cmd/compile/internal/types2: reduce number of delayed functions (optimization)

Rather than create and delay execution of a closure for each type parameter
in a type parameter list, just create one per type parameter list.

While at it, inline the small amount of code for getting the type constraint
and remove the respective function.

Change-Id: I49a00ff0a7b7e43eb53992dd7dbfac25ff23b42c
Reviewed-on: https://go-review.googlesource.com/c/go/+/348018
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

Author: griesemer

src/cmd/compile/internal/types2/decl.go

@@ -619,10 +619,25 @@ func (check *Checker) collectTypeParams(dst **TParamList, list []*syntax.Field)
 		// This also preserves the grouped output of type parameter lists
 		// when printing type strings.
 		if i == 0 || f.Type != list[i-1].Type {
-			bound = check.boundType(f.Type)
+			// The predeclared identifier "any" is visible only as a type bound in a type parameter list.
+			// If we allow "any" for general use, this if-statement can be removed (issue #33232).
+			if name, _ := unparen(f.Type).(*syntax.Name); name != nil && name.Value == "any" && check.lookup("any") == universeAny {
+				bound = universeAny.Type()
+			} else {
+				bound = check.typ(f.Type)
+			}
 		}
 		tparams[i].bound = bound
 	}
+
+	check.later(func() {
+		for i, tpar := range tparams {
+			u := under(tpar.bound)
+			if _, ok := u.(*Interface); !ok && u != Typ[Invalid] {
+				check.errorf(list[i].Type, "%s is not an interface", tpar.bound)
+			}
+		}
+	})
 }
 
 func (check *Checker) declareTypeParam(name *syntax.Name) *TypeParam {
@@ -638,25 +653,6 @@ func (check *Checker) declareTypeParam(name *syntax.Name) *TypeParam {
 	return tpar
 }
 
-// boundType type-checks the type expression e and returns its type, or Typ[Invalid].
-// The type must be an interface, including the predeclared type "any".
-func (check *Checker) boundType(e syntax.Expr) Type {
-	// The predeclared identifier "any" is visible only as a type bound in a type parameter list.
-	// If we allow "any" for general use, this if-statement can be removed (issue #33232).
-	if name, _ := unparen(e).(*syntax.Name); name != nil && name.Value == "any" && check.lookup("any") == universeAny {
-		return universeAny.Type()
-	}
-
-	bound := check.typ(e)
-	check.later(func() {
-		u := under(bound)
-		if _, ok := u.(*Interface); !ok && u != Typ[Invalid] {
-			check.errorf(e, "%s is not an interface", bound)
-		}
-	})
-	return bound
-}
-
 func (check *Checker) collectMethods(obj *TypeName) {
 	// get associated methods
 	// (Checker.collectObjects only collects methods with non-blank names;