Auto merge of #16446 - Tyrubias:literal_from_str, r=Veykril

Implement `literal_from_str` for proc macro server

Closes #16233

Todos and unanswered questions:

- [x] Is this the correct approach? Can both the legacy and `rust_analyzer_span` servers depend on the `syntax` crate?
- [ ] How should we handle suffixes for string literals? It doesn't seem like `rust-analyzer` preservers suffix information after parsing.
- [x] Why are the `expect` tests failing? Specifically `test_fn_like_macro_clone_literals`

crates/syntax/src/lib.rs

@@ -182,6 +182,28 @@ impl SourceFile {
     }
 }
 
+impl ast::Literal {
+    pub fn parse(text: &str) -> Option<Parse<ast::Literal>> {
+        let lexed = parser::LexedStr::new(text);
+        let parser_input = lexed.to_input();
+        let parser_output = parser::TopEntryPoint::Expr.parse(&parser_input);
+        let (green, mut errors, _) = parsing::build_tree(lexed, parser_output);
+        let root = SyntaxNode::new_root(green.clone());
+
+        errors.extend(validation::validate(&root));
+
+        if root.kind() == SyntaxKind::LITERAL {
+            Some(Parse {
+                green,
+                errors: if errors.is_empty() { None } else { Some(errors.into()) },
+                _ty: PhantomData,
+            })
+        } else {
+            None
+        }
+    }
+}
+
 impl ast::TokenTree {
     pub fn reparse_as_comma_separated_expr(self) -> Parse<ast::MacroEagerInput> {
         let tokens = self.syntax().descendants_with_tokens().filter_map(NodeOrToken::into_token);