parser: remove scan.go dependency on tree

We now inject the NewNumVal and NewPlaceholder functions into scan.go.
This lets us move scan.go into its own package.

Release note: None

pkg/sql/parser/parse.go

@@ -21,6 +21,7 @@ package parser
 
 import (
 	"fmt"
+	"go/constant"
 	"strings"
 
 	"github.com/cockroachdb/cockroach/pkg/sql/pgwire/pgcode"
@@ -30,6 +31,11 @@ import (
 	"github.com/cockroachdb/errors"
 )
 
+func init() {
+	NewNumValFn = func(a constant.Value, s string, b bool) interface{} { return tree.NewNumVal(a, s, b) }
+	NewPlaceholderFn = func(s string) (interface{}, error) { return tree.NewPlaceholder(s) }
+}
+
 // Statement is the result of parsing a single statement. It contains the AST
 // node along with other information.
 type Statement struct {

pkg/sql/parser/scan.go

@@ -20,7 +20,6 @@ import (
 	"unsafe"
 
 	"github.com/cockroachdb/cockroach/pkg/sql/lexbase"
-	"github.com/cockroachdb/cockroach/pkg/sql/sem/tree"
 )
 
 const eof = -1
@@ -30,6 +29,17 @@ const errInvalidHexNumeric = "invalid hexadecimal numeric literal"
 const singleQuote = '\''
 const identQuote = '"'
 
+// NewNumValFn allows us to use tree.NewNumVal without a dependency on tree.
+var NewNumValFn = func(constant.Value, string, bool) interface{} {
+	return struct{}{}
+}
+
+// NewPlaceholderFn allows us to use tree.NewPlaceholder without a dependency on
+// tree.
+var NewPlaceholderFn = func(string) (interface{}, error) {
+	return struct{}{}, nil
+}
+
 // scanner lexes SQL statements.
 type scanner struct {
 	in            string
@@ -655,7 +665,7 @@ func (s *scanner) scanNumber(lval *sqlSymType, ch int) {
 			lval.str = fmt.Sprintf("could not make constant float from literal %q", lval.str)
 			return
 		}
-		lval.union.val = tree.NewNumVal(floatConst, lval.str, false /* negative */)
+		lval.union.val = NewNumValFn(floatConst, lval.str, false /* negative */)
 	} else {
 		if isHex && s.pos == start+2 {
 			lval.id = ERROR
@@ -680,7 +690,7 @@ func (s *scanner) scanNumber(lval *sqlSymType, ch int) {
 			lval.str = fmt.Sprintf("could not make constant int from literal %q", lval.str)
 			return
 		}
-		lval.union.val = tree.NewNumVal(intConst, lval.str, false /* negative */)
+		lval.union.val = NewNumValFn(intConst, lval.str, false /* negative */)
 	}
 }
 
@@ -691,7 +701,7 @@ func (s *scanner) scanPlaceholder(lval *sqlSymType) {
 	}
 	lval.str = s.in[start:s.pos]
 
-	placeholder, err := tree.NewPlaceholder(lval.str)
+	placeholder, err := NewPlaceholderFn(lval.str)
 	if err != nil {
 		lval.id = ERROR
 		lval.str = err.Error()