highlighter: Represent Highlight as a non-max u32

`u32::MAX` is reserved for the null pointer space optimization:

    size_of::<Option<Highlight>>() == size_of::<Highlight>()

Unfortunately this kind of type is not available in std so we must roll
our own with bit-wise xor, which is an involution when used with a
constant like `u32::MAX`.

We could expose an interface like `NonZeroU32` and have `new` return an
`Option<Self>`. It is unlikely that a caller would ever pass `u32::MAX`
though so an assertion seems appropriate instead.

Author: the-mikedavis

highlighter/src/config.rs

@@ -51,7 +51,7 @@ impl LanguageConfig {
         })
     }
 
-    pub fn configure(&self, mut f: impl FnMut(&str) -> Highlight) {
+    pub fn configure(&self, mut f: impl FnMut(&str) -> Option<Highlight>) {
         self.highlight_query.configure(&mut f);
         self.injection_query.configure(&mut f);
     }

highlighter/src/highlighter.rs

@@ -1,5 +1,6 @@
 use std::borrow::Cow;
 use std::mem::replace;
+use std::num::NonZeroU32;
 use std::ops::RangeBounds;
 use std::path::Path;
 use std::slice;
@@ -24,7 +25,7 @@ use tree_sitter::{Pattern, QueryMatch};
 #[derive(Debug)]
 pub struct HighlightQuery {
     pub query: Query,
-    highlight_indices: ArcSwap<Vec<Highlight>>,
+    highlight_indices: ArcSwap<Vec<Option<Highlight>>>,
     #[allow(dead_code)]
     /// Patterns that do not match when the node is a local.
     non_local_patterns: HashSet<Pattern>,
@@ -86,10 +87,7 @@ impl HighlightQuery {
         }
 
         Ok(Self {
-            highlight_indices: ArcSwap::from_pointee(vec![
-                Highlight::NONE;
-                query.num_captures() as usize
-            ]),
+            highlight_indices: ArcSwap::from_pointee(vec![None; query.num_captures() as usize]),
             non_local_patterns,
             local_reference_capture: query.get_capture("local.reference"),
             query,
@@ -112,7 +110,7 @@ impl HighlightQuery {
     /// When highlighting, results are returned as `Highlight` values, configured by this function.
     /// The meaning of these indices is up to the user of the implementation. The highlighter
     /// treats the indices as entirely opaque.
-    pub(crate) fn configure(&self, f: &mut impl FnMut(&str) -> Highlight) {
+    pub(crate) fn configure(&self, f: &mut impl FnMut(&str) -> Option<Highlight>) {
         let highlight_indices = self
             .query
             .captures()
@@ -123,11 +121,26 @@ impl HighlightQuery {
 }
 
 /// Indicates which highlight should be applied to a region of source code.
+///
+/// This type is represented as a non-max u32 - a u32 which cannot be `u32::MAX`. This is checked
+/// at runtime with assertions in `Highlight::new`.
 #[derive(Copy, Clone, Debug, PartialEq, Eq)]
-pub struct Highlight(pub u32);
+pub struct Highlight(NonZeroU32);
 
 impl Highlight {
-    pub const NONE: Highlight = Highlight(u32::MAX);
+    pub const fn new(inner: u32) -> Self {
+        assert!(inner != u32::MAX);
+        // SAFETY: must be non-zero because `inner` is not `u32::MAX`.
+        Self(unsafe { NonZeroU32::new_unchecked(inner ^ u32::MAX) })
+    }
+
+    const fn get(&self) -> u32 {
+        self.0.get() ^ u32::MAX
+    }
+
+    pub const fn idx(&self) -> usize {
+        self.get() as usize
+    }
 }
 
 #[derive(Debug)]
@@ -340,7 +353,6 @@ impl<'a, 'tree: 'a, Loader: LanguageLoader> Highlighter<'a, 'tree, Loader> {
                 .load()
                 .get(&capture)
                 .copied()
-                .unwrap_or(Highlight::NONE)
         } else {
             config.highlight_query.highlight_indices.load()[node.capture.idx()]
         };
@@ -355,7 +367,7 @@ impl<'a, 'tree: 'a, Loader: LanguageLoader> Highlighter<'a, 'tree, Loader> {
         {
             self.active_highlights.pop();
         }
-        if highlight != Highlight::NONE {
+        if let Some(highlight) = highlight {
             self.active_highlights.push(HighlightedNode {
                 end: range.end,
                 highlight,

highlighter/src/injections_query.rs

@@ -196,13 +196,13 @@ impl InjectionsQuery {
         })
     }
 
-    pub(crate) fn configure(&self, f: &mut impl FnMut(&str) -> Highlight) {
+    pub(crate) fn configure(&self, f: &mut impl FnMut(&str) -> Option<Highlight>) {
         let local_definition_captures = self
             .local_query
             .captures()
             .filter_map(|(capture, name)| {
                 let suffix = name.strip_prefix("local.definition.")?;
-                Some((capture, f(suffix)))
+                Some((capture, f(suffix)?))
             })
             .collect();
         self.local_definition_captures

highlighter/src/tests.rs

@@ -179,7 +179,9 @@ impl LanguageLoader for TestLanguageLoader {
                 &self.overwrites[lang.idx()],
             );
             let mut theme = self.test_theme.borrow_mut();
-            config.configure(|scope| Highlight(theme.insert_full(scope.to_owned()).0 as u32));
+            config.configure(|scope| {
+                Some(Highlight::new(theme.insert_full(scope.to_owned()).0 as u32))
+            });
             config
         });
         Some(config)
@@ -206,7 +208,7 @@ fn highlight_fixture(loader: &TestLanguageLoader, fixture: impl AsRef<Path>) {
         "// ",
         lang,
         loader,
-        |highlight| loader.test_theme.borrow()[highlight.0 as usize].clone(),
+        |highlight| loader.test_theme.borrow()[highlight.idx()].clone(),
         |_| ..,
     )
 }