Fragments that keep track of type of syntax

Fixes #777.

serde_derive/src/fragment.rs

@@ -0,0 +1,67 @@
+use quote::{Tokens, ToTokens};
+
+pub enum Fragment {
+    /// Tokens that can be used as an expression.
+    Expr(Tokens),
+    /// Tokens that can be used inside a block. The surrounding curly braces are
+    /// not part of these tokens.
+    Block(Tokens),
+}
+
+macro_rules! quote_expr {
+    ($($tt:tt)*) => {
+        $crate::fragment::Fragment::Expr(quote!($($tt)*))
+    }
+}
+
+macro_rules! quote_block {
+    ($($tt:tt)*) => {
+        $crate::fragment::Fragment::Block(quote!($($tt)*))
+    }
+}
+
+/// Interpolate a fragment in place of an expression. This involves surrounding
+/// Block fragments in curly braces.
+pub struct Expr(pub Fragment);
+impl ToTokens for Expr {
+    fn to_tokens(&self, out: &mut Tokens) {
+        match self.0 {
+            Fragment::Expr(ref expr) => expr.to_tokens(out),
+            Fragment::Block(ref block) => {
+                out.append("{");
+                block.to_tokens(out);
+                out.append("}");
+            }
+        }
+    }
+}
+
+/// Interpolate a fragment as the statements of a block.
+pub struct Stmts(pub Fragment);
+impl ToTokens for Stmts {
+    fn to_tokens(&self, out: &mut Tokens) {
+        match self.0 {
+            Fragment::Expr(ref expr) => expr.to_tokens(out),
+            Fragment::Block(ref block) => block.to_tokens(out),
+        }
+    }
+}
+
+/// Interpolate a fragment as the value part of a `match` expression. This
+/// involves putting a comma after expressions and curly braces around blocks.
+pub struct Match(pub Fragment);
+impl ToTokens for Match {
+    fn to_tokens(&self, out: &mut Tokens) {
+        match self.0 {
+            Fragment::Expr(ref expr) => {
+                expr.to_tokens(out);
+                out.append(",");
+            }
+            Fragment::Block(ref block) => {
+                out.append("{");
+                block.to_tokens(out);
+                out.append("}");
+            }
+        }
+    }
+}