feat: simproc for String.singleton (#12706)

This PR adds a dsimproc which evaluates `String.singleton ' '` to `" "`.

src/Lean/Meta/Tactic/Simp/BuiltinSimprocs/String.lean

@@ -46,6 +46,11 @@ builtin_dsimproc [simp, seval] reducePush (String.push _ _) := fun e => do
   let some m ← Char.fromExpr? e.appArg! | return .continue
   return .done <| toExpr (n.push m)
 
+builtin_dsimproc [simp, seval] reduceSingleton (String.singleton _) := fun e => do
+  unless e.isAppOfArity ``String.singleton 1 do return .continue
+  let some c ← Char.fromExpr? e.appArg! | return .continue
+  return .done <| toExpr (String.singleton c)
+
 @[inline] def reduceBinPred (declName : Name) (arity : Nat) (op : String → String → Bool) (e : Expr) : SimpM Step := do
   unless e.isAppOfArity declName arity do return .continue
   let some n ← fromExpr? e.appFn!.appArg! | return .continue

tests/elab/simprocString.lean

@@ -50,5 +50,10 @@ example : "b" > "a" := by simp
 example : "abc" ≥ "abc" := by simp
 example : "abd" ≥ "abc" := by simp
 
+-- String.reduceSingleton
+example : String.singleton ' ' = " " := by simp
+example : String.singleton 'a' = "a" := by simp
+example : String.singleton '\n' = "\n" := by simp
+
 -- Combined: roundtrip toList/ofList
 example : String.ofList "hello".toList = "hello" := by simp