Optimizing AST Traversal with Adaptive Iteration (#5754)

## Description
Implements a new `adaptive_iter` method for use when traversing the
ASTs. If the size of the collection is less than 8, then regular
sequential iteration is used, otherwise a rayon parallel iterator is
used. I tested all values between 0-30 and found that 8 to be the sweet
spot. Less than this we don't see any benefit as the overhead for
spawning rayon threads isn't worth it.

This also fixes a stackoverlow error I noticed below. With this change,
we aren't overloading the rayon thread stack size when the recursion
level is quite deep.

```
thread '<unknown>' has overflowed its stack
fatal runtime error: stack overflow
error: test failed, to rerun pass `--test lib`

Caused by:
  process didn't exit successfully: `/Users/josh/Documents/rust/fuel/sway/target/debug/deps/lib-6ef1da655cf2d895 pedro --nocapture` (signal: 6, SIGABRT: process abort signal)
```

Author: JoshuaBatty

Cargo.lock

@@ -24,6 +24,7 @@ parking_lot = "0.12.1"
 proc-macro2 = "1.0.5"
 quote = "1.0.9"
 rayon = "1.5.0"
+rayon-cond = "0.3"
 ropey = "1.2"
 serde = { version = "1.0", features = ["derive"] }
 serde_json = "1.0.60"

sway-lsp/Cargo.toml

@@ -1,8 +1,8 @@
 use crate::{
     core::token::{AstToken, SymbolKind, Token},
-    traverse::{Parse, ParseContext},
+    traverse::{adaptive_iter, Parse, ParseContext},
 };
-use rayon::iter::{IntoParallelRefIterator, ParallelBridge, ParallelIterator};
+use rayon::iter::{ParallelBridge, ParallelIterator};
 use sway_ast::{
     expr::LoopControlFlow, ty::TyTupleDescriptor, Assignable, CodeBlockContents, ConfigurableField,
     Expr, ExprArrayDescriptor, ExprStructField, ExprTupleDescriptor, FnArg, FnArgs, FnSignature,
@@ -16,23 +16,18 @@ use sway_types::{Ident, Span, Spanned};
 
 pub fn parse(lexed_program: &LexedProgram, ctx: &ParseContext) {
     insert_module_kind(ctx, &lexed_program.root.tree.kind);
-    lexed_program
-        .root
-        .tree
-        .items
-        .par_iter()
-        .for_each(|item| item.value.parse(ctx));
+    adaptive_iter(&lexed_program.root.tree.items, |item| {
+        item.value.parse(ctx);
+    });
 
     lexed_program
         .root
         .submodules_recursive()
         .for_each(|(_, dep)| {
             insert_module_kind(ctx, &dep.module.tree.kind);
-            dep.module
-                .tree
-                .items
-                .par_iter()
-                .for_each(|item| item.value.parse(ctx));
+            adaptive_iter(&dep.module.tree.items, |item| {
+                item.value.parse(ctx);
+            });
         });
 }
 
@@ -157,7 +152,7 @@ impl Parse for Expr {
             } => {
                 insert_keyword(ctx, match_token.span());
                 value.parse(ctx);
-                branches.get().iter().par_bridge().for_each(|branch| {
+                adaptive_iter(branches.get(), |branch| {
                     branch.pattern.parse(ctx);
                     branch.kind.parse(ctx);
                 });
@@ -326,21 +321,15 @@ impl Parse for ItemTrait {
             insert_keyword(ctx, where_clause_opt.where_token.span());
         }
 
-        self.trait_items
-            .get()
-            .par_iter()
-            .for_each(|annotated| match &annotated.value {
-                sway_ast::ItemTraitItem::Fn(fn_sig, _) => fn_sig.parse(ctx),
-                sway_ast::ItemTraitItem::Const(item_const, _) => item_const.parse(ctx),
-                sway_ast::ItemTraitItem::Type(item_type, _) => item_type.parse(ctx),
-                sway_ast::ItemTraitItem::Error(_, _) => {}
-            });
+        adaptive_iter(self.trait_items.get(), |annotated| match &annotated.value {
+            sway_ast::ItemTraitItem::Fn(fn_sig, _) => fn_sig.parse(ctx),
+            sway_ast::ItemTraitItem::Const(item_const, _) => item_const.parse(ctx),
+            sway_ast::ItemTraitItem::Type(item_type, _) => item_type.parse(ctx),
+            sway_ast::ItemTraitItem::Error(_, _) => {}
+        });
 
         if let Some(trait_defs_opt) = &self.trait_defs_opt {
-            trait_defs_opt
-                .get()
-                .par_iter()
-                .for_each(|item| item.value.parse(ctx));
+            adaptive_iter(trait_defs_opt.get(), |item| item.value.parse(ctx));
         }
     }
 }
@@ -359,36 +348,27 @@ impl Parse for ItemImpl {
             insert_keyword(ctx, where_clause_opt.where_token.span());
         }
 
-        self.contents
-            .get()
-            .par_iter()
-            .for_each(|item| match &item.value {
-                ItemImplItem::Fn(fn_decl) => fn_decl.parse(ctx),
-                ItemImplItem::Const(const_decl) => const_decl.parse(ctx),
-                ItemImplItem::Type(type_decl) => type_decl.parse(ctx),
-            });
+        adaptive_iter(self.contents.get(), |item| match &item.value {
+            ItemImplItem::Fn(fn_decl) => fn_decl.parse(ctx),
+            ItemImplItem::Const(const_decl) => const_decl.parse(ctx),
+            ItemImplItem::Type(type_decl) => type_decl.parse(ctx),
+        });
     }
 }
 
 impl Parse for ItemAbi {
     fn parse(&self, ctx: &ParseContext) {
         insert_keyword(ctx, self.abi_token.span());
 
-        self.abi_items
-            .get()
-            .par_iter()
-            .for_each(|annotated| match &annotated.value {
-                sway_ast::ItemTraitItem::Fn(fn_sig, _) => fn_sig.parse(ctx),
-                sway_ast::ItemTraitItem::Const(item_const, _) => item_const.parse(ctx),
-                sway_ast::ItemTraitItem::Type(item_type, _) => item_type.parse(ctx),
-                sway_ast::ItemTraitItem::Error(_, _) => {}
-            });
+        adaptive_iter(self.abi_items.get(), |annotated| match &annotated.value {
+            sway_ast::ItemTraitItem::Fn(fn_sig, _) => fn_sig.parse(ctx),
+            sway_ast::ItemTraitItem::Const(item_const, _) => item_const.parse(ctx),
+            sway_ast::ItemTraitItem::Type(item_type, _) => item_type.parse(ctx),
+            sway_ast::ItemTraitItem::Error(_, _) => {}
+        });
 
         if let Some(abi_defs_opt) = self.abi_defs_opt.as_ref() {
-            abi_defs_opt
-                .get()
-                .par_iter()
-                .for_each(|item| item.value.parse(ctx));
+            adaptive_iter(abi_defs_opt.get(), |item| item.value.parse(ctx));
         }
     }
 }
@@ -576,9 +556,9 @@ impl Parse for FnArg {
 
 impl Parse for CodeBlockContents {
     fn parse(&self, ctx: &ParseContext) {
-        self.statements
-            .par_iter()
-            .for_each(|statement| statement.parse(ctx));
+        adaptive_iter(&self.statements, |statement| {
+            statement.parse(ctx);
+        });
         if let Some(expr) = self.final_expr_opt.as_ref() {
             expr.parse(ctx);
         }

sway-lsp/src/traverse/lexed_tree.rs

@@ -1,4 +1,5 @@
 use crate::core::{token::TokenIdent, token_map::TokenMap};
+use rayon_cond::CondIterator;
 use sway_core::{namespace::Module, Engines};
 
 pub(crate) mod dependency;
@@ -30,3 +31,19 @@ impl<'a> ParseContext<'a> {
 pub trait Parse {
     fn parse(&self, ctx: &ParseContext);
 }
+
+/// Determines the threshold a collection must meet to be processed in parallel.
+const PARALLEL_THRESHOLD: usize = 8;
+
+/// Iterates over items, choosing parallel or sequential execution based on size.
+pub fn adaptive_iter<T, F>(items: &[T], action: F)
+where
+    T: Sync + Send,          // Required for parallel processing
+    F: Fn(&T) + Sync + Send, // Action to be applied to each item
+{
+    // Determine if the length meets the parallel threshold
+    let use_parallel = items.len() >= PARALLEL_THRESHOLD;
+
+    // Create a conditional iterator based on the use_parallel flag
+    CondIterator::new(items, use_parallel).for_each(action);
+}