feat: Supprt parsing dotted keys

We support parsing a lot of expressions to help users to edit their toml
document from real world code.  One thing that was missing was dotted
key parsing.

Also, this will hopefully help with rust-lang/cargo#10176

src/key.rs

@@ -48,6 +48,13 @@ impl Key {
         }
     }
 
+    /// Parse a TOML key expression
+    ///
+    /// Unlike `"".parse<Key>()`, this supports dotted keys.
+    pub fn parse(repr: &str) -> Result<Vec<Self>, parser::TomlError> {
+        Self::try_parse_path(repr)
+    }
+
     pub(crate) fn with_repr_unchecked(mut self, repr: Repr) -> Self {
         self.repr = Some(repr);
         self
@@ -97,12 +104,12 @@ impl Key {
         self.decor.clear();
     }
 
-    fn try_parse(s: &str) -> Result<Key, parser::TomlError> {
+    fn try_parse_simple(s: &str) -> Result<Key, parser::TomlError> {
         use combine::stream::position::{IndexPositioner, Positioner};
         use combine::EasyParser;
 
         let b = s.as_bytes();
-        let result = parser::key_parser().easy_parse(Stream::new(b));
+        let result = parser::simple_key().easy_parse(Stream::new(b));
         match result {
             Ok((_, ref rest)) if !rest.input.is_empty() => Err(parser::TomlError::from_unparsed(
                 (&rest.positioner
@@ -114,6 +121,24 @@ impl Key {
             Err(e) => Err(parser::TomlError::new(e, b)),
         }
     }
+
+    fn try_parse_path(s: &str) -> Result<Vec<Key>, parser::TomlError> {
+        use combine::stream::position::{IndexPositioner, Positioner};
+        use combine::EasyParser;
+
+        let b = s.as_bytes();
+        let result = parser::key_path().easy_parse(Stream::new(b));
+        match result {
+            Ok((_, ref rest)) if !rest.input.is_empty() => Err(parser::TomlError::from_unparsed(
+                (&rest.positioner
+                    as &dyn Positioner<usize, Position = usize, Checkpoint = IndexPositioner>)
+                    .position(),
+                b,
+            )),
+            Ok((keys, _)) => Ok(keys),
+            Err(e) => Err(parser::TomlError::new(e, b)),
+        }
+    }
 }
 
 impl std::ops::Deref for Key {
@@ -158,7 +183,7 @@ impl FromStr for Key {
     /// if fails, tries as basic quoted key (surrounds with "")
     /// and then literal quoted key (surrounds with '')
     fn from_str(s: &str) -> Result<Self, Self::Err> {
-        Key::try_parse(s)
+        Key::try_parse_simple(s)
     }
 }
 

src/parser/mod.rs

@@ -17,7 +17,8 @@ mod value;
 
 pub use self::errors::TomlError;
 pub(crate) use self::key::is_unquoted_char;
-pub(crate) use self::key::simple_key as key_parser;
+pub(crate) use self::key::key as key_path;
+pub(crate) use self::key::simple_key;
 pub(crate) use self::value::value as value_parser;
 
 use self::table::duplicate_key;