gopls/completion: tweak fuzz handling to make its own MethodSet

I'm going to try changing methodsAndFields() to not use
types.MethodSet anymore. The `*testing.F` handling depends on the
MethodSet, so let's change it to make its own.

Change-Id: Iba81a58eb66142ede87f2be9125ae08753fb1700
Reviewed-on: https://go-review.googlesource.com/c/tools/+/594235
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Joedian Reid <[email protected]>
Reviewed-by: Robert Findley <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Run-TryBot: Muir Manders <[email protected]>

Author: muirdm

gopls/internal/golang/completion/completion.go

@@ -1496,6 +1496,13 @@ func ignoreUnimportedCompletion(fix *imports.ImportFix) bool {
 }
 
 func (c *completer) methodsAndFields(typ types.Type, addressable bool, imp *importInfo, cb func(candidate)) {
+	if isStarTestingDotF(typ) {
+		// is that a sufficient test? (or is more care needed?)
+		if c.fuzz(typ, imp, cb) {
+			return
+		}
+	}
+
 	mset := c.methodSetCache[methodSetKey{typ, addressable}]
 	if mset == nil {
 		if addressable && !types.IsInterface(typ) && !isPointer(typ) {
@@ -1508,13 +1515,6 @@ func (c *completer) methodsAndFields(typ types.Type, addressable bool, imp *impo
 		c.methodSetCache[methodSetKey{typ, addressable}] = mset
 	}
 
-	if isStarTestingDotF(typ) && addressable {
-		// is that a sufficient test? (or is more care needed?)
-		if c.fuzz(mset, imp, cb) {
-			return
-		}
-	}
-
 	for i := 0; i < mset.Len(); i++ {
 		obj := mset.At(i).Obj()
 		// to the other side of the cb() queue?

gopls/internal/golang/completion/fuzz.go

@@ -20,12 +20,14 @@ import (
 // PJW: are there other packages where we can deduce usage constraints?
 
 // if we find fuzz completions, then return true, as those are the only completions to offer
-func (c *completer) fuzz(mset *types.MethodSet, imp *importInfo, cb func(candidate)) bool {
+func (c *completer) fuzz(testingF types.Type, imp *importInfo, cb func(candidate)) bool {
 	// 1. inside f.Fuzz? (only f.Failed and f.Name)
 	// 2. possible completing f.Fuzz?
 	//    [Ident,SelectorExpr,Callexpr,ExprStmt,BlockiStmt,FuncDecl(Fuzz...)]
 	// 3. before f.Fuzz, same (for 2., offer choice when looking at an F)
 
+	mset := types.NewMethodSet(testingF)
+
 	// does the path contain FuncLit as arg to f.Fuzz CallExpr?
 	inside := false
 Loop: