feat(semantic): expose Stats (#5755)

Expose `Stats` type, and provide a method `Semantic::Stats` to obtain them.

Note: If we move counting into parser later, parser can use this same `Stats` type (we'd need to move it into `oxc_parser` or `oxc_ast` crate, but `oxc_semantic` can still re-export it.

crates/oxc_semantic/src/builder.rs

@@ -244,12 +244,12 @@ impl<'a> SemanticBuilder<'a> {
             #[cfg(debug_assertions)]
             {
                 #[allow(clippy::cast_possible_truncation)]
-                let actual_stats = Stats {
-                    nodes: self.nodes.len() as u32,
-                    scopes: self.scope.len() as u32,
-                    symbols: self.symbols.len() as u32,
-                    references: self.symbols.references.len() as u32,
-                };
+                let actual_stats = Stats::new(
+                    self.nodes.len() as u32,
+                    self.scope.len() as u32,
+                    self.symbols.len() as u32,
+                    self.symbols.references.len() as u32,
+                );
                 Stats::assert_accurate(&actual_stats, &stats);
             }
 

crates/oxc_semantic/src/lib.rs

@@ -40,6 +40,7 @@ pub use crate::{
     node::{AstNode, AstNodes, NodeId},
     reference::{Reference, ReferenceFlags, ReferenceId},
     scope::ScopeTree,
+    stats::Stats,
     symbol::{IsGlobalReference, SymbolTable},
 };
 use class::ClassTable;
@@ -162,6 +163,17 @@ impl<'a> Semantic<'a> {
         self.cfg.as_ref()
     }
 
+    /// Get statistics about data held in `Semantic`.
+    pub fn stats(&self) -> Stats {
+        #[allow(clippy::cast_possible_truncation)]
+        Stats::new(
+            self.nodes.len() as u32,
+            self.scopes.len() as u32,
+            self.symbols.len() as u32,
+            self.symbols.references.len() as u32,
+        )
+    }
+
     pub fn is_unresolved_reference(&self, node_id: NodeId) -> bool {
         let reference_node = self.nodes.get_node(node_id);
         let AstKind::IdentifierReference(id) = reference_node.kind() else {

crates/oxc_semantic/src/stats.rs

@@ -1,7 +1,3 @@
-//! Visitor to count nodes, scopes, symbols and references in AST.
-//! These counts can be used to pre-allocate sufficient capacity in `AstNodes`,
-//! `ScopeTree`, and `SymbolTable` to store info for all these items.
-
 use std::cell::Cell;
 
 use oxc_ast::{
@@ -13,22 +9,67 @@ use oxc_ast::{
 };
 use oxc_syntax::scope::{ScopeFlags, ScopeId};
 
+/// Statistics about data held in [`Semantic`].
+///
+/// Comprises number of AST nodes, scopes, symbols, and references.
+///
+/// These counts can be used to pre-allocate sufficient capacity in `AstNodes`,
+/// `ScopeTree`, and `SymbolTable` to store info for all these items.
+///
+/// * Obtain `Stats` from an existing [`Semantic`] with [`Semantic::stats`].
+/// * Use [`Stats::count`] to visit AST and obtain accurate counts.
+///
+/// # Example
+/// ```
+/// use oxc_ast::ast::Program;
+/// use oxc_semantic::{Semantic, Stats};
+///
+/// fn print_stats_from_semantic(semantic: &Semantic) {
+///     dbg!(semantic.stats());
+/// }
+///
+/// fn print_stats_from_ast(program: &Program) {
+///     dbg!(Stats::count(program));
+/// }
+/// ```
+///
+/// [`Semantic`]: super::Semantic
+/// [`Semantic::stats`]: super::Semantic::stats
 #[derive(Default, Debug)]
-pub(crate) struct Stats {
+pub struct Stats {
     pub nodes: u32,
     pub scopes: u32,
     pub symbols: u32,
     pub references: u32,
 }
 
 impl Stats {
+    /// Create new [`Stats`] from specified counts.
+    pub fn new(nodes: u32, scopes: u32, symbols: u32, references: u32) -> Self {
+        Stats { nodes, scopes, symbols, references }
+    }
+
+    /// Gather [`Stats`] by visiting AST and counting nodes, scopes, symbols, and references.
+    ///
+    /// Nodes, scopes and references counts will be exactly accurate.
+    /// Symbols count may be an over-estimate if there are multiple declarations for a single symbol.
+    /// e.g. `var x; var x;` will produce a count of 2 symbols, but this is actually only 1 symbol.
+    ///
+    /// If semantic analysis has already been run on AST, prefer getting counts with [`Semantic::stats`].
+    /// They will be 100% accurate, and very cheap to obtain, whereas this method performs a complete
+    /// AST traversal.
+    ///
+    /// [`Semantic::stats`]: super::Semantic::stats
     pub fn count(program: &Program) -> Self {
-        let mut stats = Stats::default();
-        stats.visit_program(program);
-        stats
+        let mut counter = Counter::default();
+        counter.visit_program(program);
+        counter.stats
     }
 
-    #[cfg_attr(not(debug_assertions), expect(dead_code))]
+    /// Check that estimated [`Stats`] match actual.
+    ///
+    /// # Panics
+    /// Panics if stats are not accurate.
     pub fn assert_accurate(actual: &Self, estimated: &Self) {
         assert_eq!(actual.nodes, estimated.nodes, "nodes count mismatch");
         assert_eq!(actual.scopes, estimated.scopes, "scopes count mismatch");
@@ -48,40 +89,46 @@ impl Stats {
     }
 }
 
-impl<'a> Visit<'a> for Stats {
+#[derive(Default)]
+struct Counter {
+    stats: Stats,
+}
+
+/// Visitor to count nodes, scopes, symbols and references in AST
+impl<'a> Visit<'a> for Counter {
     #[inline]
     fn enter_node(&mut self, _: AstKind<'a>) {
-        self.nodes += 1;
+        self.stats.nodes += 1;
     }
 
     #[inline]
     fn enter_scope(&mut self, _: ScopeFlags, _: &Cell<Option<ScopeId>>) {
-        self.scopes += 1;
+        self.stats.scopes += 1;
     }
 
     #[inline]
     fn visit_binding_identifier(&mut self, _: &BindingIdentifier<'a>) {
-        self.nodes += 1;
-        self.symbols += 1;
+        self.stats.nodes += 1;
+        self.stats.symbols += 1;
     }
 
     #[inline]
     fn visit_identifier_reference(&mut self, _: &IdentifierReference<'a>) {
-        self.nodes += 1;
-        self.references += 1;
+        self.stats.nodes += 1;
+        self.stats.references += 1;
     }
 
     #[inline]
     fn visit_ts_enum_member_name(&mut self, it: &TSEnumMemberName<'a>) {
         if !it.is_expression() {
-            self.symbols += 1;
+            self.stats.symbols += 1;
         }
         walk_ts_enum_member_name(self, it);
     }
 
     #[inline]
     fn visit_ts_module_declaration_name(&mut self, it: &TSModuleDeclarationName<'a>) {
-        self.symbols += 1;
+        self.stats.symbols += 1;
         walk_ts_module_declaration_name(self, it);
     }
 }