Make Buffer::edit and MultiBuffer::edit resilient to inverted ranges

Previously, we would accept edits containing out-of-order ranges. When
generating such ranges in our randomized tests, many invariants started
breaking causing e.g. undo/redo to misbehave and operation application
to panic.

In theory, we should never pass inverted ranges, but this commit changes
the above functions to swap the start and the end when that occurs to avoid
breaking the entire system and panicking.

crates/language/src/buffer.rs

@@ -23,6 +23,7 @@ use std::{
     ffi::OsString,
     future::Future,
     iter::{Iterator, Peekable},
+    mem,
     ops::{Deref, DerefMut, Range},
     path::{Path, PathBuf},
     str,
@@ -1108,7 +1109,10 @@ impl Buffer {
         // Skip invalid edits and coalesce contiguous ones.
         let mut edits: Vec<(Range<usize>, Arc<str>)> = Vec::new();
         for (range, new_text) in edits_iter {
-            let range = range.start.to_offset(self)..range.end.to_offset(self);
+            let mut range = range.start.to_offset(self)..range.end.to_offset(self);
+            if range.start > range.end {
+                mem::swap(&mut range.start, &mut range.end);
+            }
             let new_text = new_text.into();
             if !new_text.is_empty() || !range.is_empty() {
                 if let Some((prev_range, prev_text)) = edits.last_mut() {
@@ -1452,7 +1456,10 @@ impl Buffer {
             }
 
             let new_start = last_end.map_or(0, |last_end| last_end + 1);
-            let range = self.random_byte_range(new_start, rng);
+            let mut range = self.random_byte_range(new_start, rng);
+            if rng.gen_bool(0.2) {
+                mem::swap(&mut range.start, &mut range.end);
+            }
             last_end = Some(range.end);
 
             let new_text_len = rng.gen_range(0..10);