introduce completion presentation

This module should remove completion rendering boilerplate from the
"brains" of completion engine.

crates/ra_ide_api/src/completion/presentation.rs

@@ -0,0 +1,33 @@
+//! This modules takes care of rendering various defenitions as completion items.
+use hir::Docs;
+
+use crate::completion::{Completions, CompletionKind, CompletionItemKind, CompletionContext, CompletionItem};
+
+impl Completions {
+    pub(crate) fn add_field(
+        &mut self,
+        kind: CompletionKind,
+        ctx: &CompletionContext,
+        field: hir::StructField,
+        substs: &hir::Substs,
+    ) {
+        CompletionItem::new(kind, ctx.source_range(), field.name(ctx.db).to_string())
+            .kind(CompletionItemKind::Field)
+            .detail(field.ty(ctx.db).subst(substs).to_string())
+            .set_documentation(field.docs(ctx.db))
+            .add_to(self);
+    }
+
+    pub(crate) fn add_pos_field(
+        &mut self,
+        kind: CompletionKind,
+        ctx: &CompletionContext,
+        field: usize,
+        ty: &hir::Ty,
+    ) {
+        CompletionItem::new(kind, ctx.source_range(), field.to_string())
+            .kind(CompletionItemKind::Field)
+            .detail(ty.to_string())
+            .add_to(self);
+    }
+}