perf(parser): disambiguate single-identifier arrow heads (#28491)

Use bounded lookahead for `(identifier)` and `async(identifier)` to
avoid speculative parameter parsing, preserving TypeScript return-type
fallback.

Validated parser conformance, bindings, and allocations. AI-assisted.

crates/oxc_parser/src/js/arrow.rs

@@ -161,7 +161,18 @@ impl<'a, C: Config> ParserImpl<'a, C> {
                         }
                         Tristate::False
                     }
-                    // If we have "(a," or "(a=" or "(a)" this *could* be an arrow function
+                    // A single identifier needs no speculative parameter parse. Keep `await`,
+                    // `yield`, and `this` on the existing contextual/error-recovery paths.
+                    // A TS return type can still be ambiguous with a conditional's colon.
+                    Kind::RParen if second.is_identifier() => {
+                        match self.lexer.next_token().kind() {
+                            Kind::Arrow => Tristate::True,
+                            Kind::Colon if self.is_ts => Tristate::Maybe,
+                            _ => Tristate::False,
+                        }
+                    }
+                    // If we have "(a," or "(a=" or a context-sensitive keyword in "(a)",
+                    // this *could* be an arrow function.
                     Kind::Comma | Kind::Eq | Kind::RParen => Tristate::Maybe,
                     // It is definitely not an arrow function
                     _ => Tristate::False,

tasks/track_memory_allocations/allocs_formatter.yaml

@@ -40,7 +40,7 @@ pdf.mjs:
   sys peak growth: 1156488 # 1.16 MB
   arena allocs: 21652
   arena reallocs: 402
-  arena size: 11006048 # 11.01 MB
+  arena size: 10987960 # 10.99 MB
 
 antd.js:
   file size: 6686316 # 6.69 MB
@@ -51,7 +51,7 @@ antd.js:
   sys peak growth: 13710144 # 13.71 MB
   arena allocs: 120676
   arena reallocs: 1889
-  arena size: 114354248 # 114.35 MB
+  arena size: 114187808 # 114.19 MB
 
 binder.ts:
   file size: 193077 # 193.08 kB

tasks/track_memory_allocations/allocs_minifier.yaml

@@ -40,7 +40,7 @@ pdf.mjs:
   sys peak growth: 3693355 # 3.69 MB
   arena allocs: 19582
   arena reallocs: 2574
-  arena size: 5684472 # 5.68 MB
+  arena size: 5666384 # 5.67 MB
 
 antd.js:
   file size: 6686316 # 6.69 MB
@@ -51,7 +51,7 @@ antd.js:
   sys peak growth: 19934725 # 19.93 MB
   arena allocs: 175842
   arena reallocs: 34169
-  arena size: 43638784 # 43.64 MB
+  arena size: 43472344 # 43.47 MB
 
 binder.ts:
   file size: 193077 # 193.08 kB

tasks/track_memory_allocations/allocs_parser.yaml

@@ -33,25 +33,25 @@ RadixUIAdoptionSection.jsx:
 
 pdf.mjs:
   file size: 567296 # 567.30 kB
-  sys allocs: 67
+  sys allocs: 61
   sys reallocs: 67
-  sys deallocs: 67
+  sys deallocs: 61
   sys alloc bytes: 24020 # 24.02 kB
   sys peak growth: 12140 # 12.14 kB
-  arena allocs: 89436
+  arena allocs: 89079
   arena reallocs: 5904
-  arena size: 4510576 # 4.51 MB
+  arena size: 4492488 # 4.49 MB
 
 antd.js:
   file size: 6686316 # 6.69 MB
-  sys allocs: 185
+  sys allocs: 183
   sys reallocs: 221
-  sys deallocs: 185
-  sys alloc bytes: 109952 # 109.95 kB
-  sys peak growth: 48264 # 48.26 kB
-  arena allocs: 523271
+  sys deallocs: 183
+  sys alloc bytes: 94120 # 94.12 kB
+  sys peak growth: 38896 # 38.90 kB
+  arena allocs: 519986
   arena reallocs: 35629
-  arena size: 30069720 # 30.07 MB
+  arena size: 29903280 # 29.90 MB
 
 binder.ts:
   file size: 193077 # 193.08 kB

tasks/track_memory_allocations/allocs_semantic.yaml

@@ -40,7 +40,7 @@ pdf.mjs:
   sys peak growth: 832762 # 832.76 kB
   arena allocs: 0
   arena reallocs: 0
-  arena size: 4510576 # 4.51 MB
+  arena size: 4492488 # 4.49 MB
 
 antd.js:
   file size: 6686316 # 6.69 MB
@@ -51,7 +51,7 @@ antd.js:
   sys peak growth: 5441278 # 5.44 MB
   arena allocs: 0
   arena reallocs: 0
-  arena size: 30069720 # 30.07 MB
+  arena size: 29903280 # 29.90 MB
 
 binder.ts:
   file size: 193077 # 193.08 kB

tasks/track_memory_allocations/allocs_transformer.yaml

@@ -40,7 +40,7 @@ pdf.mjs:
   sys peak growth: 1610 # 1.61 kB
   arena allocs: 2
   arena reallocs: 0
-  arena size: 4510600 # 4.51 MB
+  arena size: 4492512 # 4.49 MB
 
 antd.js:
   file size: 6686316 # 6.69 MB
@@ -51,7 +51,7 @@ antd.js:
   sys peak growth: 1611 # 1.61 kB
   arena allocs: 2
   arena reallocs: 0
-  arena size: 30069744 # 30.07 MB
+  arena size: 29903304 # 29.90 MB
 
 binder.ts:
   file size: 193077 # 193.08 kB