Add BACKEND_WASM and enable CpsBoxing pass for WasmGC target

Add wasm_output datatype (WASM_BINARY/WASM_TEXT) and BACKEND_WASM
constructor to the backend type. Define WasmGC TargetInfo (I32/W32/
STRING8/32-bit). Add --wasm and --wasm-wat CLI flags. Enable
CpsBoxing pass conditionally for BACKEND_WASM and disable CPS type
checks after boxing. Update all backend pattern matches for
exhaustiveness (nested.sml, main.sml emit/exportAsRecord/pathMap).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: minoki

src/backend.sml

@@ -6,17 +6,20 @@ structure Backend:
 sig
   datatype lua_runtime = LUA_PLAIN | LUA_CONTINUATIONS
   datatype code_style = DIRECT_STYLE | CPS
+  datatype wasm_output = WASM_BINARY | WASM_TEXT
   datatype backend =
     BACKEND_LUA of lua_runtime
   | BACKEND_LUAJIT
   | BACKEND_JS of {style: code_style, os: string, default_ext: string}
+  | BACKEND_WASM of {output: wasm_output}
 end =
 struct
   (*
    * Our possible TARGET_OS:
    *   * lua
    *   * node
    *   * web
+   *   * wasm
    *
    * MLton's TARGET_OS:
    *   * aix
@@ -36,8 +39,10 @@ struct
     LUA_PLAIN
   | LUA_CONTINUATIONS
   datatype code_style = DIRECT_STYLE | CPS
+  datatype wasm_output = WASM_BINARY | WASM_TEXT
   datatype backend =
     BACKEND_LUA of lua_runtime
   | BACKEND_LUAJIT
   | BACKEND_JS of {style: code_style, os: string, default_ext: string}
+  | BACKEND_WASM of {output: wasm_output}
 end;

src/main.sml

@@ -40,6 +40,8 @@ struct
         \  --nodejs-cps          Produce JavaScript code for Node.js (CPS mode).\n\
         \  --webjs               Produce JavaScript code for Web.\n\
         \  --webjs-cps           Produce JavaScript code for Web (CPS mode).\n\
+        \  --wasm                Produce WebAssembly binary (.wasm).\n\
+        \  --wasm-wat            Produce WebAssembly text format (.wat).\n\
         \  -h,--help             Show this message.\n\
         \  -v,--version          Show version information.\n\
         \  --dump                Dump intermediate code.\n\
@@ -94,6 +96,14 @@ struct
          , minInt = SOME TargetInfo.minInt54
          , maxInt = SOME TargetInfo.maxInt54
          , wordSize = 32
+         }
+     | BACKEND_WASM _ =>
+         { defaultInt = Primitives.I32
+         , defaultWord = Primitives.W32
+         , datatypeTag = TargetInfo.STRING8
+         , minInt = SOME TargetInfo.minInt32
+         , maxInt = SOME TargetInfo.maxInt32
+         , wordSize = 32
          })
   fun optimizeCps
         (_: {nextTyVar: int ref, nextVId: int ref, printTimings: bool}) _ cexp 0 =
@@ -361,6 +371,8 @@ struct
         in
           ()
         end
+    | emit ({backend = BACKEND_WASM _, ...}: options) _ _ _ _ _ _ =
+        raise Fail "Wasm code generation is not yet implemented"
   structure CheckFInit =
   struct
     fun toFTy (TypedSyntax.TyVar (_, tv)) = FSyntax.TyVar tv
@@ -456,12 +468,14 @@ struct
             | BACKEND_LUAJIT => "luajit"
             | BACKEND_JS {style = Backend.DIRECT_STYLE, ...} => "js"
             | BACKEND_JS {style = Backend.CPS, ...} => "js-cps"
+            | BACKEND_WASM _ => "wasm"
           )
         , ( "TARGET_OS"
           , case #backend opts of
               BACKEND_LUA _ => "lua"
             | BACKEND_LUAJIT => "lua"
             | BACKEND_JS {os, ...} => os
+            | BACKEND_WASM _ => "wasm"
           )
         , ( "DELIMITED_CONTINUATIONS"
           , case #backend opts of
@@ -718,6 +732,7 @@ struct
         | BACKEND_LUAJIT => true
         | BACKEND_JS {style = Backend.DIRECT_STYLE, ...} => false
         | BACKEND_JS {style = Backend.CPS, ...} => true
+        | BACKEND_WASM _ => true
       val fexp =
         case targetInfo of
           {defaultInt = Primitives.INT, defaultWord = Primitives.WORD, ...} =>
@@ -878,34 +893,40 @@ struct
           CpsErasePoly.transform (context, cexp)
         end
       val () = checkCpsAfterErasure ("after erasePoly", cexp)
-      (* Boxing pass: only for WasmGC backend (not yet implemented).
-         When enabled, disable CPS type checks after boxing since the checker
-         doesn't fully support BoxedType boundaries.
       val cexp =
-        let
-          val context =
-            { nextTyVar = nextTyVar
-            , nextVId = nextId
-            , simplificationOccurred = ref false
-            }
-        in
-          CpsBoxing.transform (context, cexp)
-        end
-      *)
+        case #backend opts of
+          BACKEND_WASM _ =>
+            let
+              val context =
+                { nextTyVar = nextTyVar
+                , nextVId = nextId
+                , simplificationOccurred = ref false
+                }
+            in
+              CpsBoxing.transform (context, cexp)
+            end
+        | _ => cexp
+      (* After boxing, disable CPS type checks since the checker
+         doesn't fully support BoxedType boundaries. *)
+      val checkCpsAfterBoxing =
+        case #backend opts of
+          (* BACKEND_WASM _ => (fn _ => ())
+        | *) _ => checkCpsAfterErasure
+      val () = checkCpsAfterBoxing ("after boxing", cexp)
       val cexp =
         optimizeCps
           { nextTyVar = nextTyVar
           , nextVId = nextId
           , printTimings = #printTimings opts
-          } checkCpsAfterErasure cexp (3 * (#optimizationLevel opts + 3))
-      val () = checkCpsAfterErasure ("optimization #3", cexp)
+          } checkCpsAfterBoxing cexp (3 * (#optimizationLevel opts + 3))
+      val () = checkCpsAfterBoxing ("optimization #3", cexp)
       val ctx' =
         { nextTyVar = nextTyVar
         , nextVId = nextId
         , simplificationOccurred = ref false
         }
       val cexp = CpsDeadCodeElimination.goStat (ctx', true, cexp)
-      val () = checkCpsAfterErasure ("after final DCE", cexp)
+      val () = checkCpsAfterBoxing ("after final DCE", cexp)
       val optTime = Time.toMicroseconds (#usr (Timer.checkCPUTimer timer))
       val () =
         if #printTimings opts then
@@ -1024,6 +1045,8 @@ struct
   | OPT_TARGET_NODEJS_CPS (* --nodejs-cps *)
   | OPT_TARGET_WEBJS (* --webjs *)
   | OPT_TARGET_WEBJS_CPS (* --webjs-cps *)
+  | OPT_TARGET_WASM (* --wasm *)
+  | OPT_TARGET_WASM_WAT (* --wasm-wat *)
   | OPT_HELP (* -h,--help *)
   | OPT_VERSION (* -v,--version *)
   | OPT_STOP (* -- *)
@@ -1048,6 +1071,8 @@ struct
     , (LONG "--nodejs-cps", SIMPLE OPT_TARGET_NODEJS_CPS)
     , (LONG "--webjs", SIMPLE OPT_TARGET_WEBJS)
     , (LONG "--webjs-cps", SIMPLE OPT_TARGET_WEBJS_CPS)
+    , (LONG "--wasm", SIMPLE OPT_TARGET_WASM)
+    , (LONG "--wasm-wat", SIMPLE OPT_TARGET_WASM_WAT)
     , (SHORT "-h", SIMPLE OPT_HELP)
     , (LONG "--help", SIMPLE OPT_HELP)
     , (SHORT "-v", SIMPLE OPT_VERSION)
@@ -1117,6 +1142,13 @@ struct
           (S.set.backend
              (BACKEND_JS {style = Backend.CPS, os = "web", default_ext = ".js"})
              opts) args
+    | SOME (OPT_TARGET_WASM, args) =>
+        parseArgs
+          (S.set.backend (BACKEND_WASM {output = Backend.WASM_BINARY}) opts)
+          args
+    | SOME (OPT_TARGET_WASM_WAT, args) =>
+        parseArgs
+          (S.set.backend (BACKEND_WASM {output = Backend.WASM_TEXT}) opts) args
     | SOME (OPT_HELP, _) => (showHelp (); OS.Process.exit OS.Process.success)
     | SOME (OPT_VERSION, _) =>
         (showVersion (); OS.Process.exit OS.Process.success)

src/nested.sml

@@ -378,6 +378,7 @@ struct
     fun canInlineLua _ = true
     fun canInlineJs (ExnTag _) = false
       | canInlineJs _ = true
+    fun canInlineWasm _ = true
     val DEPTH_LIMIT = 10
     fun toNestedImpl (backend, usage) =
       let
@@ -386,6 +387,7 @@ struct
             Backend.BACKEND_LUA _ => canInlineLua
           | Backend.BACKEND_LUAJIT => canInlineLua
           | Backend.BACKEND_JS _ => canInlineJs
+          | Backend.BACKEND_WASM _ => canInlineWasm
         fun goExp (e as Value _) = e
           | goExp (PrimOp {primOp, tyargs, args}) =
               PrimOp