Add way of getting docs from the code model and use for completion

crates/ra_hir/src/docs.rs

@@ -0,0 +1,35 @@
+use ra_syntax::ast;
+
+use crate::HirDatabase;
+
+#[derive(Debug, Clone)]
+pub struct Documentation(String);
+
+impl Documentation {
+    pub fn new(s: &str) -> Self {
+        Self(s.into())
+    }
+
+    pub fn contents(&self) -> &str {
+        &self.0
+    }
+}
+
+impl Into<String> for Documentation {
+    fn into(self) -> String {
+        self.contents().into()
+    }
+}
+
+pub trait Docs {
+    fn docs(&self, db: &impl HirDatabase) -> Option<Documentation>;
+}
+
+pub(crate) fn docs_from_ast(node: &impl ast::DocCommentsOwner) -> Option<Documentation> {
+    let comments = node.doc_comment_text();
+    if comments.is_empty() {
+        None
+    } else {
+        Some(Documentation::new(&comments))
+    }
+}