move function rendering to presentation

crates/ra_ide_api/src/completion/presentation.rs

@@ -1,7 +1,11 @@
 //! This modules takes care of rendering various defenitions as completion items.
+use test_utils::tested_by;
 use hir::Docs;
 
-use crate::completion::{Completions, CompletionKind, CompletionItemKind, CompletionContext, CompletionItem};
+use crate::completion::{
+    Completions, CompletionKind, CompletionItemKind, CompletionContext, CompletionItem,
+    function_label,
+};
 
 impl Completions {
     pub(crate) fn add_field(
@@ -30,4 +34,39 @@ impl Completions {
             .detail(ty.to_string())
             .add_to(self);
     }
+
+    pub(crate) fn add_function(
+        &mut self,
+        kind: CompletionKind,
+        ctx: &CompletionContext,
+        func: hir::Function,
+    ) {
+        let sig = func.signature(ctx.db);
+
+        let mut builder = CompletionItem::new(kind, ctx.source_range(), sig.name().to_string())
+            .kind(if sig.has_self_param() {
+                CompletionItemKind::Method
+            } else {
+                CompletionItemKind::Function
+            })
+            .set_documentation(func.docs(ctx.db))
+            .set_detail(function_item_label(ctx, func));
+        // If not an import, add parenthesis automatically.
+        if ctx.use_item_syntax.is_none() && !ctx.is_call {
+            tested_by!(inserts_parens_for_function_calls);
+            let snippet =
+                if sig.params().is_empty() || sig.has_self_param() && sig.params().len() == 1 {
+                    format!("{}()$0", sig.name())
+                } else {
+                    format!("{}($0)", sig.name())
+                };
+            builder = builder.insert_snippet(snippet);
+        }
+        self.add(builder)
+    }
+}
+
+fn function_item_label(ctx: &CompletionContext, function: hir::Function) -> Option<String> {
+    let node = function.source(ctx.db).1;
+    function_label(&node)
 }