    fn hash<H: std::hash::Hasher>(&self, state: &mut H) {
+        // SAFETY: The user must verify the invariants
+        // specified in the struct definition.
+        unsafe {
+            self.as_str().hash(state);
+        }
+    }
+}
+
+impl Eq for InternedStr {}
+
+impl PartialEq for InternedStr {
+    fn eq(&self, other: &Self) -> bool {
+        // SAFETY: The user must verify the invariants
+        // specified in the struct definition.
+        unsafe { self.as_str() == other.as_str() }
+    }
+}
+
+impl Borrow<str> for InternedStr {
+    fn borrow(&self) -> &str {
+        // SAFETY: The user must verify the invariants
+        // specified in the struct definition.
+        unsafe { self.as_str() }
+    }
+}