go/ir: support unsafe.Add and unsafe.Slice

This is a backport of x/tool's
640c1dea83015e5271a001c99370762fc63dc280.

Updates gh-1045

(cherry picked from commit e5c61cf)

go/ir/UPSTREAM

@@ -5,6 +5,5 @@ The changes are too many to list here, and it is best to consider this package i
 Upstream changes still get applied when they address bugs in portions of code we have inherited.
 
 The last upstream commit we've looked at was:
-20dafe5d6055a2382de349face79f568e574d9fd
-
+640c1dea83015e5271a001c99370762fc63dc280
 

go/ir/builder.go

@@ -654,6 +654,10 @@ func (b *builder) expr0(fn *Function, e ast.Expr, tv types.TypeAndValue) Value {
 	case *ast.SelectorExpr:
 		sel, ok := fn.Pkg.info.Selections[e]
 		if !ok {
+			// builtin unsafe.{Add,Slice}
+			if obj, ok := fn.Pkg.info.Uses[e.Sel].(*types.Builtin); ok {
+				return &Builtin{name: "Unsafe" + obj.Name(), sig: tv.Type.(*types.Signature)}
+			}
 			// qualified identifier
 			return b.expr(fn, e.Sel)
 		}

go/ir/builder_go117_test.go

@@ -9,6 +9,7 @@ package ir_test
 
 import (
 	"go/ast"
+	"go/importer"
 	"go/parser"
 	"go/token"
 	"go/types"
@@ -18,23 +19,33 @@ import (
 	"honnef.co/go/tools/go/ir/irutil"
 )
 
-func TestSliceToArrayPtr(t *testing.T) {
-	src := `package p
-func f() {
-	var s []byte
-	_ = (*[4]byte)(s)
-}
-`
-	fset := token.NewFileSet()
-	f, err := parser.ParseFile(fset, "p.go", src, parser.ParseComments)
-	if err != nil {
-		t.Fatal(err)
+func TestBuildPackageGo117(t *testing.T) {
+	tests := []struct {
+		name     string
+		src      string
+		importer types.Importer
+	}{
+		{"slice to array pointer", "package p; var s []byte; var _ = (*[4]byte)(s)", nil},
+		{"unsafe slice", `package p; import "unsafe"; var _ = unsafe.Add(nil, 0)`, importer.Default()},
+		{"unsafe add", `package p; import "unsafe"; var _ = unsafe.Slice((*int)(nil), 0)`, importer.Default()},
 	}
-	files := []*ast.File{f}
 
-	pkg := types.NewPackage("p", "")
-	conf := &types.Config{}
-	if _, _, err := irutil.BuildPackage(conf, fset, pkg, files, ir.SanityCheckFunctions); err != nil {
-		t.Fatalf("unexpected error: %v", err)
+	for _, tc := range tests {
+		tc := tc
+		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+			fset := token.NewFileSet()
+			f, err := parser.ParseFile(fset, "p.go", tc.src, parser.ParseComments)
+			if err != nil {
+				t.Error(err)
+			}
+			files := []*ast.File{f}
+
+			pkg := types.NewPackage("p", "")
+			conf := &types.Config{Importer: tc.importer}
+			if _, _, err := irutil.BuildPackage(conf, fset, pkg, files, ir.SanityCheckFunctions); err != nil {
+				t.Errorf("unexpected error: %v", err)
+			}
+		})
 	}
 }