Revert "reflect: support multiple keys in struct tags"

Proposal #40281 was initially accepted, but has now been declined.
This CL removes most of the work done to implement it.

Specifically this reverts CLs 248341, 274448, 274474, and 278392.

For #40281
For #43226

Change-Id: I5a9ebb4d9cb5fb0962434b64c59beb8343030be5
Reviewed-on: https://go-review.googlesource.com/c/go/+/281515
Trust: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Emmanuel Odeke <emmanuel@orijtech.com>

2 files changed, 9 insertions, 204 deletions

diff --git a/src/reflect/all_test.go b/src/reflect/all_test.go
index b01158635f..1225d6177d 100644
--- a/src/reflect/all_test.go
+++ b/src/reflect/all_test.go
@@ -7168,176 +7168,6 @@ func TestMapIterDelete1(t *testing.T) {
 	}
 }
 
-func TestStructTagLookup(t *testing.T) {
-	var tests = []struct {
-		tag           StructTag
-		key           string
-		expectedValue string
-		expectedOK    bool
-	}{
-		{
-			tag:           `json:"json_value_1"`,
-			key:           "json",
-			expectedValue: "json_value_1",
-			expectedOK:    true,
-		},
-		{
-			tag:           `json:"json_value_2" xml:"xml_value_2"`,
-			key:           "json",
-			expectedValue: "json_value_2",
-			expectedOK:    true,
-		},
-		{
-			tag:           `json:"json_value_3" xml:"xml_value_3"`,
-			key:           "xml",
-			expectedValue: "xml_value_3",
-			expectedOK:    true,
-		},
-		{
-			tag:           `bson json:"shared_value_4"`,
-			key:           "json",
-			expectedValue: "shared_value_4",
-			expectedOK:    true,
-		},
-		{
-			tag:           `bson json:"shared_value_5"`,
-			key:           "bson",
-			expectedValue: "shared_value_5",
-			expectedOK:    true,
-		},
-		{
-			tag:           `json bson xml form:"field_1,omitempty" other:"value_1"`,
-			key:           "xml",
-			expectedValue: "field_1,omitempty",
-			expectedOK:    true,
-		},
-		{
-			tag:           `json bson xml form:"field_2,omitempty" other:"value_2"`,
-			key:           "form",
-			expectedValue: "field_2,omitempty",
-			expectedOK:    true,
-		},
-		{
-			tag:           `json bson xml form:"field_3,omitempty" other:"value_3"`,
-			key:           "other",
-			expectedValue: "value_3",
-			expectedOK:    true,
-		},
-		{
-			tag:           `json    bson    xml    form:"field_4" other:"value_4"`,
-			key:           "json",
-			expectedValue: "field_4",
-			expectedOK:    true,
-		},
-		{
-			tag:           `json    bson    xml    form:"field_5" other:"value_5"`,
-			key:           "non_existing",
-			expectedValue: "",
-			expectedOK:    false,
-		},
-		{
-			tag:           `json "json_6"`,
-			key:           "json",
-			expectedValue: "",
-			expectedOK:    false,
-		},
-		{
-			tag:           `json:"json_7" bson "bson_7"`,
-			key:           "json",
-			expectedValue: "json_7",
-			expectedOK:    true,
-		},
-		{
-			tag:           `json:"json_8" xml "xml_8"`,
-			key:           "xml",
-			expectedValue: "",
-			expectedOK:    false,
-		},
-		{
-			tag:           `json    bson    xml    form "form_9" other:"value_9"`,
-			key:           "bson",
-			expectedValue: "",
-			expectedOK:    false,
-		},
-		{
-			tag:           `json bson xml form "form_10" other:"value_10"`,
-			key:           "other",
-			expectedValue: "",
-			expectedOK:    false,
-		},
-		{
-			tag:           `json bson xml form:"form_11" other "value_11"`,
-			key:           "json",
-			expectedValue: "form_11",
-			expectedOK:    true,
-		},
-		{
-			tag:           `tag1`,
-			key:           "tag1",
-			expectedValue: "",
-			expectedOK:    false,
-		},
-		{
-			tag:           `tag2 :"hello_2"`,
-			key:           "tag2",
-			expectedValue: "",
-			expectedOK:    false,
-		},
-		{
-			tag:           `tag3: "hello_3"`,
-			key:           "tag3",
-			expectedValue: "",
-			expectedOK:    false,
-		},
-		{
-			tag:           "json\x7fbson: \"hello_4\"",
-			key:           "json",
-			expectedValue: "",
-			expectedOK:    false,
-		},
-		{
-			tag:           "json\x7fbson: \"hello_5\"",
-			key:           "bson",
-			expectedValue: "",
-			expectedOK:    false,
-		},
-		{
-			tag:           "json bson:\x7f\"hello_6\"",
-			key:           "json",
-			expectedValue: "",
-			expectedOK:    false,
-		},
-		{
-			tag:           "json bson:\x7f\"hello_7\"",
-			key:           "bson",
-			expectedValue: "",
-			expectedOK:    false,
-		},
-		{
-			tag:           "json\x09bson:\"hello_8\"",
-			key:           "json",
-			expectedValue: "",
-			expectedOK:    false,
-		},
-		{
-			tag:           "a\x7fb json:\"val\"",
-			key:           "json",
-			expectedValue: "",
-			expectedOK:    false,
-		},
-	}
-
-	for _, test := range tests {
-		v, ok := test.tag.Lookup(test.key)
-		if v != test.expectedValue {
-			t.Errorf("struct tag lookup failed, got %s, want %s", v, test.expectedValue)
-		}
-		if ok != test.expectedOK {
-			t.Errorf("struct tag lookup failed, got %t, want %t", ok, test.expectedOK)
-		}
-	}
-}
-
 // iterateToString returns the set of elements
 // returned by an iterator in readable form.
 func iterateToString(it *MapIter) string {
diff --git a/src/reflect/type.go b/src/reflect/type.go
index 1f1e70d485..a1cdf45e15 100644
--- a/src/reflect/type.go
+++ b/src/reflect/type.go
@@ -1104,16 +1104,12 @@ type StructField struct {
 
 // A StructTag is the tag string in a struct field.
 //
-// By convention, tag strings are a mapping of keys to values.
-// The format is key:"value". Each key is a non-empty string consisting
-// of non-control characters other than space (U+0020 ' '),
-// quote (U+0022 '"'), and colon (U+003A ':'). Each value is quoted
-// using U+0022 '"' characters and Go string literal syntax.
-// Multiple key-value mappings are separated by zero or more spaces, as in
-//   key1:"value1" key2:"value2"
-// Multiple keys may map to a single shared value by separating the keys
-// with spaces, as in
-//   key1 key2:"value"
+// By convention, tag strings are a concatenation of
+// optionally space-separated key:"value" pairs.
+// Each key is a non-empty string consisting of non-control
+// characters other than space (U+0020 ' '), quote (U+0022 '"'),
+// and colon (U+003A ':').  Each value is quoted using U+0022 '"'
+// characters and Go string literal syntax.
 type StructTag string
 
 // Get returns the value associated with key in the tag string.
@@ -1136,9 +1132,6 @@ func (tag StructTag) Lookup(key string) (value string, ok bool) {
 	// When modifying this code, also update the validateStructTag code
 	// in cmd/vet/structtag.go.
 
-	// keyFound indicates that such key on the left side has already been found.
-	var keyFound bool
-
 	for tag != "" {
 		// Skip leading space.
 		i := 0
@@ -1158,29 +1151,11 @@ func (tag StructTag) Lookup(key string) (value string, ok bool) {
 		for i < len(tag) && tag[i] > ' ' && tag[i] != ':' && tag[i] != '"' && tag[i] != 0x7f {
 			i++
 		}
-		if i == 0 || i+1 >= len(tag) || tag[i] < ' ' || tag[i] == 0x7f {
+		if i == 0 || i+1 >= len(tag) || tag[i] != ':' || tag[i+1] != '"' {
 			break
 		}
 		name := string(tag[:i])
-		tag = tag[i:]
-
-		// If we found a space char here - assume that we have a tag with
-		// multiple keys.
-		if tag[0] == ' ' {
-			if name == key {
-				keyFound = true
-			}
-			continue
-		}
-
-		// Spaces were filtered above so we assume that here we have
-		// only valid tag value started with `:"`.
-		if tag[0] != ':' || tag[1] != '"' {
-			break
-		}
-
-		// Remove the colon leaving tag at the start of the quoted string.
-		tag = tag[1:]
+		tag = tag[i+1:]
 
 		// Scan quoted string to find value.
 		i = 1
@@ -1196,7 +1171,7 @@ func (tag StructTag) Lookup(key string) (value string, ok bool) {
 		qvalue := string(tag[:i+1])
 		tag = tag[i+1:]
 
-		if key == name || keyFound {
+		if key == name {
 			value, err := strconv.Unquote(qvalue)
 			if err != nil {
 				break