Question 9: metavariables are opaque

questions/009-opaque-metavariable.md

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+Answer: 21
+Difficulty: 2
+
+# Hint
+
+Upon being matched as a `$:expr`, the matched expression becomes a single opaque
+token tree.
+
+# Explanation
+
+This question involves the behavior of macro matchers as regards matching macro
+metavariables.
+
+Starting from the bottom of the quiz code, the invocation `t!(1)` matches the
+first rule of `t!` and expands to `e!(1); m!(1);`.
+
+The invocation `e!(1)` matches the first rule of `e!`. As part of this match,
+the expression `1` is packaged into an opaque expression token called `$e`. At
+no subsequent point will it be possible for any `macro_rules!` macro to look
+inside of `$e`. All that can be known is that `$e` is *some* expression.
+
+In any case, `e!(1)` expands to `m!($e)` where `$e` is an opaque expression
+containing `1`. That `m!($e)` *does not* match the first rule of `m!` because
+`$e` is opaque. Instead it matches the second rule of `m!` and prints `2`.
+
+After `e!(1)` there is an invocation `m!(1)` coming from the expansion of `t!`.
+That one *does* match the first rule of `m!` and prints `1`. The output of this
+program is `21`.
+
+Most fragment specifiers have this behavior of becoming opaque token boxes, but
+some do not. Specifiers that are opaque once matched:
+
+- `$:block`
+- `$:expr`
+- `$:item`
+- `$:literal`
+- `$:meta`
+- `$:pat`
+- `$:path`
+- `$:stmt`
+- `$:ty`
+
+The rest of the specifiers do not become opaque and can be inspected by
+subsequent rules:
+
+- `$:ident`
+- `$:lifetime`
+- `$:tt`
+
+For example:
+
+```rust
+macro_rules! m {
+    ('a) => {};
+}
+
+macro_rules! l {
+    ($l:lifetime) => {
+        // $l is not opaque.
+        m!($l);
+    }
+}
+
+l!('a);
+```

questions/009-opaque-metavariable.rs

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+macro_rules! m {
+    (1) => { print!("1") };
+    ($tt:tt) => { print!("2") };
+}
+
+macro_rules! e {
+    ($e:expr) => { m!($e) };
+}
+
+macro_rules! t {
+    ($tt:tt) => { e!($tt); m!($tt); };
+}
+
+fn main() {
+    t!(1);
+}