diff --git a/crates/ra_syntax/src/parser_api.rs b/crates/ra_syntax/src/parser_api.rs
index 0f740963ded..3487aef8508 100644
--- a/crates/ra_syntax/src/parser_api.rs
+++ b/crates/ra_syntax/src/parser_api.rs
@@ -142,11 +142,13 @@ fn new(pos: u32) -> Marker {
         }
     }
 
-    /// Finishes the syntax tree node and assigns `kind` to it.
+    /// Finishes the syntax tree node and assigns `kind` to it,
+    /// and mark the create a `CompletedMarker` for possible future
+    /// operation like `.precede()` to deal with forward_parent.
     pub(crate) fn complete(mut self, p: &mut Parser, kind: SyntaxKind) -> CompletedMarker {
         self.bomb.defuse();
         p.0.complete(self.pos, kind);
-        CompletedMarker(self.pos, kind)
+        CompletedMarker::new(self.pos, kind)
     }
 
     /// Abandons the syntax tree node. All its children
@@ -160,13 +162,22 @@ pub(crate) fn abandon(mut self, p: &mut Parser) {
 pub(crate) struct CompletedMarker(u32, SyntaxKind);
 
 impl CompletedMarker {
-    /// This one is tricky :-)
+    fn new(pos: u32, kind: SyntaxKind) -> Self {
+        CompletedMarker(pos, kind)
+    }
+
     /// This method allows to create a new node which starts
     /// *before* the current one. That is, parser could start
     /// node `A`, then complete it, and then after parsing the
     /// whole `A`, decide that it should have started some node
     /// `B` before starting `A`. `precede` allows to do exactly
     /// that. See also docs about `forward_parent` in `Event::Start`.
+    ///
+    /// Given completed events `[START, FINISH]` and its corresponding
+    /// `CompletedMarker(pos: 0, _)`.
+    /// Append a new `START` events as `[START, FINISH, NEWSTART]`,
+    /// then mark `NEWSTART` as `START`'s parent with saving its relative
+    /// distance to `NEWSTART` into forward_parent(=2 in this case);
     pub(crate) fn precede(self, p: &mut Parser) -> Marker {
         Marker::new(p.0.precede(self.0))
     }
diff --git a/crates/ra_syntax/src/parser_impl.rs b/crates/ra_syntax/src/parser_impl.rs
index ce321aecb37..01a51cd8d6d 100644
--- a/crates/ra_syntax/src/parser_impl.rs
+++ b/crates/ra_syntax/src/parser_impl.rs
@@ -63,7 +63,7 @@ pub(crate) fn parse_with<S: Sink>(
 /// to a separate struct in order not to pollute
 /// the public API of the `Parser`.
 pub(crate) struct ParserImpl<'t> {
-    inp: &'t ParserInput<'t>,
+    parser_input: &'t ParserInput<'t>,
     pos: InputPosition,
     events: Vec<Event>,
     steps: Cell<u32>,
@@ -72,7 +72,7 @@ pub(crate) struct ParserImpl<'t> {
 impl<'t> ParserImpl<'t> {
     pub(crate) fn new(inp: &'t ParserInput<'t>) -> ParserImpl<'t> {
         ParserImpl {
-            inp,
+            parser_input: inp,
             pos: InputPosition::new(),
             events: Vec::new(),
             steps: Cell::new(0),
@@ -85,10 +85,10 @@ pub(crate) fn into_events(self) -> Vec<Event> {
     }
 
     pub(super) fn next2(&self) -> Option<(SyntaxKind, SyntaxKind)> {
-        let c1 = self.inp.kind(self.pos);
-        let c2 = self.inp.kind(self.pos + 1);
-        if self.inp.token_start_at(self.pos + 1)
-            == self.inp.token_start_at(self.pos) + self.inp.len(self.pos)
+        let c1 = self.parser_input.kind(self.pos);
+        let c2 = self.parser_input.kind(self.pos + 1);
+        if self.parser_input.token_start_at(self.pos + 1)
+            == self.parser_input.token_start_at(self.pos) + self.parser_input.token_len(self.pos)
         {
             Some((c1, c2))
         } else {
@@ -97,13 +97,14 @@ pub(super) fn next2(&self) -> Option<(SyntaxKind, SyntaxKind)> {
     }
 
     pub(super) fn next3(&self) -> Option<(SyntaxKind, SyntaxKind, SyntaxKind)> {
-        let c1 = self.inp.kind(self.pos);
-        let c2 = self.inp.kind(self.pos + 1);
-        let c3 = self.inp.kind(self.pos + 2);
-        if self.inp.token_start_at(self.pos + 1)
-            == self.inp.token_start_at(self.pos) + self.inp.len(self.pos)
-            && self.inp.token_start_at(self.pos + 2)
-                == self.inp.token_start_at(self.pos + 1) + self.inp.len(self.pos + 1)
+        let c1 = self.parser_input.kind(self.pos);
+        let c2 = self.parser_input.kind(self.pos + 1);
+        let c3 = self.parser_input.kind(self.pos + 2);
+        if self.parser_input.token_start_at(self.pos + 1)
+            == self.parser_input.token_start_at(self.pos) + self.parser_input.token_len(self.pos)
+            && self.parser_input.token_start_at(self.pos + 2)
+                == self.parser_input.token_start_at(self.pos + 1)
+                    + self.parser_input.token_len(self.pos + 1)
         {
             Some((c1, c2, c3))
         } else {
@@ -111,29 +112,27 @@ pub(super) fn next3(&self) -> Option<(SyntaxKind, SyntaxKind, SyntaxKind)> {
         }
     }
 
+    /// Get the syntax kind of the nth token.
     pub(super) fn nth(&self, n: u32) -> SyntaxKind {
         let steps = self.steps.get();
-        if steps > 10_000_000 {
-            panic!("the parser seems stuck");
-        }
+        assert!(steps <= 10_000_000, "the parser seems stuck");
         self.steps.set(steps + 1);
 
-        self.inp.kind(self.pos + n)
+        self.parser_input.kind(self.pos + n)
     }
 
     pub(super) fn at_kw(&self, t: &str) -> bool {
-        self.inp.text(self.pos) == t
+        self.parser_input.token_text(self.pos) == t
     }
 
+    /// Start parsing right behind the last event.
     pub(super) fn start(&mut self) -> u32 {
         let pos = self.events.len() as u32;
-        self.event(Event::Start {
-            kind: TOMBSTONE,
-            forward_parent: None,
-        });
+        self.push_event(Event::tombstone());
         pos
     }
 
+    /// Advances the parser by one token unconditionally.
     pub(super) fn bump(&mut self) {
         let kind = self.nth(0);
         if kind == EOF {
@@ -156,15 +155,17 @@ pub(super) fn bump_compound(&mut self, kind: SyntaxKind, n: u8) {
 
     fn do_bump(&mut self, kind: SyntaxKind, n_raw_tokens: u8) {
         self.pos += u32::from(n_raw_tokens);
-        self.event(Event::Token { kind, n_raw_tokens });
+        self.push_event(Event::Token { kind, n_raw_tokens });
     }
 
+    /// Append one Error event to the back of events.
     pub(super) fn error(&mut self, msg: String) {
-        self.event(Event::Error {
+        self.push_event(Event::Error {
             msg: ParseError(msg),
         })
     }
 
+    /// Complete an event with appending a `Finish` event.
     pub(super) fn complete(&mut self, pos: u32, kind: SyntaxKind) {
         match self.events[pos as usize] {
             Event::Start {
@@ -174,9 +175,10 @@ pub(super) fn complete(&mut self, pos: u32, kind: SyntaxKind) {
             }
             _ => unreachable!(),
         }
-        self.event(Event::Finish);
+        self.push_event(Event::Finish);
     }
 
+    /// Ignore the dummy `Start` event.
     pub(super) fn abandon(&mut self, pos: u32) {
         let idx = pos as usize;
         if idx == self.events.len() - 1 {
@@ -190,6 +192,7 @@ pub(super) fn abandon(&mut self, pos: u32) {
         }
     }
 
+    /// Save the relative distance of a completed event to its forward_parent.
     pub(super) fn precede(&mut self, pos: u32) -> u32 {
         let new_pos = self.start();
         match self.events[pos as usize] {
@@ -204,7 +207,7 @@ pub(super) fn precede(&mut self, pos: u32) -> u32 {
         new_pos
     }
 
-    fn event(&mut self, event: Event) {
+    fn push_event(&mut self, event: Event) {
         self.events.push(event)
     }
 }
diff --git a/crates/ra_syntax/src/parser_impl/event.rs b/crates/ra_syntax/src/parser_impl/event.rs
index d6299b5e37f..835b2c78dc7 100644
--- a/crates/ra_syntax/src/parser_impl/event.rs
+++ b/crates/ra_syntax/src/parser_impl/event.rs
@@ -36,7 +36,7 @@ pub(crate) enum Event {
     ///
     /// For left-recursive syntactic constructs, the parser produces
     /// a child node before it sees a parent. `forward_parent`
-    /// exists to allow to tweak parent-child relationships.
+    /// saves the position of current event's parent.
     ///
     /// Consider this path
     ///
@@ -84,6 +84,15 @@ pub(crate) enum Event {
     },
 }
 
+impl Event {
+    pub(crate) fn tombstone() -> Self {
+        Event::Start {
+            kind: TOMBSTONE,
+            forward_parent: None,
+        }
+    }
+}
+
 pub(super) struct EventProcessor<'a, S: Sink> {
     sink: S,
     text_pos: TextUnit,
@@ -110,17 +119,12 @@ pub(super) fn new(
         }
     }
 
+    /// Generate the syntax tree with the control of events.
     pub(super) fn process(mut self) -> S {
-        fn tombstone() -> Event {
-            Event::Start {
-                kind: TOMBSTONE,
-                forward_parent: None,
-            }
-        }
         let mut forward_parents = Vec::new();
 
         for i in 0..self.events.len() {
-            match mem::replace(&mut self.events[i], tombstone()) {
+            match mem::replace(&mut self.events[i], Event::tombstone()) {
                 Event::Start {
                     kind: TOMBSTONE, ..
                 } => (),
@@ -129,12 +133,18 @@ fn tombstone() -> Event {
                     kind,
                     forward_parent,
                 } => {
+                    // For events[A, B, C], B is A's forward_parent, C is B's forward_parent,
+                    // in the normal control flow, the parent-child relation: `A -> B -> C`,
+                    // while with the magic forward_parent, it writes: `C <- B <- A`.
+
+                    // append `A` into parents.
                     forward_parents.push(kind);
                     let mut idx = i;
                     let mut fp = forward_parent;
                     while let Some(fwd) = fp {
                         idx += fwd as usize;
-                        fp = match mem::replace(&mut self.events[idx], tombstone()) {
+                        // append `A`'s forward_parent `B`
+                        fp = match mem::replace(&mut self.events[idx], Event::tombstone()) {
                             Event::Start {
                                 kind,
                                 forward_parent,
@@ -144,14 +154,16 @@ fn tombstone() -> Event {
                             }
                             _ => unreachable!(),
                         };
+                        // append `B`'s forward_parent `C` in the next stage.
                     }
+
                     for kind in forward_parents.drain(..).rev() {
                         self.start(kind);
                     }
                 }
                 Event::Finish => {
-                    let last = i == self.events.len() - 1;
-                    self.finish(last);
+                    let is_last = i == self.events.len() - 1;
+                    self.finish(is_last);
                 }
                 Event::Token { kind, n_raw_tokens } => {
                     self.eat_trivias();
@@ -171,6 +183,7 @@ fn tombstone() -> Event {
         self.sink
     }
 
+    /// Add the node into syntax tree but discard the comments/whitespaces.
     fn start(&mut self, kind: SyntaxKind) {
         if kind == SOURCE_FILE {
             self.sink.start_branch(kind);
@@ -198,8 +211,8 @@ fn start(&mut self, kind: SyntaxKind) {
         self.eat_n_trivias(n_attached_trivias);
     }
 
-    fn finish(&mut self, last: bool) {
-        if last {
+    fn finish(&mut self, is_last: bool) {
+        if is_last {
             self.eat_trivias()
         }
         self.sink.finish_branch();
@@ -235,6 +248,7 @@ fn n_attached_trivias<'a>(
     kind: SyntaxKind,
     trivias: impl Iterator<Item = (SyntaxKind, &'a str)>,
 ) -> usize {
+    // FIXME: parse attached trivias of CONST_DEF/TYPE_DEF
     match kind {
         STRUCT_DEF | ENUM_DEF | FN_DEF | TRAIT_DEF | MODULE => {
             let mut res = 0;
diff --git a/crates/ra_syntax/src/parser_impl/input.rs b/crates/ra_syntax/src/parser_impl/input.rs
index 083a7aa152c..7fde5b3ab19 100644
--- a/crates/ra_syntax/src/parser_impl/input.rs
+++ b/crates/ra_syntax/src/parser_impl/input.rs
@@ -70,7 +70,7 @@ pub fn token_start_at(&self, pos: InputPosition) -> TextUnit {
         self.start_offsets[idx]
     }
 
-    /// Get the raw text of a toen at given input position.
+    /// Get the raw text of a token at given input position.
     pub fn token_text(&self, pos: InputPosition) -> &'t str {
         let idx = pos.0 as usize;
         if !(idx < self.tokens.len()) {