1 files changed, 31 insertions, 1 deletions

diff --git a/crates/ra_syntax/src/parsing.rs b/crates/ra_syntax/src/parsing.rs
index e5eb80850..0ed3c20ef 100644
--- a/crates/ra_syntax/src/parsing.rs
+++ b/crates/ra_syntax/src/parsing.rs
@@ -6,13 +6,14 @@ mod text_token_source;
 mod text_tree_sink;
 mod reparsing;
 
-use crate::{syntax_node::GreenNode, SyntaxError};
+use crate::{syntax_node::GreenNode, AstNode, SyntaxError, SyntaxNode};
 use text_token_source::TextTokenSource;
 use text_tree_sink::TextTreeSink;
 
 pub use lexer::*;
 
 pub(crate) use self::reparsing::incremental_reparse;
+use ra_parser::SyntaxKind;
 
 pub(crate) fn parse_text(text: &str) -> (GreenNode, Vec<SyntaxError>) {
     let (tokens, lexer_errors) = tokenize(&text);
@@ -27,3 +28,32 @@ pub(crate) fn parse_text(text: &str) -> (GreenNode, Vec<SyntaxError>) {
 
     (tree, parser_errors)
 }
+
+/// Returns `text` parsed as a `T` provided there are no parse errors.
+pub(crate) fn parse_text_fragment<T: AstNode>(
+    text: &str,
+    fragment_kind: ra_parser::FragmentKind,
+) -> Result<T, ()> {
+    let (tokens, lexer_errors) = tokenize(&text);
+    if !lexer_errors.is_empty() {
+        return Err(());
+    }
+
+    let mut token_source = TextTokenSource::new(text, &tokens);
+    let mut tree_sink = TextTreeSink::new(text, &tokens);
+
+    // TextTreeSink assumes that there's at least some root node to which it can attach errors and
+    // tokens. We arbitrarily give it a SourceFile.
+    use ra_parser::TreeSink;
+    tree_sink.start_node(SyntaxKind::SOURCE_FILE);
+    ra_parser::parse_fragment(&mut token_source, &mut tree_sink, fragment_kind);
+    tree_sink.finish_node();
+
+    let (tree, parser_errors) = tree_sink.finish();
+    use ra_parser::TokenSource;
+    if !parser_errors.is_empty() || token_source.current().kind != SyntaxKind::EOF {
+        return Err(());
+    }
+
+    SyntaxNode::new_root(tree).first_child().and_then(T::cast).ok_or(())
+}