Add fuzz test for reparsing

crates/ra_syntax/src/fuzz.rs

@@ -1,4 +1,6 @@
-use crate::{SourceFile, validation, AstNode};
+use crate::{SourceFile, validation, TextUnit, TextRange, AstNode};
+use ra_text_edit::AtomTextEdit;
+use std::str::{self, FromStr};
 
 fn check_file_invariants(file: &SourceFile) {
     let root = file.syntax();
@@ -10,3 +12,41 @@ pub fn check_parser(text: &str) {
     let file = SourceFile::parse(text);
     check_file_invariants(&file);
 }
+
+#[derive(Debug, Clone)]
+pub struct CheckReparse {
+    text: String,
+    edit: AtomTextEdit,
+    edited_text: String,
+}
+
+impl CheckReparse {
+    pub fn from_data(data: &[u8]) -> Option<Self> {
+        let data = str::from_utf8(data).ok()?;
+        let mut lines = data.lines();
+        let delete_start = usize::from_str(lines.next()?).ok()?;
+        let delete_len = usize::from_str(lines.next()?).ok()?;
+        let insert = lines.next()?.to_string();
+        let text = lines.collect::<Vec<_>>().join("\n");
+        text.get(delete_start..delete_start.checked_add(delete_len)?)?; // make sure delete is a valid range
+        let delete = TextRange::offset_len(
+            TextUnit::from_usize(delete_start),
+            TextUnit::from_usize(delete_len),
+        );
+        let edited_text =
+            format!("{}{}{}", &text[..delete_start], &insert, &text[delete_start + delete_len..]);
+        let edit = AtomTextEdit { delete, insert };
+        Some(CheckReparse { text, edit, edited_text })
+    }
+
+    pub fn run(&self) {
+        let file = SourceFile::parse(&self.text);
+        let new_file = file.reparse(&self.edit);
+        check_file_invariants(&new_file);
+        assert_eq!(&new_file.syntax().text().to_string(), &self.edited_text);
+        let full_reparse = SourceFile::parse(&self.edited_text);
+        for (a, b) in new_file.syntax().descendants().zip(full_reparse.syntax().descendants()) {
+            assert_eq!(a.kind(), b.kind(), "different syntax tree produced by a full reparse");
+        }
+    }
+}