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

@@ -7,16 +7,13 @@ extern crate syntax;
 use itertools::Itertools;
 use rustc_plugin::Registry;
 use syntax::ast::{self, Ident, TraitRef, Ty, TyKind};
-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;
-use syntax::parse::{self, token, PResult};
+use syntax::parse::{self, token, str_lit, PResult};
 use syntax::parse::parser::{Parser, PathStyle};
 use syntax::symbol::Symbol;
 use syntax::ptr::P;
-use syntax::tokenstream::TokenTree;
+use syntax::tokenstream::{TokenTree, TokenStream};
 use syntax::util::small_vector::SmallVector;
 
 fn snake_to_camel(cx: &mut ExtCtxt, sp: Span, tts: &[TokenTree]) -> Box<MacResult + 'static> {
@@ -43,15 +40,44 @@ fn snake_to_camel(cx: &mut ExtCtxt, sp: Span, tts: &[TokenTree]) -> Box<MacResul
     // so this is the hacky workaround.
     //
     // This code looks intimidating, but it's just iterating through the trait item's attributes
-    // (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));
-        } else {
-            unreachable!()
-        };
-    }
+    // copying non-doc attributes, and modifying doc attributes such that replacing any {} in the
+    // doc string instead holds the original, snake_case ident.
+    let attrs: Vec<_> = item.attrs
+        .drain(..)
+        .map(|mut attr| {
+            if !attr.is_sugared_doc {
+                return attr;
+            }
+
+            // Getting at the underlying doc comment is surprisingly painful.
+            // The call-chain goes something like:
+            //
+            //  - https://github.com/rust-lang/rust/blob/9c15de4fd59bee290848b5443c7e194fd5afb02c/src/libsyntax/attr.rs#L283
+            //  - https://github.com/rust-lang/rust/blob/9c15de4fd59bee290848b5443c7e194fd5afb02c/src/libsyntax/attr.rs#L1067
+            //  - https://github.com/rust-lang/rust/blob/9c15de4fd59bee290848b5443c7e194fd5afb02c/src/libsyntax/attr.rs#L1196
+            //  - https://github.com/rust-lang/rust/blob/9c15de4fd59bee290848b5443c7e194fd5afb02c/src/libsyntax/parse/mod.rs#L399
+            //  - https://github.com/rust-lang/rust/blob/9c15de4fd59bee290848b5443c7e194fd5afb02c/src/libsyntax/parse/mod.rs#L268
+            //
+            // Note that a docstring (i.e., something with is_sugared_doc) *always* has exactly two
+            // tokens: an Eq followed by a Literal, where the Literal contains a Str_. We therefore
+            // match against that, modifying the inner Str with our modified Symbol.
+            let mut tokens = attr.tokens.clone().into_trees();
+            if let Some(tt @ TokenTree::Token(_, token::Eq)) = tokens.next() {
+                let mut docstr = tokens.next().expect("Docstrings must have literal docstring");
+                if let TokenTree::Token(_, token::Literal(token::Str_(ref mut doc), _)) = docstr {
+                    *doc = Symbol::intern(&str_lit(&doc.as_str()).replace("{}", &old_ident));
+                } else {
+                    unreachable!();
+                }
+                attr.tokens = TokenStream::concat(vec![tt.into(), docstr.into()]);
+            } else {
+                unreachable!();
+            }
+
+            attr
+        })
+        .collect();
+    item.attrs.extend(attrs.into_iter());
 
     MacEager::trait_items(SmallVector::one(item))
 }