1 files changed, 195 insertions, 0 deletions

diff --git a/crates/ra_syntax/src/parsing/parser_api.rs b/crates/ra_syntax/src/parsing/parser_api.rs
new file mode 100644
index 000000000..781c407de
--- /dev/null
+++ b/crates/ra_syntax/src/parsing/parser_api.rs
@@ -0,0 +1,195 @@
+use drop_bomb::DropBomb;
+
+use crate::{
+    SyntaxKind::{self, ERROR},
+    parsing::{
+        token_set::TokenSet,
+        parser_impl::ParserImpl
+    },
+};
+
+/// `Parser` struct provides the low-level API for
+/// navigating through the stream of tokens and
+/// constructing the parse tree. The actual parsing
+/// happens in the `grammar` module.
+///
+/// However, the result of this `Parser` is not a real
+/// tree, but rather a flat stream of events of the form
+/// "start expression, consume number literal,
+/// finish expression". See `Event` docs for more.
+pub(crate) struct Parser<'t>(pub(super) ParserImpl<'t>);
+
+impl<'t> Parser<'t> {
+    /// Returns the kind of the current token.
+    /// If parser has already reached the end of input,
+    /// the special `EOF` kind is returned.
+    pub(crate) fn current(&self) -> SyntaxKind {
+        self.nth(0)
+    }
+
+    /// Returns the kinds of the current two tokens, if they are not separated
+    /// by trivia.
+    ///
+    /// Useful for parsing things like `>>`.
+    pub(crate) fn current2(&self) -> Option<(SyntaxKind, SyntaxKind)> {
+        self.0.current2()
+    }
+
+    /// Returns the kinds of the current three tokens, if they are not separated
+    /// by trivia.
+    ///
+    /// Useful for parsing things like `=>>`.
+    pub(crate) fn current3(&self) -> Option<(SyntaxKind, SyntaxKind, SyntaxKind)> {
+        self.0.current3()
+    }
+
+    /// Lookahead operation: returns the kind of the next nth
+    /// token.
+    pub(crate) fn nth(&self, n: u32) -> SyntaxKind {
+        self.0.nth(n)
+    }
+
+    /// Checks if the current token is `kind`.
+    pub(crate) fn at(&self, kind: SyntaxKind) -> bool {
+        self.current() == kind
+    }
+
+    /// Checks if the current token is in `kinds`.
+    pub(crate) fn at_ts(&self, kinds: TokenSet) -> bool {
+        kinds.contains(self.current())
+    }
+
+    /// Checks if the current token is contextual keyword with text `t`.
+    pub(crate) fn at_contextual_kw(&self, t: &str) -> bool {
+        self.0.at_kw(t)
+    }
+
+    /// Starts a new node in the syntax tree. All nodes and tokens
+    /// consumed between the `start` and the corresponding `Marker::complete`
+    /// belong to the same node.
+    pub(crate) fn start(&mut self) -> Marker {
+        Marker::new(self.0.start())
+    }
+
+    /// Advances the parser by one token unconditionally.
+    pub(crate) fn bump(&mut self) {
+        self.0.bump();
+    }
+
+    /// Advances the parser by one token, remapping its kind.
+    /// This is useful to create contextual keywords from
+    /// identifiers. For example, the lexer creates an `union`
+    /// *identifier* token, but the parser remaps it to the
+    /// `union` keyword, and keyword is what ends up in the
+    /// final tree.
+    pub(crate) fn bump_remap(&mut self, kind: SyntaxKind) {
+        self.0.bump_remap(kind);
+    }
+
+    /// Advances the parser by `n` tokens, remapping its kind.
+    /// This is useful to create compound tokens from parts. For
+    /// example, an `<<` token is two consecutive remapped `<` tokens
+    pub(crate) fn bump_compound(&mut self, kind: SyntaxKind, n: u8) {
+        self.0.bump_compound(kind, n);
+    }
+
+    /// Emit error with the `message`
+    /// TODO: this should be much more fancy and support
+    /// structured errors with spans and notes, like rustc
+    /// does.
+    pub(crate) fn error<T: Into<String>>(&mut self, message: T) {
+        self.0.error(message.into())
+    }
+
+    /// Consume the next token if `kind` matches.
+    pub(crate) fn eat(&mut self, kind: SyntaxKind) -> bool {
+        if !self.at(kind) {
+            return false;
+        }
+        self.bump();
+        true
+    }
+
+    /// Consume the next token if it is `kind` or emit an error
+    /// otherwise.
+    pub(crate) fn expect(&mut self, kind: SyntaxKind) -> bool {
+        if self.eat(kind) {
+            return true;
+        }
+        self.error(format!("expected {:?}", kind));
+        false
+    }
+
+    /// Create an error node and consume the next token.
+    pub(crate) fn err_and_bump(&mut self, message: &str) {
+        self.err_recover(message, TokenSet::empty());
+    }
+
+    /// Create an error node and consume the next token.
+    pub(crate) fn err_recover(&mut self, message: &str, recovery: TokenSet) {
+        if self.at(SyntaxKind::L_CURLY) || self.at(SyntaxKind::R_CURLY) || self.at_ts(recovery) {
+            self.error(message);
+        } else {
+            let m = self.start();
+            self.error(message);
+            self.bump();
+            m.complete(self, ERROR);
+        };
+    }
+}
+
+/// See `Parser::start`.
+pub(crate) struct Marker {
+    pos: u32,
+    bomb: DropBomb,
+}
+
+impl Marker {
+    fn new(pos: u32) -> Marker {
+        Marker { pos, bomb: DropBomb::new("Marker must be either completed or abandoned") }
+    }
+
+    /// 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::new(self.pos, kind)
+    }
+
+    /// Abandons the syntax tree node. All its children
+    /// are attached to its parent instead.
+    pub(crate) fn abandon(mut self, p: &mut Parser) {
+        self.bomb.defuse();
+        p.0.abandon(self.pos);
+    }
+}
+
+pub(crate) struct CompletedMarker(u32, SyntaxKind);
+
+impl CompletedMarker {
+    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))
+    }
+
+    pub(crate) fn kind(&self) -> SyntaxKind {
+        self.1
+    }
+}