fix: Prevent add_missing_match_arms assist from ignoring existing v…

…ariants prefixed with `Self::`

This is an ad hoc fix of rust-lang#4650 by making only the last identifier
segment used for comparison when the variant is prefixed, so that the
comparison is not affected by differences in prefix, such as type
names, type aliases, or `Self::`.

(The appropriate fix would be to use `hir_ty::diagnostics::match_check`,
as discussed in rust-lang#8493.)

Closes rust-lang#4650.

crates/ide-assists/src/utils.rs

@@ -289,16 +289,22 @@ pub(crate) fn next_prev() -> impl Iterator<Item = Direction> {
 
 pub(crate) fn does_pat_match_variant(pat: &ast::Pat, var: &ast::Pat) -> bool {
     let first_node_text = |pat: &ast::Pat| pat.syntax().first_child().map(|node| node.text());
+    let last_segment_text =
+        |path: &ast::Path| path.segments().last().map(|segment| segment.syntax().text());
+    let normalize_pat = |pat: &ast::Pat| match pat {
+        ast::Pat::PathPat(p) => p.path().and_then(|p| last_segment_text(&p)),
+        ast::Pat::TupleStructPat(p) => p.path().and_then(|p| last_segment_text(&p)),
+        _ => first_node_text(&pat),
+    };
 
     let pat_head = match pat {
         ast::Pat::IdentPat(bind_pat) => match bind_pat.pat() {
-            Some(p) => first_node_text(&p),
+            Some(p) => normalize_pat(&p),
             None => return pat.syntax().text() == var.syntax().text(),
         },
-        pat => first_node_text(pat),
+        _ => normalize_pat(pat),
     };
-
-    let var_head = first_node_text(var);
+    let var_head = normalize_pat(var);
 
     pat_head == var_head
 }