syntax::ext: Make type errors in deriving point to the field itself.

This rearranges the deriving code so that #[deriving] a trait on a field
that doesn't implement that trait will point to the field in question,
e.g.

    struct NotEq; // doesn't implement Eq

    #[deriving(Eq)]
    struct Foo {
        ok: int,
        also_ok: ~str,
        bad: NotEq // error points here.
    }

Unfortunately, this means the error is disconnected from the `deriving`
itself but there's no current way to pass that information through to
rustc except via the spans, at the moment.

Fixes #7724.

src/test/compile-fail/deriving-field-span-enum-struct-variant.rs

@@ -0,0 +1,23 @@
+// Copyright 2013 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#[feature(struct_variant)];
+
+struct NotEq;
+
+#[deriving(Eq)]
+enum Foo {
+    Bar {
+        x: NotEq //~ ERROR mismatched types
+        //~^ ERROR failed to find an implementation of trait std::cmp::Eq for NotEq
+    }
+}
+
+pub fn main() {}

src/test/compile-fail/deriving-field-span-enum.rs

@@ -0,0 +1,22 @@
+// Copyright 2013 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#[feature(struct_variant)];
+
+struct NotEq;
+
+#[deriving(Eq)]
+enum Foo {
+    Bar(NotEq), //~ ERROR mismatched types
+        //~^ ERROR failed to find an implementation of trait std::cmp::Eq for NotEq
+    Baz { x: NotEq }
+}
+
+pub fn main() {}

src/test/compile-fail/deriving-field-span-struct.rs

@@ -0,0 +1,19 @@
+// Copyright 2013 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+struct NotEq;
+
+#[deriving(Eq)]
+struct Foo {
+    x: NotEq //~ ERROR mismatched types
+        //~^ ERROR failed to find an implementation of trait std::cmp::Eq for NotEq
+}
+
+pub fn main() {}

src/test/compile-fail/deriving-field-span-tuple-struct.rs

@@ -0,0 +1,19 @@
+// Copyright 2013 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+struct NotEq;
+
+#[deriving(Eq)]
+struct Foo (
+    NotEq //~ ERROR mismatched types
+        //~^ ERROR failed to find an implementation of trait std::cmp::Eq for NotEq
+        );
+
+pub fn main() {}