Merge #11691

11691: feat: Suggest union literals, suggest union fields within an empty union literal r=Veykril a=m0rg-dev

Adds a `Union {…}` completion in contexts where a union is expected, expanding to a choice of available fields (if snippets are supported):

![image](https://user-images.githubusercontent.com/38578268/158023335-84c03e39-daf0-4a52-b969-f40b01501cc8.png)
![image](https://user-images.githubusercontent.com/38578268/158023354-db49d0bb-034c-49d3-bc02-07414179cb61.png)

Also, adds support for listing possible fields in an empty union literal.

![image](https://user-images.githubusercontent.com/38578268/158023398-4695ae34-ce64-4f40-8494-68731a3030c6.png)
![image](https://user-images.githubusercontent.com/38578268/158023406-be96dd95-125a-47ac-9628-0bce634ca2eb.png)

Closes #11568.

Co-authored-by: Morgan Thomas <corp@m0rg.dev>

crates/ide_completion/src/render.rs

@@ -9,6 +9,7 @@ pub(crate) mod pattern;
 pub(crate) mod type_alias;
 pub(crate) mod struct_literal;
 pub(crate) mod compound;
+pub(crate) mod union_literal;
 
 use hir::{AsAssocItem, HasAttrs, HirDisplay, ScopeDef};
 use ide_db::{helpers::item_name, RootDatabase, SnippetCap, SymbolKind};