Align config's API with usage

The config now is mostly immutable, optimize for that.

crates/ide_db/src/helpers.rs

@@ -1,9 +1,10 @@
 //! A module with ide helpers for high-level ide features.
-use crate::RootDatabase;
+pub mod insert_use;
+
 use hir::{Crate, Enum, Module, ScopeDef, Semantics, Trait};
 use syntax::ast::{self, make};
 
-pub mod insert_use;
+use crate::RootDatabase;
 
 /// Converts the mod path struct into its ast representation.
 pub fn mod_path_to_ast(path: &hir::ModPath) -> ast::Path {
@@ -201,3 +202,18 @@ pub use prelude::*;
         Some(def)
     }
 }
+
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+pub struct SnippetCap {
+    _private: (),
+}
+
+impl SnippetCap {
+    pub const fn new(allow_snippets: bool) -> Option<SnippetCap> {
+        if allow_snippets {
+            Some(SnippetCap { _private: () })
+        } else {
+            None
+        }
+    }
+}