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 compared 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
 }