add macro by example ide

crates/ra_hir/src/macros.rs

@@ -1,5 +1,7 @@
 #[allow(unused)]
 mod tt;
+#[allow(unused)]
+mod mbe;
 
 /// Machinery for macro expansion.
 ///

crates/ra_hir/src/macros/mbe.rs

@@ -0,0 +1,50 @@
+use ra_syntax::SmolStr;
+
+struct MacroRules {
+    rules: Vec<Rule>,
+}
+
+struct Rule {
+    lhs: TokenTree,
+    rhs: TokenTree,
+}
+
+enum TokenTree {
+    Leaf(Leaf),
+    Subtree(Subtree),
+}
+
+enum Leaf {
+    Literal(Literal),
+    Punct(Punct),
+    Ident(Ident),
+    Var(Var),
+}
+
+struct Subtree {
+    delimiter: Delimiter,
+    token_trees: Vec<TokenTree>,
+}
+
+enum Delimiter {
+    Parenthesis,
+    Brace,
+    Bracket,
+    None,
+}
+
+struct Literal {
+    text: SmolStr,
+}
+
+struct Punct {
+    char: char,
+}
+
+struct Ident {
+    text: SmolStr,
+}
+
+struct Var {
+    text: SmolStr,
+}