go/analysis/passes/nilness: fixed slice not being considered as non-nil

The initialized slice will always be non-nil.
But, nilness could not detect it, so I fixed it.

Fixes golang/go#45177

Change-Id: I3bad2b8dee16331c4a24d2e93fb4baacd98f30ec
GitHub-Last-Rev: f3a532e
GitHub-Pull-Request: #291
Reviewed-on: https://go-review.googlesource.com/c/tools/+/303890
Run-TryBot: Tim King <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
Reviewed-by: Tim King <[email protected]>
gopls-CI: kokoro <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Run-TryBot: Dmitri Shuralyov <[email protected]>

Author: komem3

go/analysis/passes/nilness/nilness.go

@@ -251,7 +251,7 @@ func (n nilness) String() string { return nilnessStrings[n+1] }
 // or unknown given the dominating stack of facts.
 func nilnessOf(stack []fact, v ssa.Value) nilness {
 	switch v := v.(type) {
-	// unwrap ChangeInterface values recursively, to detect if underlying
+	// unwrap ChangeInterface and Slice values recursively, to detect if underlying
 	// values have any facts recorded or are otherwise known with regard to nilness.
 	//
 	// This work must be in addition to expanding facts about
@@ -265,6 +265,10 @@ func nilnessOf(stack []fact, v ssa.Value) nilness {
 		if underlying := nilnessOf(stack, v.X); underlying != unknown {
 			return underlying
 		}
+	case *ssa.Slice:
+		if underlying := nilnessOf(stack, v.X); underlying != unknown {
+			return underlying
+		}
 	case *ssa.SliceToArrayPointer:
 		nn := nilnessOf(stack, v.X)
 		if slice2ArrayPtrLen(v) > 0 {

go/analysis/passes/nilness/testdata/src/a/a.go

@@ -130,7 +130,6 @@ func f9(x interface {
 	b()
 	c()
 }) {
-
 	x.b() // we don't catch this panic because we don't have any facts yet
 	xx := interface {
 		a()
@@ -155,11 +154,27 @@ func f9(x interface {
 	}
 }
 
+func f10() {
+	s0 := make([]string, 0)
+	if s0 == nil { // want "impossible condition: non-nil == nil"
+		print(0)
+	}
+
+	var s1 []string
+	if s1 == nil { // want "tautological condition: nil == nil"
+		print(0)
+	}
+	s2 := s1[:][:]
+	if s2 == nil { // want "tautological condition: nil == nil"
+		print(0)
+	}
+}
+
 func unknown() bool {
 	return false
 }
 
-func f10(a interface{}) {
+func f11(a interface{}) {
 	switch a.(type) {
 	case nil:
 		return
@@ -170,7 +185,7 @@ func f10(a interface{}) {
 	}
 }
 
-func f11(a interface{}) {
+func f12(a interface{}) {
 	switch a {
 	case nil:
 		return
@@ -190,7 +205,7 @@ type innerY struct {
 	value int
 }
 
-func f12() {
+func f13() {
 	var d *Y
 	print(d.value) // want "nil dereference in field selection"
 }