fix(toml): Track dotted key for spans (#1118)

As this is just `toml`, this is for display purposes only and this can
help with visualizing issues, see rust-lang/cargo#16600

Author: epage

crates/toml/src/de/parser/key.rs

@@ -86,12 +86,14 @@ fn more_key(input: &Input<'_>) -> bool {
 
 struct State {
     current_key: Option<toml_parser::parser::Event>,
+    start: usize,
 }
 
 impl State {
     fn new(key_event: &toml_parser::parser::Event) -> Self {
         Self {
             current_key: Some(*key_event),
+            start: key_event.span().start(),
         }
     }
 
@@ -108,8 +110,9 @@ impl State {
             return;
         };
 
-        let key_span = key.span();
-        let key_span = key_span.start()..key_span.end();
+        let key_start = self.start;
+        let key_end = key.span().end();
+        let key_span = key_start..key_end;
 
         let raw = source.get(key).unwrap();
         let mut decoded = alloc::borrow::Cow::Borrowed("");