Introduce internal snippet cap

crates/ra_ide/src/completion/completion_config.rs

@@ -1,8 +1,20 @@
+//! Settings for tweaking completion.
+//!
+//! The fun thing here is `SnippetCap` -- this type can only be created in this
+//! module, and we use to statically check that we only produce snippet
+//! completions if we are allowed to.
+
 #[derive(Clone, Debug, PartialEq, Eq)]
 pub struct CompletionConfig {
     pub enable_postfix_completions: bool,
     pub add_call_parenthesis: bool,
     pub add_call_argument_snippets: bool,
+    pub snippet_cap: Option<SnippetCap>,
+}
+
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+pub struct SnippetCap {
+    _private: (),
 }
 
 impl Default for CompletionConfig {
@@ -11,6 +23,7 @@ impl Default for CompletionConfig {
             enable_postfix_completions: true,
             add_call_parenthesis: true,
             add_call_argument_snippets: true,
+            snippet_cap: Some(SnippetCap { _private: () }),
         }
     }
 }

crates/rust-analyzer/src/config.rs

@@ -104,6 +104,7 @@ impl Default for Config {
                 enable_postfix_completions: true,
                 add_call_parenthesis: true,
                 add_call_argument_snippets: true,
+                ..CompletionConfig::default()
             },
             call_info_full: true,
         }