Fix following removal of Attribute.value

Since rust-lang/rust#40346 has now been merged, Attribute no longer has
a .value field. Instead, we must follow the token stream and modify the
tokens directly. For Docstring attributes, there should only be one
token, the docstring value.

src/plugins/src/lib.rs

@@ -7,8 +7,8 @@ extern crate syntax;
 use itertools::Itertools;
 use rustc_plugin::Registry;
 use syntax::ast::{self, Ident, TraitRef, Ty, TyKind};
+use syntax::ast::{MetaItem, MetaItemKind};
 use syntax::ast::LitKind::Str;
-use syntax::ast::MetaItemKind::NameValue;
 use syntax::codemap::Spanned;
 use syntax::ext::base::{ExtCtxt, MacResult, DummyResult, MacEager};
 use syntax::ext::quote::rt::Span;
@@ -46,11 +46,17 @@ fn snake_to_camel(cx: &mut ExtCtxt, sp: Span, tts: &[TokenTree]) -> Box<MacResul
     // (NameValues), filtering out non-doc attributes, and replacing any {} in the doc string with
     // the original, snake_case ident.
     for attr in item.attrs.iter_mut().filter(|attr| attr.is_sugared_doc) {
-        if let NameValue(Spanned { node: Str(ref mut doc, _), .. }) = attr.value.node {
-            *doc = Symbol::intern(&doc.as_str().replace("{}", &old_ident));
+        // We need to extract the firsts element in the token stream.
+        let meta = attr.meta();
+        if let Some(MetaItem { mut node, .. }) = meta {
+            if let MetaItemKind::NameValue(Spanned { node: Str(ref mut doc, _), .. }) = node {
+                *doc = Symbol::intern(&doc.as_str().replace("{}", &old_ident));
+            } else {
+                unreachable!();
+            }
         } else {
-            unreachable!()
-        };
+            unreachable!();
+        }
     }
 
     MacEager::trait_items(SmallVector::one(item))