Revert "go/analysis/passes/structtag: recognize multiple keys per tag"

This reverts CL 277092.

Proposal golang/go#40281 was initially accepted, then declined.  Stop accepting
the new, now once again invalid, format.

For golang/go#40281
For golang/go#43083
For golang/go#43226

Change-Id: Ida97263048f3a048f904a844f577d8353e3a1afa
Reviewed-on: https://go-review.googlesource.com/c/tools/+/281973
Trust: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Heschi Kreinick <heschi@google.com>

go/analysis/passes/structtag/structtag.go

@@ -207,12 +207,12 @@ var (
 )
 
 // validateStructTag parses the struct tag and returns an error if it is not
-// in the canonical format, as defined by reflect.StructTag.
+// in the canonical format, which is a space-separated list of key:"value"
+// settings. The value may contain spaces.
 func validateStructTag(tag string) error {
 	// This code is based on the StructTag.Get code in package reflect.
 
 	n := 0
-	var keys []string
 	for ; tag != ""; n++ {
 		if n > 0 && tag != "" && tag[0] != ' ' {
 			// More restrictive than reflect, but catches likely mistakes
@@ -240,27 +240,14 @@ func validateStructTag(tag string) error {
 		if i == 0 {
 			return errTagKeySyntax
 		}
-		if i+1 >= len(tag) || tag[i] < ' ' || tag[i] == 0x7f {
+		if i+1 >= len(tag) || tag[i] != ':' {
 			return errTagSyntax
 		}
-		key := tag[:i]
-		keys = append(keys, key)
-		tag = tag[i:]
-
-		// If we found a space char here - assume that we have a tag with
-		// multiple keys.
-		if tag[0] == ' ' {
-			continue
-		}
-
-		// Spaces were filtered above so we assume that here we have
-		// only valid tag value started with `:"`.
-		if tag[0] != ':' || tag[1] != '"' {
+		if tag[i+1] != '"' {
 			return errTagValueSyntax
 		}
-
-		// Remove the colon leaving tag at the start of the quoted string.
-		tag = tag[1:]
+		key := tag[:i]
+		tag = tag[i+1:]
 
 		// Scan quoted string to find value.
 		i = 1
@@ -276,56 +263,51 @@ func validateStructTag(tag string) error {
 		qvalue := tag[:i+1]
 		tag = tag[i+1:]
 
-		wholeValue, err := strconv.Unquote(qvalue)
+		value, err := strconv.Unquote(qvalue)
 		if err != nil {
 			return errTagValueSyntax
 		}
 
-		for _, key := range keys {
-			if !checkTagSpaces[key] {
-				continue
-			}
-
-			value := wholeValue
-			switch key {
-			case "xml":
-				// If the first or last character in the XML tag is a space, it is
-				// suspicious.
-				if strings.Trim(value, " ") != value {
-					return errTagValueSpace
-				}
+		if !checkTagSpaces[key] {
+			continue
+		}
 
-				// If there are multiple spaces, they are suspicious.
-				if strings.Count(value, " ") > 1 {
-					return errTagValueSpace
-				}
+		switch key {
+		case "xml":
+			// If the first or last character in the XML tag is a space, it is
+			// suspicious.
+			if strings.Trim(value, " ") != value {
+				return errTagValueSpace
+			}
 
-				// If there is no comma, skip the rest of the checks.
-				comma := strings.IndexRune(value, ',')
-				if comma < 0 {
-					continue
-				}
+			// If there are multiple spaces, they are suspicious.
+			if strings.Count(value, " ") > 1 {
+				return errTagValueSpace
+			}
 
-				// If the character before a comma is a space, this is suspicious.
-				if comma > 0 && value[comma-1] == ' ' {
-					return errTagValueSpace
-				}
-				value = value[comma+1:]
-			case "json":
-				// JSON allows using spaces in the name, so skip it.
-				comma := strings.IndexRune(value, ',')
-				if comma < 0 {
-					continue
-				}
-				value = value[comma+1:]
+			// If there is no comma, skip the rest of the checks.
+			comma := strings.IndexRune(value, ',')
+			if comma < 0 {
+				continue
 			}
 
-			if strings.IndexByte(value, ' ') >= 0 {
+			// If the character before a comma is a space, this is suspicious.
+			if comma > 0 && value[comma-1] == ' ' {
 				return errTagValueSpace
 			}
+			value = value[comma+1:]
+		case "json":
+			// JSON allows using spaces in the name, so skip it.
+			comma := strings.IndexRune(value, ',')
+			if comma < 0 {
+				continue
+			}
+			value = value[comma+1:]
 		}
 
-		keys = keys[:0]
+		if strings.IndexByte(value, ' ') >= 0 {
+			return errTagValueSpace
+		}
 	}
 	return nil
 }