Allow empty table keys

Quoting 68076ffc6d (user-content-keys)

> A bare key must be non-empty, but an empty quoted key is allowed (though discouraged).

See also the discussion in https://github.com/toml-lang/toml/issues/432

src/de.rs

@@ -1978,7 +1978,6 @@ impl<'a> Deserializer<'a> {
                 expected,
                 found,
             } => self.error(at, ErrorKind::Wanted { expected, found }),
-            TokenError::EmptyTableKey(at) => self.error(at, ErrorKind::EmptyTableKey),
             TokenError::MultilineStringKey(at) => self.error(at, ErrorKind::MultilineStringKey),
         }
     }

src/ser.rs

@@ -509,7 +509,8 @@ impl<'a> Serializer<'a> {
     }
 
     fn escape_key(&mut self, key: &str) -> Result<(), Error> {
-        let ok = key.chars().all(|c| match c {
+        let ok = key.len() > 0
+            && key.chars().all(|c| match c {
                 'a'..='z' | 'A'..='Z' | '0'..='9' | '-' | '_' => true,
                 _ => false,
             });

src/tokens.rs

@@ -56,7 +56,6 @@ pub enum Error {
     UnterminatedString(usize),
     NewlineInTableKey(usize),
     MultilineStringKey(usize),
-    EmptyTableKey(usize),
     Wanted {
         at: usize,
         expected: &'static str,
@@ -194,9 +193,6 @@ impl<'a> Tokenizer<'a> {
                 if multiline {
                     return Err(Error::MultilineStringKey(offset));
                 }
-                if val == "" {
-                    return Err(Error::EmptyTableKey(offset));
-                }
                 match src.find('\n') {
                     None => Ok((span, val)),
                     Some(i) => Err(Error::NewlineInTableKey(offset + i)),

test-suite/tests/invalid-misc.rs

@@ -14,7 +14,7 @@ fn bad() {
     bad!("a = 01", "invalid number at line 1 column 6");
     bad!("a = 1__1", "invalid number at line 1 column 5");
     bad!("a = 1_", "invalid number at line 1 column 5");
-    bad!("''", "empty table key found at line 1 column 1");
+    bad!("''", "expected an equals, found eof at line 1 column 3");
     bad!("a = 9e99999", "invalid number at line 1 column 5");
 
     bad!(

test-suite/tests/parser.rs

@@ -342,12 +342,14 @@ fn bad_keys() {
         "key|=3",
         "unexpected character found: `|` at line 1 column 4"
     );
-    bad!("\"\"=3", "empty table key found at line 1 column 1");
     bad!(
         "=3",
         "expected a table key, found an equals at line 1 column 1"
     );
-    bad!("\"\"|=3", "empty table key found at line 1 column 1");
+    bad!(
+        "\"\"|=3",
+        "unexpected character found: `|` at line 1 column 3"
+    );
     bad!("\"\n\"|=3", "newline in string found at line 1 column 2");
     bad!(
         "\"\r\"|=3",
@@ -381,12 +383,10 @@ fn bad_table_names() {
         "[.]",
         "expected a table key, found a period at line 1 column 2"
     );
-    bad!("[\"\".\"\"]", "empty table key found at line 1 column 2");
     bad!(
         "[a.]",
         "expected a table key, found a right bracket at line 1 column 4"
     );
-    bad!("[\"\"]", "empty table key found at line 1 column 2");
     bad!("[!]", "unexpected character found: `!` at line 1 column 2");
     bad!("[\"\n\"]", "newline in string found at line 1 column 3");
     bad!(

test-suite/tests/valid.rs

@@ -231,6 +231,11 @@ test!(
     include_str!("valid/key-with-pound.toml"),
     include_str!("valid/key-with-pound.json")
 );
+test!(
+    key_empty,
+    include_str!("valid/key-empty.toml"),
+    include_str!("valid/key-empty.json")
+);
 test!(
     long_float,
     include_str!("valid/long-float.toml"),

test-suite/tests/valid/key-empty.json

@@ -0,0 +1,3 @@
+{
+  "": {"type": "integer", "value": "1"}
+}

test-suite/tests/valid/key-empty.toml

@@ -0,0 +1 @@
+"" = 1