Keep Parmatch's fresh pattern names per compiler request

Concurrent requests shared the counter and could reuse a name within one
exhaustiveness check.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Christoph Knittel <ck@cca.io>

Author: cknitt

compiler/ext/compiler_request_state.ml

@@ -7,6 +7,7 @@ type t = {
   mutable lambda_raise_count: int;
   mutable lambda_negative_raise_count: int;
   mutable builtin_ppx_local_module_counter: int;
+  mutable parmatch_name_counter: int;
   mutable substitution_saved_id: int;
   mutable type_node_id: int;
   mutable type_node_reset_id: int option;
@@ -26,6 +27,7 @@ let create ~active cwd =
     lambda_raise_count = 0;
     lambda_negative_raise_count = 0;
     builtin_ppx_local_module_counter = 0;
+    parmatch_name_counter = 0;
     substitution_saved_id = -1;
     type_node_id = -1;
     type_node_reset_id = None;

compiler/ext/compiler_request_state.mli

@@ -7,6 +7,7 @@ type t = {
   mutable lambda_raise_count: int;
   mutable lambda_negative_raise_count: int;
   mutable builtin_ppx_local_module_counter: int;
+  mutable parmatch_name_counter: int;
   mutable substitution_saved_id: int;
   mutable type_node_id: int;
   mutable type_node_reset_id: int option;

compiler/ml/parmatch.ml

@@ -1872,10 +1872,10 @@ module Conv = struct
   open Parsetree
   let mkpat desc = Ast_helper.Pat.mk desc
 
-  let name_counter = ref 0
   let fresh name =
-    let current = !name_counter in
-    name_counter := !name_counter + 1;
+    let state = Compiler_request_state.current () in
+    let current = state.parmatch_name_counter in
+    state.parmatch_name_counter <- current + 1;
     "#$" ^ name ^ string_of_int current
 
   let conv_record_rest (rest : Typedtree.record_pat_rest) =