Merge #116

116: Collapse comments upon join r=matklad a=aochagavia

Todo:

- [x] Write tests
- [x] Resolve fixmes
- [x] Implement `comment_start_length` using the parser

I left a bunch of questions as fixmes. Can someone take a look at them? Also, I would love to use the parser to calculate the length of the leading characters in a comment (`//`, `///`, `//!`, `/*`), so any hints are greatly appreciated.

Co-authored-by: Adolfo Ochagavía <aochagavia92@gmail.com>
Co-authored-by: Adolfo Ochagavía <github@adolfo.ochagavia.xyz>

crates/ra_syntax/src/ast/generated.rs

@@ -231,6 +231,24 @@ impl<'a> AstNode<'a> for CastExpr<'a> {
 
 impl<'a> CastExpr<'a> {}
 
+// Comment
+#[derive(Debug, Clone, Copy)]
+pub struct Comment<'a> {
+    syntax: SyntaxNodeRef<'a>,
+}
+
+impl<'a> AstNode<'a> for Comment<'a> {
+    fn cast(syntax: SyntaxNodeRef<'a>) -> Option<Self> {
+        match syntax.kind() {
+            COMMENT => Some(Comment { syntax }),
+            _ => None,
+        }
+    }
+    fn syntax(self) -> SyntaxNodeRef<'a> { self.syntax }
+}
+
+impl<'a> Comment<'a> {}
+
 // Condition
 #[derive(Debug, Clone, Copy)]
 pub struct Condition<'a> {

crates/ra_syntax/src/ast/mod.rs

@@ -99,6 +99,49 @@ impl<'a> Lifetime<'a> {
     }
 }
 
+impl<'a> Comment<'a> {
+    pub fn text(&self) -> SmolStr {
+        self.syntax().leaf_text().unwrap().clone()
+    }
+
+    pub fn flavor(&self) -> CommentFlavor {
+        let text = self.text();
+        if text.starts_with("///") {
+            CommentFlavor::Doc
+        } else if text.starts_with("//!") {
+            CommentFlavor::ModuleDoc
+        } else if text.starts_with("//") {
+            CommentFlavor::Line
+        } else {
+            CommentFlavor::Multiline
+        }
+    }
+
+    pub fn prefix(&self) -> &'static str {
+        self.flavor().prefix()
+    }
+}
+
+#[derive(Debug)]
+pub enum CommentFlavor {
+    Line,
+    Doc,
+    ModuleDoc,
+    Multiline
+}
+
+impl CommentFlavor {
+    pub fn prefix(&self) -> &'static str {
+        use self::CommentFlavor::*;
+        match *self {
+            Line => "//",
+            Doc => "///",
+            ModuleDoc => "//!",
+            Multiline => "/*"
+        }
+    }
+}
+
 impl<'a> Name<'a> {
     pub fn text(&self) -> SmolStr {
         let ident = self.syntax().first_child()