go/types: make Interface.Complete a no-op

This is a partial port of CL 340255 to go/types. Of course we can't
delete Interface.Complete, but make it a no-op.

Completing interfaces is no longer necessary.

Change-Id: Ida3c84cc94713f14a646c7682f5d4ae5339a0faa
Reviewed-on: https://go-review.googlesource.com/c/go/+/342489
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

src/go/internal/gcimporter/iimport.go

@@ -174,10 +174,6 @@ func iImportData(fset *token.FileSet, imports map[string]*types.Package, dataRea
 		p.doDecl(localpkg, name)
 	}
 
-	for _, typ := range p.interfaceList {
-		typ.Complete()
-	}
-
 	// record all referenced packages as imports
 	list := append(([]*types.Package)(nil), pkgList[1:]...)
 	sort.Sort(byPath(list))

src/go/types/interface.go

@@ -104,22 +104,12 @@ func (t *Interface) IsComparable() bool { return t.typeSet().IsComparable() }
 // IsConstraint reports whether interface t is not just a method set.
 func (t *Interface) IsConstraint() bool { return !t.typeSet().IsMethodSet() }
 
-// Complete computes the interface's type set. It must be called by users of
-// NewInterfaceType and NewInterface after the interface's embedded types are
-// fully defined and before using the interface type in any way other than to
-// form other types. The interface must not contain duplicate methods or a
-// panic occurs. Complete returns the receiver.
+// Complete just returns its receiver. There's no other effect.
 //
-// Deprecated: Type sets are now computed lazily, on demand; this function
-//             is only here for backward-compatibility. It does not have to
-//             be called explicitly anymore.
+// Deprecated: Interfaces are now completed on demand; this function is only
+// here for backward-compatibility. It does not have to be called explicitly
+// anymore.
 func (t *Interface) Complete() *Interface {
-	// Some tests are still depending on the state change
-	// (string representation of an Interface not containing an
-	// /* incomplete */ marker) caused by the explicit Complete
-	// call, so we compute the type set eagerly here.
-	t.complete = true
-	t.typeSet()
 	return t
 }
 

src/go/types/issues_test.go

@@ -405,8 +405,9 @@ func TestIssue28282(t *testing.T) {
 	// create type interface { error }
 	et := Universe.Lookup("error").Type()
 	it := NewInterfaceType(nil, []Type{et})
-	it.Complete()
 	// verify that after completing the interface, the embedded method remains unchanged
+	// (interfaces are "completed" lazily now, so the completion happens implicitly when
+	// accessing Method(0))
 	want := et.Underlying().(*Interface).Method(0)
 	got := it.Method(0)
 	if got != want {