Rework diff rendering to allow putting the cursor into deleted text, soft-wrapping and scrolling deleted text correctly (#22994)

Closes #12553

* [x] Fix `diff_hunk_before`
* [x] Fix failure to show deleted text when expanding hunk w/ cursor on
second line of the hunk
* [x] Failure to expand diff hunk below the cursor.
* [x] Delete the whole file, and expand the diff. Backspace over the
deleted hunk, panic!
* [x] Go-to-line now counts the diff hunks, but it should not
* [x] backspace at the beginning of a deleted hunk deletes too much text
* [x] Indent guides are rendered incorrectly 
* [ ] Fix randomized multi buffer tests

Maybe:
* [ ] Buffer search should include deleted text (in vim mode it turns
out I use `/x` all the time to jump to the next x I can see).
* [ ] vim: should refuse to switch into insert mode if selection is
fully within a diff.
* [ ] vim `o` command when cursor is on last line of deleted hunk.
* [ ] vim `shift-o` on first line of deleted hunk moves cursor but
doesn't insert line
* [x] `enter` at end of diff hunk inserts a new line but doesn't move
cursor
* [x] (`shift-enter` at start of diff hunk does nothing)
* [ ] Inserting a line just before an expanded hunk collapses it

Release Notes:


- Improved diff rendering, allowing you to navigate with your cursor
inside of deleted text in diff hunks.

---------

Co-authored-by: Conrad <conrad@zed.dev>
Co-authored-by: Cole <cole@zed.dev>
Co-authored-by: Mikayla <mikayla@zed.dev>
Co-authored-by: Conrad Irwin <conrad.irwin@gmail.com>
Co-authored-by: Michael <michael@zed.dev>
Co-authored-by: Agus <agus@zed.dev>
Co-authored-by: João <joao@zed.dev>

crates/multi_buffer/src/anchor.rs

@@ -12,14 +12,38 @@ pub struct Anchor {
     pub buffer_id: Option<BufferId>,
     pub excerpt_id: ExcerptId,
     pub text_anchor: text::Anchor,
+    pub diff_base_anchor: Option<text::Anchor>,
 }
 
 impl Anchor {
+    pub fn in_buffer(
+        excerpt_id: ExcerptId,
+        buffer_id: BufferId,
+        text_anchor: text::Anchor,
+    ) -> Self {
+        Self {
+            buffer_id: Some(buffer_id),
+            excerpt_id,
+            text_anchor,
+            diff_base_anchor: None,
+        }
+    }
+
+    pub fn range_in_buffer(
+        excerpt_id: ExcerptId,
+        buffer_id: BufferId,
+        range: Range<text::Anchor>,
+    ) -> Range<Self> {
+        Self::in_buffer(excerpt_id, buffer_id, range.start)
+            ..Self::in_buffer(excerpt_id, buffer_id, range.end)
+    }
+
     pub fn min() -> Self {
         Self {
             buffer_id: None,
             excerpt_id: ExcerptId::min(),
             text_anchor: text::Anchor::MIN,
+            diff_base_anchor: None,
         }
     }
 
@@ -28,22 +52,47 @@ impl Anchor {
             buffer_id: None,
             excerpt_id: ExcerptId::max(),
             text_anchor: text::Anchor::MAX,
+            diff_base_anchor: None,
         }
     }
 
     pub fn cmp(&self, other: &Anchor, snapshot: &MultiBufferSnapshot) -> Ordering {
         let excerpt_id_cmp = self.excerpt_id.cmp(&other.excerpt_id, snapshot);
-        if excerpt_id_cmp.is_eq() {
-            if self.excerpt_id == ExcerptId::min() || self.excerpt_id == ExcerptId::max() {
-                Ordering::Equal
-            } else if let Some(excerpt) = snapshot.excerpt(self.excerpt_id) {
-                self.text_anchor.cmp(&other.text_anchor, &excerpt.buffer)
-            } else {
-                Ordering::Equal
-            }
-        } else {
-            excerpt_id_cmp
+        if excerpt_id_cmp.is_ne() {
+            return excerpt_id_cmp;
         }
+        if self.excerpt_id == ExcerptId::min() || self.excerpt_id == ExcerptId::max() {
+            return Ordering::Equal;
+        }
+        if let Some(excerpt) = snapshot.excerpt(self.excerpt_id) {
+            let text_cmp = self.text_anchor.cmp(&other.text_anchor, &excerpt.buffer);
+            if text_cmp.is_ne() {
+                return text_cmp;
+            }
+            if self.diff_base_anchor.is_some() || other.diff_base_anchor.is_some() {
+                if let Some(diff_base) = snapshot.diffs.get(&excerpt.buffer_id) {
+                    let self_anchor = self
+                        .diff_base_anchor
+                        .filter(|a| diff_base.base_text.can_resolve(a));
+                    let other_anchor = other
+                        .diff_base_anchor
+                        .filter(|a| diff_base.base_text.can_resolve(a));
+                    return match (self_anchor, other_anchor) {
+                        (Some(a), Some(b)) => a.cmp(&b, &diff_base.base_text),
+                        (Some(_), None) => match other.text_anchor.bias {
+                            Bias::Left => Ordering::Greater,
+                            Bias::Right => Ordering::Less,
+                        },
+                        (None, Some(_)) => match self.text_anchor.bias {
+                            Bias::Left => Ordering::Less,
+                            Bias::Right => Ordering::Greater,
+                        },
+                        (None, None) => Ordering::Equal,
+                    };
+                }
+            }
+        }
+        Ordering::Equal
     }
 
     pub fn bias(&self) -> Bias {
@@ -57,6 +106,14 @@ impl Anchor {
                     buffer_id: self.buffer_id,
                     excerpt_id: self.excerpt_id,
                     text_anchor: self.text_anchor.bias_left(&excerpt.buffer),
+                    diff_base_anchor: self.diff_base_anchor.map(|a| {
+                        if let Some(base) = snapshot.diffs.get(&excerpt.buffer_id) {
+                            if a.buffer_id == Some(base.base_text.remote_id()) {
+                                return a.bias_left(&base.base_text);
+                            }
+                        }
+                        a
+                    }),
                 };
             }
         }
@@ -70,6 +127,14 @@ impl Anchor {
                     buffer_id: self.buffer_id,
                     excerpt_id: self.excerpt_id,
                     text_anchor: self.text_anchor.bias_right(&excerpt.buffer),
+                    diff_base_anchor: self.diff_base_anchor.map(|a| {
+                        if let Some(base) = snapshot.diffs.get(&excerpt.buffer_id) {
+                            if a.buffer_id == Some(base.base_text.remote_id()) {
+                                return a.bias_right(&base.base_text);
+                            }
+                        }
+                        a
+                    }),
                 };
             }
         }