1 files changed, 247 insertions, 0 deletions

diff --git a/crates/ra_syntax/src/parsing/event.rs b/crates/ra_syntax/src/parsing/event.rs
new file mode 100644
index 000000000..f6f020eab
--- /dev/null
+++ b/crates/ra_syntax/src/parsing/event.rs
@@ -0,0 +1,247 @@
+//! This module provides a way to construct a `File`.
+//! It is intended to be completely decoupled from the
+//! parser, so as to allow to evolve the tree representation
+//! and the parser algorithm independently.
+//!
+//! The `TreeSink` trait is the bridge between the parser and the
+//! tree builder: the parser produces a stream of events like
+//! `start node`, `finish node`, and `FileBuilder` converts
+//! this stream to a real tree.
+use std::mem;
+
+use crate::{
+    SmolStr,
+    SyntaxKind::{self, *},
+    TextRange, TextUnit,
+    parsing::{
+        ParseError, TreeSink,
+        lexer::Token,
+    },
+};
+
+/// `Parser` produces a flat list of `Event`s.
+/// They are converted to a tree-structure in
+/// a separate pass, via `TreeBuilder`.
+#[derive(Debug)]
+pub(crate) enum Event {
+    /// This event signifies the start of the node.
+    /// It should be either abandoned (in which case the
+    /// `kind` is `TOMBSTONE`, and the event is ignored),
+    /// or completed via a `Finish` event.
+    ///
+    /// All tokens between a `Start` and a `Finish` would
+    /// become the children of the respective node.
+    ///
+    /// For left-recursive syntactic constructs, the parser produces
+    /// a child node before it sees a parent. `forward_parent`
+    /// saves the position of current event's parent.
+    ///
+    /// Consider this path
+    ///
+    /// foo::bar
+    ///
+    /// The events for it would look like this:
+    ///
+    ///
+    /// START(PATH) IDENT('foo') FINISH START(PATH) COLONCOLON IDENT('bar') FINISH
+    ///       |                          /\
+    ///       |                          |
+    ///       +------forward-parent------+
+    ///
+    /// And the tree would look like this
+    ///
+    ///    +--PATH---------+
+    ///    |   |           |
+    ///    |   |           |
+    ///    |  '::'       'bar'
+    ///    |
+    ///   PATH
+    ///    |
+    ///   'foo'
+    ///
+    /// See also `CompletedMarker::precede`.
+    Start {
+        kind: SyntaxKind,
+        forward_parent: Option<u32>,
+    },
+
+    /// Complete the previous `Start` event
+    Finish,
+
+    /// Produce a single leaf-element.
+    /// `n_raw_tokens` is used to glue complex contextual tokens.
+    /// For example, lexer tokenizes `>>` as `>`, `>`, and
+    /// `n_raw_tokens = 2` is used to produced a single `>>`.
+    Token {
+        kind: SyntaxKind,
+        n_raw_tokens: u8,
+    },
+
+    Error {
+        msg: ParseError,
+    },
+}
+
+impl Event {
+    pub(crate) fn tombstone() -> Self {
+        Event::Start { kind: TOMBSTONE, forward_parent: None }
+    }
+}
+
+pub(super) struct EventProcessor<'a, S: TreeSink> {
+    sink: S,
+    text_pos: TextUnit,
+    text: &'a str,
+    token_pos: usize,
+    tokens: &'a [Token],
+    events: &'a mut [Event],
+}
+
+impl<'a, S: TreeSink> EventProcessor<'a, S> {
+    pub(super) fn new(
+        sink: S,
+        text: &'a str,
+        tokens: &'a [Token],
+        events: &'a mut [Event],
+    ) -> EventProcessor<'a, S> {
+        EventProcessor { sink, text_pos: 0.into(), text, token_pos: 0, tokens, events }
+    }
+
+    /// Generate the syntax tree with the control of events.
+    pub(crate) fn process(mut self) -> S {
+        let mut forward_parents = Vec::new();
+
+        for i in 0..self.events.len() {
+            match mem::replace(&mut self.events[i], Event::tombstone()) {
+                Event::Start { kind: TOMBSTONE, .. } => (),
+
+                Event::Start { 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;
+                        // append `A`'s forward_parent `B`
+                        fp = match mem::replace(&mut self.events[idx], Event::tombstone()) {
+                            Event::Start { kind, forward_parent } => {
+                                forward_parents.push(kind);
+                                forward_parent
+                            }
+                            _ => unreachable!(),
+                        };
+                        // append `B`'s forward_parent `C` in the next stage.
+                    }
+
+                    for kind in forward_parents.drain(..).rev() {
+                        self.start(kind);
+                    }
+                }
+                Event::Finish => {
+                    let is_last = i == self.events.len() - 1;
+                    self.finish(is_last);
+                }
+                Event::Token { kind, n_raw_tokens } => {
+                    self.eat_trivias();
+                    let n_raw_tokens = n_raw_tokens as usize;
+                    let len = self.tokens[self.token_pos..self.token_pos + n_raw_tokens]
+                        .iter()
+                        .map(|it| it.len)
+                        .sum::<TextUnit>();
+                    self.leaf(kind, len, n_raw_tokens);
+                }
+                Event::Error { msg } => self.sink.error(msg),
+            }
+        }
+        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);
+            return;
+        }
+        let n_trivias =
+            self.tokens[self.token_pos..].iter().take_while(|it| it.kind.is_trivia()).count();
+        let leading_trivias = &self.tokens[self.token_pos..self.token_pos + n_trivias];
+        let mut trivia_end =
+            self.text_pos + leading_trivias.iter().map(|it| it.len).sum::<TextUnit>();
+
+        let n_attached_trivias = {
+            let leading_trivias = leading_trivias.iter().rev().map(|it| {
+                let next_end = trivia_end - it.len;
+                let range = TextRange::from_to(next_end, trivia_end);
+                trivia_end = next_end;
+                (it.kind, &self.text[range])
+            });
+            n_attached_trivias(kind, leading_trivias)
+        };
+        self.eat_n_trivias(n_trivias - n_attached_trivias);
+        self.sink.start_branch(kind);
+        self.eat_n_trivias(n_attached_trivias);
+    }
+
+    fn finish(&mut self, is_last: bool) {
+        if is_last {
+            self.eat_trivias()
+        }
+        self.sink.finish_branch();
+    }
+
+    fn eat_trivias(&mut self) {
+        while let Some(&token) = self.tokens.get(self.token_pos) {
+            if !token.kind.is_trivia() {
+                break;
+            }
+            self.leaf(token.kind, token.len, 1);
+        }
+    }
+
+    fn eat_n_trivias(&mut self, n: usize) {
+        for _ in 0..n {
+            let token = self.tokens[self.token_pos];
+            assert!(token.kind.is_trivia());
+            self.leaf(token.kind, token.len, 1);
+        }
+    }
+
+    fn leaf(&mut self, kind: SyntaxKind, len: TextUnit, n_tokens: usize) {
+        let range = TextRange::offset_len(self.text_pos, len);
+        let text: SmolStr = self.text[range].into();
+        self.text_pos += len;
+        self.token_pos += n_tokens;
+        self.sink.leaf(kind, text);
+    }
+}
+
+fn n_attached_trivias<'a>(
+    kind: SyntaxKind,
+    trivias: impl Iterator<Item = (SyntaxKind, &'a str)>,
+) -> usize {
+    match kind {
+        CONST_DEF | TYPE_DEF | STRUCT_DEF | ENUM_DEF | ENUM_VARIANT | FN_DEF | TRAIT_DEF
+        | MODULE | NAMED_FIELD_DEF => {
+            let mut res = 0;
+            for (i, (kind, text)) in trivias.enumerate() {
+                match kind {
+                    WHITESPACE => {
+                        if text.contains("\n\n") {
+                            break;
+                        }
+                    }
+                    COMMENT => {
+                        res = i + 1;
+                    }
+                    _ => (),
+                }
+            }
+            res
+        }
+        _ => 0,
+    }
+}