Merge #1013

1013: Fuzz reparsing and fix found bugs r=matklad a=pcpthm

Add fuzz test for reparsing which:
- Checks reparsing doesn't panic and validate result syntax tree.
- Checks that incremental reparsing produces the same syntax tree as full reparse.
- Check for that errors are the same as full reparsing is disabled because errors are less important than syntax tree and produce failures which I couldn't figure out how to fix immediately (FIXME comment).

I guess the current input generation is inefficient but still found several bugs:
- Arithmetic overflow (negative result on an unsigned type). I changed the signature of `SyntaxError::add_offset` to solve this problem.
- When reparsing a leaf, the token of the leaf can be joined to the next characters. Such case was not considered.
- UNDERSCORE token was not produced when text length is exactly 1 (not a reparsing bug).
- When reparsing a block, *inner* curly braces should be balanced. i.e. `{}{}` is invalid.
- Effects of deleting newlines were not considered.

Co-authored-by: pcpthm <[email protected]>

1 files changed, 3 insertions, 3 deletions

diff --git a/crates/ra_syntax/src/syntax_error.rs b/crates/ra_syntax/src/syntax_error.rs
index bdd431742..4b8c22a57 100644
--- a/crates/ra_syntax/src/syntax_error.rs
+++ b/crates/ra_syntax/src/syntax_error.rs
@@ -48,10 +48,10 @@ impl SyntaxError {
         }
     }
 
-    pub fn add_offset(mut self, plus_offset: TextUnit) -> SyntaxError {
+    pub fn add_offset(mut self, plus_offset: TextUnit, minus_offset: TextUnit) -> SyntaxError {
         self.location = match self.location {
-            Location::Range(range) => Location::Range(range + plus_offset),
-            Location::Offset(offset) => Location::Offset(offset + plus_offset),
+            Location::Range(range) => Location::Range(range + plus_offset - minus_offset),
+            Location::Offset(offset) => Location::Offset(offset + plus_offset - minus_offset),
         };
 
         self