5 files changed, 104 insertions, 13 deletions

diff --git a/crates/libsyntax2/src/grammar/mod.rs b/crates/libsyntax2/src/grammar/mod.rs
index 46ba8a89a..496d28349 100644
--- a/crates/libsyntax2/src/grammar/mod.rs
+++ b/crates/libsyntax2/src/grammar/mod.rs
@@ -40,11 +40,11 @@ pub(crate) use self::{
     items::named_field_def_list,
 };
 
-pub(crate) fn file(p: &mut Parser) {
-    let file = p.start();
+pub(crate) fn root(p: &mut Parser) {
+    let m = p.start();
     p.eat(SHEBANG);
     items::mod_contents(p, false);
-    file.complete(p, ROOT);
+    m.complete(p, ROOT);
 }
 
 
diff --git a/crates/libsyntax2/src/lib.rs b/crates/libsyntax2/src/lib.rs
index bb060cbae..d43d26c4c 100644
--- a/crates/libsyntax2/src/lib.rs
+++ b/crates/libsyntax2/src/lib.rs
@@ -70,13 +70,15 @@ impl File {
     }
     pub fn parse(text: &str) -> File {
         let tokens = tokenize(&text);
-        let (green, errors) = parser_impl::parse::<yellow::GreenBuilder>(text, &tokens);
+        let (green, errors) = parser_impl::parse_with::<yellow::GreenBuilder>(
+            text, &tokens, grammar::root,
+        );
         File::new(green, errors)
     }
     pub fn reparse(&self, edit: &AtomEdit) -> File {
         self.incremental_reparse(edit).unwrap_or_else(|| self.full_reparse(edit))
     }
-    fn incremental_reparse(&self, edit: &AtomEdit) -> Option<File> {
+    pub fn incremental_reparse(&self, edit: &AtomEdit) -> Option<File> {
         let (node, reparser) = find_reparsable_node(self.syntax(), edit.delete)?;
         let text = replace_range(
             node.text(),
@@ -87,7 +89,12 @@ impl File {
         if !is_balanced(&tokens) {
             return None;
         }
-        None
+        let (green, new_errors) = parser_impl::parse_with::<yellow::GreenBuilder>(
+            &text, &tokens, reparser,
+        );
+        let green_root = node.replace_with(green);
+        let errors = merge_errors(self.errors(), new_errors, edit, node.range().start());
+        Some(File::new(green_root, errors))
     }
     fn full_reparse(&self, edit: &AtomEdit) -> File {
         let text = replace_range(self.syntax().text(), edit.delete, &edit.insert);
@@ -173,7 +180,7 @@ fn find_reparsable_node(node: SyntaxNodeRef, range: TextRange) -> Option<(Syntax
     }
 }
 
-fn replace_range(mut text: String, range: TextRange, replace_with: &str) -> String {
+pub /*(meh)*/ fn replace_range(mut text: String, range: TextRange, replace_with: &str) -> String {
     let start = u32::from(range.start()) as usize;
     let end = u32::from(range.end()) as usize;
     text.replace_range(start..end, replace_with);
@@ -199,3 +206,29 @@ fn is_balanced(tokens: &[Token]) -> bool {
     }
     balance == 0
 }
+
+fn merge_errors(
+    old_errors: Vec<SyntaxError>,
+    new_errors: Vec<SyntaxError>,
+    edit: &AtomEdit,
+    node_offset: TextUnit,
+) -> Vec<SyntaxError> {
+    let mut res = Vec::new();
+    for e in old_errors {
+        if e.offset < edit.delete.start() {
+            res.push(e)
+        } else if e.offset > edit.delete.end() {
+            res.push(SyntaxError {
+                msg: e.msg,
+                offset: e.offset + TextUnit::of_str(&edit.insert) - edit.delete.len(),
+            })
+        }
+    }
+    for e in new_errors {
+        res.push(SyntaxError {
+            msg: e.msg,
+            offset: e.offset + node_offset,
+        })
+    }
+    res
+}
diff --git a/crates/libsyntax2/src/parser_impl/mod.rs b/crates/libsyntax2/src/parser_impl/mod.rs
index 14cceced5..f60ef80f0 100644
--- a/crates/libsyntax2/src/parser_impl/mod.rs
+++ b/crates/libsyntax2/src/parser_impl/mod.rs
@@ -2,7 +2,6 @@ mod event;
 mod input;
 
 use {
-    grammar,
     lexer::Token,
     parser_api::Parser,
     parser_impl::{
@@ -27,12 +26,16 @@ pub(crate) trait Sink<'a> {
 }
 
 /// Parse a sequence of tokens into the representative node tree
-pub(crate) fn parse<'a, S: Sink<'a>>(text: &'a str, tokens: &[Token]) -> S::Tree {
+pub(crate) fn parse_with<'a, S: Sink<'a>>(
+    text: &'a str,
+    tokens: &[Token],
+    parser: fn(&mut Parser),
+) -> S::Tree {
     let events = {
         let input = input::ParserInput::new(text, tokens);
         let parser_impl = ParserImpl::new(&input);
         let mut parser_api = Parser(parser_impl);
-        grammar::file(&mut parser_api);
+        parser(&mut parser_api);
         parser_api.0.into_events()
     };
     let mut sink = S::new(text);
diff --git a/crates/libsyntax2/src/yellow/syntax.rs b/crates/libsyntax2/src/yellow/syntax.rs
index 8f1b1d79a..0045598d4 100644
--- a/crates/libsyntax2/src/yellow/syntax.rs
+++ b/crates/libsyntax2/src/yellow/syntax.rs
@@ -3,7 +3,7 @@ use std::{fmt, sync::Arc};
 use smol_str::SmolStr;
 
 use {
-    yellow::{RedNode, TreeRoot, SyntaxRoot, RedPtr, RefRoot, OwnedRoot},
+    yellow::{GreenNode, RedNode, TreeRoot, SyntaxRoot, RedPtr, RefRoot, OwnedRoot},
     SyntaxKind::{self, *},
     TextRange, TextUnit,
 };
@@ -141,6 +141,27 @@ impl<R: TreeRoot> SyntaxNode<R> {
         self.red().green().leaf_text()
     }
 
+    pub(crate) fn replace_with(&self, green: GreenNode) -> GreenNode {
+        assert_eq!(self.kind(), green.kind());
+        match self.parent() {
+            None => green,
+            Some(parent) => {
+                let children: Vec<_> = parent.children().map(|child| {
+                    if child == *self {
+                        green.clone()
+                    } else {
+                        child.red().green().clone()
+                    }
+                }).collect();
+                let new_parent = GreenNode::new_branch(
+                    parent.kind(),
+                    children.into_boxed_slice(),
+                );
+                parent.replace_with(new_parent)
+            },
+        }
+    }
+
     fn red(&self) -> &RedNode {
         unsafe { self.red.get(&self.root) }
     }
diff --git a/crates/libsyntax2/tests/test/main.rs b/crates/libsyntax2/tests/test/main.rs
index cb8a52c98..e7ae4d601 100644
--- a/crates/libsyntax2/tests/test/main.rs
+++ b/crates/libsyntax2/tests/test/main.rs
@@ -9,7 +9,11 @@ use std::{
     fmt::Write,
 };
 
-use libsyntax2::File;
+use test_utils::extract_range;
+use libsyntax2::{
+    File, AtomEdit,
+    utils::dump_tree,
+};
 
 #[test]
 fn lexer_tests() {
@@ -23,10 +27,40 @@ fn lexer_tests() {
 fn parser_tests() {
     dir_tests(&["parser/inline", "parser/ok", "parser/err"], |text| {
         let file = File::parse(text);
-        libsyntax2::utils::dump_tree(file.syntax())
+        dump_tree(file.syntax())
     })
 }
 
+#[test]
+fn reparse_test() {
+    fn do_check(before: &str, replace_with: &str) {
+        let (range, before) = extract_range(before);
+        let after = libsyntax2::replace_range(before.clone(), range, replace_with);
+
+        let fully_reparsed = File::parse(&after);
+        let incrementally_reparsed = {
+            let f = File::parse(&before);
+            let edit = AtomEdit { delete: range, insert: replace_with.to_string() };
+            f.incremental_reparse(&edit).unwrap()
+        };
+        assert_eq_text!(
+            &dump_tree(fully_reparsed.syntax()),
+            &dump_tree(incrementally_reparsed.syntax()),
+        )
+    }
+
+    do_check(r"
+fn foo() {
+    let x = foo + <|>bar<|>
+}
+", "baz");
+    do_check(r"
+struct Foo {
+    f: foo<|><|>
+}
+", ",\n    g: (),");
+}
+
 
 /// Read file and normalize newlines.
 ///