Cache hash code in TypeStructure to fix O(n) GetHashCode perf regression

src/Compiler/Checking/OverloadResolutionCache.fs

@@ -83,12 +83,12 @@ let tryGetTypeStructureForOverloadCache (g: TcGlobals) (ty: TType) : TypeStructu
     let ty = stripTyEqns g ty
 
     match tryGetTypeStructureOfStrippedType ty with
-    | ValueSome(Stable tokens) -> ValueSome(Stable tokens)
-    | ValueSome(Unstable tokens) ->
+    | ValueSome(Stable(h, tokens)) -> ValueSome(Stable(h, tokens))
+    | ValueSome(Unstable(h, tokens)) ->
         if hasUnsolvedTokens tokens then
             ValueNone
         else
-            ValueSome(Stable tokens)
+            ValueSome(Stable(h, tokens))
     | ValueSome PossiblyInfinite -> ValueNone
     | ValueNone -> ValueNone
 
@@ -162,8 +162,8 @@ let tryComputeOverloadCacheKey
                     | Some retTy ->
                         match tryGetTypeStructureForOverloadCache g retTy with
                         | ValueSome ts -> ValueSome ts
-                        | ValueNone -> if anyHasOutArgs then ValueNone else ValueSome(Stable [||])
-                    | None -> ValueSome(Stable [||])
+                        | ValueNone -> if anyHasOutArgs then ValueNone else ValueSome(Stable(0, [||]))
+                    | None -> ValueSome(Stable(0, [||]))
 
                 match retTyStructure with
                 | ValueNone -> ValueNone

src/Compiler/Utilities/TypeHashing.fs

@@ -407,12 +407,35 @@ module StructuralUtilities =
         | Unsolved of int
         | Rigid of int
 
+    let private hashTokenArray (tokens: TypeToken[]) =
+        let mutable acc = 0
+        for t in tokens do
+            acc <- combineHash acc (hash t)
+        acc
+
+    [<CustomEquality; NoComparison>]
     type TypeStructure =
-        | Stable of TypeToken[]
+        | Stable of hash: int * tokens: TypeToken[]
         // Unstable means that the type structure of a given TType may change because of constraint solving or Trace.Undo.
-        | Unstable of TypeToken[]
+        | Unstable of hash: int * tokens: TypeToken[]
         | PossiblyInfinite
 
+        override this.GetHashCode() =
+            match this with
+            | Stable(h, _) -> h
+            | Unstable(h, _) -> h ||| 0x40000000
+            | PossiblyInfinite -> 0
+
+        override this.Equals(obj) =
+            match obj with
+            | :? TypeStructure as other ->
+                match this, other with
+                | Stable(h1, a), Stable(h2, b) -> h1 = h2 && a = b
+                | Unstable(h1, a), Unstable(h2, b) -> h1 = h2 && a = b
+                | PossiblyInfinite, PossiblyInfinite -> true
+                | _ -> false
+            | _ -> false
+
     type private GenerationContext() =
         member val TyparMap = System.Collections.Generic.Dictionary<Stamp, int>(4)
         member val Tokens = ResizeArray<TypeToken>(MaxTokenCount)
@@ -566,8 +589,11 @@ module StructuralUtilities =
 
         // If the sequence got too long, just drop it, we could be dealing with an infinite type.
         if out.Count >= MaxTokenCount then PossiblyInfinite
-        elif not ctx.Stable then Unstable(out.ToArray())
-        else Stable(out.ToArray())
+        else
+            let tokens = out.ToArray()
+            let h = hashTokenArray tokens
+            if not ctx.Stable then Unstable(h, tokens)
+            else Stable(h, tokens)
 
     // Speed up repeated calls by memoizing results for types that yield a stable structure.
     let private getTypeStructureOfStrippedType =