Warn the user when a rename will change the meaning of the program

Specifically, when a rename of a local will change some code that refers it to refer another local, or some code that refer another local to refer to it.

We do it by introducing a dummy edit with an annotation. I'm not a fond of this approach, but I don't think LSP has a better way.

crates/ide-db/src/source_change.rs

@@ -3,7 +3,7 @@
 //!
 //! It can be viewed as a dual for `Change`.
 
-use std::{collections::hash_map::Entry, iter, mem};
+use std::{collections::hash_map::Entry, fmt, iter, mem};
 
 use crate::text_edit::{TextEdit, TextEditBuilder};
 use crate::{assists::Command, syntax_helpers::tree_diff::diff, SnippetCap};
@@ -18,23 +18,33 @@ use syntax::{
     AstNode, SyntaxElement, SyntaxNode, SyntaxNodePtr, SyntaxToken, TextRange, TextSize,
 };
 
+/// An annotation ID associated with an indel, to describe changes.
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
+pub struct ChangeAnnotationId(u32);
+
+impl fmt::Display for ChangeAnnotationId {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        fmt::Display::fmt(&self.0, f)
+    }
+}
+
+#[derive(Debug, Clone)]
+pub struct ChangeAnnotation {
+    pub label: String,
+    pub needs_confirmation: bool,
+    pub description: Option<String>,
+}
+
 #[derive(Default, Debug, Clone)]
 pub struct SourceChange {
     pub source_file_edits: IntMap<FileId, (TextEdit, Option<SnippetEdit>)>,
     pub file_system_edits: Vec<FileSystemEdit>,
     pub is_snippet: bool,
+    pub annotations: FxHashMap<ChangeAnnotationId, ChangeAnnotation>,
+    next_annotation_id: u32,
 }
 
 impl SourceChange {
-    /// Creates a new SourceChange with the given label
-    /// from the edits.
-    pub fn from_edits(
-        source_file_edits: IntMap<FileId, (TextEdit, Option<SnippetEdit>)>,
-        file_system_edits: Vec<FileSystemEdit>,
-    ) -> Self {
-        SourceChange { source_file_edits, file_system_edits, is_snippet: false }
-    }
-
     pub fn from_text_edit(file_id: impl Into<FileId>, edit: TextEdit) -> Self {
         SourceChange {
             source_file_edits: iter::once((file_id.into(), (edit, None))).collect(),
@@ -42,6 +52,13 @@ impl SourceChange {
         }
     }
 
+    pub fn insert_annotation(&mut self, annotation: ChangeAnnotation) -> ChangeAnnotationId {
+        let id = ChangeAnnotationId(self.next_annotation_id);
+        self.next_annotation_id += 1;
+        self.annotations.insert(id, annotation);
+        id
+    }
+
     /// Inserts a [`TextEdit`] for the given [`FileId`]. This properly handles merging existing
     /// edits for a file if some already exist.
     pub fn insert_source_edit(&mut self, file_id: impl Into<FileId>, edit: TextEdit) {
@@ -120,7 +137,12 @@ impl From<IntMap<FileId, TextEdit>> for SourceChange {
     fn from(source_file_edits: IntMap<FileId, TextEdit>) -> SourceChange {
         let source_file_edits =
             source_file_edits.into_iter().map(|(file_id, edit)| (file_id, (edit, None))).collect();
-        SourceChange { source_file_edits, file_system_edits: Vec::new(), is_snippet: false }
+        SourceChange {
+            source_file_edits,
+            file_system_edits: Vec::new(),
+            is_snippet: false,
+            ..SourceChange::default()
+        }
     }
 }
 
@@ -482,6 +504,7 @@ impl From<FileSystemEdit> for SourceChange {
             source_file_edits: Default::default(),
             file_system_edits: vec![edit],
             is_snippet: false,
+            ..SourceChange::default()
         }
     }
 }