Add expression interpolation with #{expr}

One annoyance I frequently run into with the `quote!` macro is that I
need to lift out all variable bindings that are being interpolated. For
example I'll often do:

    let name = &foo.name;
    let abi = &foo.abi;

    quote! {
        extern #abi fn #name() { /* ... */ }
    }

but I've recently had the idea that in addition to lighteight `#name`
interpolation there's also available syntax (I believe) to support
expression interpolation as well, implemented in this PR. The above
snippet could now be rewritten as:

    quote! {
        extern #{foo.abi} fn #{foo.name} { /* ... */ }
    }

Author: alexcrichton

src/lib.rs

@@ -254,6 +254,10 @@ pub mod __rt {
 /// [`ToTokens`]: trait.ToTokens.html
 /// [Syn]: https://github.com/dtolnay/syn
 ///
+/// You can also use `#{expr}` to interpolate an arbitrary expression, so long
+/// as the result implements the [`ToTokens`] trait. For example `#{1 + 3}` will
+/// interpolate `4i32`.
+///
 /// Repetition is done using `#(...)*` or `#(...),*` again similar to
 /// `macro_rules!`. This iterates through the elements of any variable
 /// interpolated within the repetition and inserts a copy of the repetition body
@@ -579,6 +583,11 @@ macro_rules! quote_each_token {
         quote_each_token!($tokens $span $($rest)*);
     };
 
+    ($tokens:ident $span:ident # { $val:expr } $($rest:tt)*) => {
+        $crate::ToTokens::to_tokens(&$val, &mut $tokens);
+        quote_each_token!($tokens $span $($rest)*);
+    };
+
     ($tokens:ident $span:ident # $first:ident $($rest:tt)*) => {
         $crate::ToTokens::to_tokens(&$first, &mut $tokens);
         quote_each_token!($tokens $span $($rest)*);

tests/test.rs

@@ -293,3 +293,19 @@ fn test_append_tokens() {
     a.append_all(b);
     assert_eq!("a b", a.to_string());
 }
+
+#[test]
+fn whole_exprs() {
+    let x = X;
+    let tokens = quote!(#{x});
+    assert_eq!(tokens.to_string(), "X");
+
+    let tokens = quote!(#{1 + 2});
+    assert_eq!(tokens.to_string(), "3i32");
+
+    struct A { b: X }
+    let x = A { b: X };
+
+    let tokens = quote!(#{x.b});
+    assert_eq!(tokens.to_string(), "X");
+}