Keep tab stop-less snippets in completion list (zed-industries#45227)

Closes zed-industries#45083

cc @agu-z 

Release Notes:

- Fixed certain rust-analyzer snippets not shown

Author: SomeoneToIgnore

crates/languages/src/rust.rs

@@ -355,7 +355,7 @@ impl LspAdapter for RustLspAdapter {
                         | lsp::CompletionTextEdit::Edit(lsp::TextEdit { new_text, .. }),
                     ) = completion.text_edit.as_ref()
                     && let Ok(mut snippet) = snippet::Snippet::parse(new_text)
-                    && !snippet.tabstops.is_empty()
+                    && snippet.tabstops.len() > 1
                 {
                     label = String::new();
 
@@ -421,7 +421,9 @@ impl LspAdapter for RustLspAdapter {
                             0..label.rfind('(').unwrap_or(completion.label.len()),
                             highlight_id,
                         ));
-                    } else if detail_left.is_none() {
+                    } else if detail_left.is_none()
+                        && kind != Some(lsp::CompletionItemKind::SNIPPET)
+                    {
                         return None;
                     }
                 }
@@ -1597,6 +1599,40 @@ mod tests {
             ))
         );
 
+        // Postfix completion without actual tabstops (only implicit final $0)
+        // The label should use completion.label so it can be filtered by "ref"
+        let ref_completion = adapter
+            .label_for_completion(
+                &lsp::CompletionItem {
+                    kind: Some(lsp::CompletionItemKind::SNIPPET),
+                    label: "ref".to_string(),
+                    filter_text: Some("ref".to_string()),
+                    label_details: Some(CompletionItemLabelDetails {
+                        detail: None,
+                        description: Some("&expr".to_string()),
+                    }),
+                    detail: Some("&expr".to_string()),
+                    insert_text_format: Some(lsp::InsertTextFormat::SNIPPET),
+                    text_edit: Some(lsp::CompletionTextEdit::Edit(lsp::TextEdit {
+                        range: lsp::Range::default(),
+                        new_text: "&String::new()".to_string(),
+                    })),
+                    ..Default::default()
+                },
+                &language,
+            )
+            .await;
+        assert!(
+            ref_completion.is_some(),
+            "ref postfix completion should have a label"
+        );
+        let ref_label = ref_completion.unwrap();
+        let filter_text = &ref_label.text[ref_label.filter_range.clone()];
+        assert!(
+            filter_text.contains("ref"),
+            "filter range text '{filter_text}' should contain 'ref' for filtering to work",
+        );
+
         // Test for correct range calculation with mixed empty and non-empty tabstops.(See https://github.com/zed-industries/zed/issues/44825)
         let res = adapter
             .label_for_completion(