Skip StmtId entries in execution graph when not debugging (#1328)

This change is an optimization that configures the lowerer to know when
debugging is the intended scenario for the lowered FIR. When not
debugging, no `StmtId` nodes are included in the execution graph, which
skips the (minimal but non-zero) overhead of checking each statement for
possible breakpoints. Only the debugger creates the interpreter and the
underlying lowerer with these debug entries enabled, preserving behavior
for those scenarios.

compiler/qsc/src/interpret.rs

@@ -144,6 +144,44 @@ impl Interpreter {
         package_type: PackageType,
         capabilities: RuntimeCapabilityFlags,
         language_features: LanguageFeatures,
+    ) -> std::result::Result<Self, Vec<Error>> {
+        Self::new_internal(
+            false,
+            std,
+            sources,
+            package_type,
+            capabilities,
+            language_features,
+        )
+    }
+
+    /// Creates a new incremental compiler with debugging stmts enabled, compiling the passed in sources.
+    /// # Errors
+    /// If compiling the sources fails, compiler errors are returned.
+    pub fn new_with_debug(
+        std: bool,
+        sources: SourceMap,
+        package_type: PackageType,
+        capabilities: RuntimeCapabilityFlags,
+        language_features: LanguageFeatures,
+    ) -> std::result::Result<Self, Vec<Error>> {
+        Self::new_internal(
+            true,
+            std,
+            sources,
+            package_type,
+            capabilities,
+            language_features,
+        )
+    }
+
+    fn new_internal(
+        dbg: bool,
+        std: bool,
+        sources: SourceMap,
+        package_type: PackageType,
+        capabilities: RuntimeCapabilityFlags,
+        language_features: LanguageFeatures,
     ) -> std::result::Result<Self, Vec<Error>> {
         let compiler = Compiler::new(std, sources, package_type, capabilities, language_features)
             .map_err(into_errors)?;
@@ -152,7 +190,9 @@ impl Interpreter {
         for (id, unit) in compiler.package_store() {
             fir_store.insert(
                 map_hir_package_to_fir(id),
-                qsc_eval::lower::Lowerer::new().lower_package(&unit.package),
+                qsc_eval::lower::Lowerer::new()
+                    .with_debug(dbg)
+                    .lower_package(&unit.package),
             );
         }
 
@@ -163,7 +203,7 @@ impl Interpreter {
             lines: 0,
             capabilities,
             fir_store,
-            lowerer: qsc_eval::lower::Lowerer::new(),
+            lowerer: qsc_eval::lower::Lowerer::new().with_debug(dbg),
             env: Env::default(),
             sim: BackendChain::new(
                 SparseSim::new(),
@@ -490,7 +530,7 @@ impl Debugger {
         position_encoding: Encoding,
         language_features: LanguageFeatures,
     ) -> std::result::Result<Self, Vec<Error>> {
-        let interpreter = Interpreter::new(
+        let interpreter = Interpreter::new_with_debug(
             true,
             sources,
             PackageType::Exe,

compiler/qsc_eval/src/lower.rs

@@ -21,6 +21,7 @@ pub struct Lowerer {
     blocks: IndexMap<BlockId, Block>,
     assigner: Assigner,
     exec_graph: Vec<ExecGraphNode>,
+    enable_debug: bool,
 }
 
 impl Default for Lowerer {
@@ -41,9 +42,16 @@ impl Lowerer {
             blocks: IndexMap::new(),
             assigner: Assigner::new(),
             exec_graph: Vec::new(),
+            enable_debug: false,
         }
     }
 
+    #[must_use]
+    pub fn with_debug(mut self, dbg: bool) -> Self {
+        self.enable_debug = dbg;
+        self
+    }
+
     pub fn take_exec_graph(&mut self) -> Vec<ExecGraphNode> {
         self.exec_graph
             .drain(..)
@@ -281,7 +289,9 @@ impl Lowerer {
     fn lower_stmt(&mut self, stmt: &hir::Stmt) -> fir::StmtId {
         let id = self.assigner.next_stmt();
         let graph_start_idx = self.exec_graph.len();
-        self.exec_graph.push(ExecGraphNode::Stmt(id));
+        if self.enable_debug {
+            self.exec_graph.push(ExecGraphNode::Stmt(id));
+        }
         let kind = match &stmt.kind {
             hir::StmtKind::Expr(expr) => fir::StmtKind::Expr(self.lower_expr(expr)),
             hir::StmtKind::Item(item) => fir::StmtKind::Item(lower_local_item_id(*item)),