refactor: use a separate getter for fvar values in toIR (#9978)

src/Lean/Compiler/IR/ToIR.lean

@@ -28,6 +28,7 @@ inductive FVarClassification where
   | var (id : VarId)
   | joinPoint (id : JoinPointId)
   | erased
+  deriving Inhabited
 
 structure BuilderState where
   fvars : Std.HashMap FVarId FVarClassification := {}
@@ -38,6 +39,9 @@ abbrev M := StateRefT BuilderState CoreM
 def M.run (x : M α) : CoreM α := do
   x.run' {}
 
+def getFVarValue (fvarId : FVarId) : M FVarClassification := do
+  return (← get).fvars.get! fvarId
+
 def bindVar (fvarId : FVarId) : M VarId := do
   modifyGet fun s =>
     let varId := { idx := s.nextId }
@@ -82,10 +86,10 @@ def lowerLitValue (v : LCNF.LitValue) : LitVal × IRType :=
 def lowerArg (a : LCNF.Arg) : M Arg := do
   match a with
   | .fvar fvarId =>
-    match (← get).fvars[fvarId]? with
-    | some (.var varId) => return .var varId
-    | some .erased => return .erased
-    | some (.joinPoint ..) | none => panic! "unexpected value"
+    match (← getFVarValue fvarId) with
+    | .var varId => return .var varId
+    | .erased => return .erased
+    | .joinPoint .. => panic! "unexpected value"
   | .erased | .type .. => return .erased
 
 inductive TranslatedProj where
@@ -116,18 +120,18 @@ partial def lowerCode (c : LCNF.Code) : M FnBody := do
     let body ← lowerCode decl.value
     return .jdecl joinPoint params body (← lowerCode k)
   | .jmp fvarId args =>
-    match (← get).fvars[fvarId]? with
-    | some (.joinPoint joinPointId) =>
+    match (← getFVarValue fvarId) with
+    | .joinPoint joinPointId =>
       return .jmp joinPointId (← args.mapM lowerArg)
-    | some (.var ..) | some .erased | none => panic! "unexpected value"
+    | .var .. | .erased => panic! "unexpected value"
   | .cases cases =>
-    match (← get).fvars[cases.discr]? with
-    | some (.var varId) =>
+    match (← getFVarValue cases.discr) with
+    | .var varId =>
       return .case cases.typeName
                    varId
                    (← nameToIRType cases.typeName)
                    (← cases.alts.mapM (lowerAlt varId))
-    | some .erased =>
+    | .erased =>
       let #[alt] := cases.alts | panic! "erased inductive should only have one case"
       match alt with
       | .alt _ params code =>
@@ -135,12 +139,12 @@ partial def lowerCode (c : LCNF.Code) : M FnBody := do
         lowerCode code
       | .default code =>
         lowerCode code
-    | some (.joinPoint ..) | none => panic! "unexpected value"
+    | .joinPoint .. => panic! "unexpected value"
   | .return fvarId =>
-    let arg := match (← get).fvars[fvarId]? with
-    | some (.var varId) => .var varId
-    | some .erased => .erased
-    | some (.joinPoint ..) | none => panic! "unexpected value"
+    let arg := match (← getFVarValue fvarId) with
+    | .var varId => .var varId
+    | .erased => .erased
+    | .joinPoint .. => panic! "unexpected value"
     return .ret arg
   | .unreach .. => return .unreachable
   | .fun .. => panic! "all local functions should be λ-lifted"
@@ -152,8 +156,8 @@ partial def lowerLet (decl : LCNF.LetDecl) (k : LCNF.Code) : M FnBody := do
     let ⟨litValue, type⟩ := lowerLitValue litValue
     return .vdecl var type (.lit litValue) (← lowerCode k)
   | .proj typeName i fvarId =>
-    match (← get).fvars[fvarId]? with
-    | some (.var varId) =>
+    match (← getFVarValue fvarId) with
+    | .var varId =>
       let some (.inductInfo { ctors := [ctorName], .. }) := (← Lean.getEnv).find? typeName
         | panic! "projection of non-structure type"
       let ⟨ctorInfo, fields⟩ ← getCtorLayout ctorName
@@ -165,10 +169,10 @@ partial def lowerLet (decl : LCNF.LetDecl) (k : LCNF.Code) : M FnBody := do
       | .erased =>
         bindErased decl.fvarId
         lowerCode k
-    | some .erased =>
+    | .erased =>
       bindErased decl.fvarId
       lowerCode k
-    | some (.joinPoint ..) | none => panic! "unexpected value"
+    | .joinPoint .. => panic! "unexpected value"
   | .const name _ args =>
     let irArgs ← args.mapM lowerArg
     if let some code ← tryIrDecl? name irArgs then
@@ -219,12 +223,12 @@ partial def lowerLet (decl : LCNF.LetDecl) (k : LCNF.Code) : M FnBody := do
       throwError f!"code generator does not support recursor '{name}' yet, consider using 'match ... with' and/or structural recursion"
     | none => panic! "reference to unbound name"
   | .fvar fvarId args =>
-    match (← get).fvars[fvarId]? with
-    | some (.var id) =>
+    match (← getFVarValue fvarId) with
+    | .var id =>
       let irArgs ← args.mapM lowerArg
       mkAp id irArgs
-    | some .erased => mkErased ()
-    | some (.joinPoint ..) | none => panic! "unexpected value"
+    | .erased => mkErased ()
+    | .joinPoint .. => panic! "unexpected value"
   | .erased => mkErased ()
 where
   mkVar (v : VarId) : M FnBody := do