Use the span of #[derive_Eq] for #[structural_match]

Author: jseyfried

src/libsyntax_ext/deriving/mod.rs

@@ -94,7 +94,7 @@ fn expand_derive(cx: &mut ExtCtxt,
             }
 
             let mut found_partial_eq = false;
-            let mut found_eq = false;
+            let mut eq_span = None;
 
             for titem in traits.iter().rev() {
                 let tname = match titem.node {
@@ -114,12 +114,6 @@ fn expand_derive(cx: &mut ExtCtxt,
                     continue;
                 }
 
-                if &tname[..] == "Eq" {
-                    found_eq = true;
-                } else if &tname[..] == "PartialEq" {
-                    found_partial_eq = true;
-                }
-
                 let span = Span {
                     expn_id: cx.codemap().record_expansion(codemap::ExpnInfo {
                         call_site: titem.span,
@@ -131,48 +125,26 @@ fn expand_derive(cx: &mut ExtCtxt,
                     }), ..titem.span
                 };
 
+                if &tname[..] == "Eq" {
+                    eq_span = Some(span);
+                } else if &tname[..] == "PartialEq" {
+                    found_partial_eq = true;
+                }
+
                 // #[derive(Foo, Bar)] expands to #[derive_Foo] #[derive_Bar]
                 item.attrs.push(cx.attribute(span, cx.meta_word(titem.span,
                     intern_and_get_ident(&format!("derive_{}", tname)))));
             }
 
             // RFC #1445. `#[derive(PartialEq, Eq)]` adds a (trusted)
             // `#[structural_match]` attribute.
-            if found_partial_eq && found_eq {
-                // This span is **very** sensitive and crucial to
-                // getting the stability behavior we want. What we are
-                // doing is marking `#[structural_match]` with the
-                // span of the `#[deriving(...)]` attribute (the
-                // entire attribute, not just the `PartialEq` or `Eq`
-                // part), but with the current backtrace. The current
-                // backtrace will contain a topmost entry that IS this
-                // `#[deriving(...)]` attribute and with the
-                // "allow-unstable" flag set to true.
-                //
-                // Note that we do NOT use the span of the `Eq`
-                // text itself. You might think this is
-                // equivalent, because the `Eq` appears within the
-                // `#[deriving(Eq)]` attribute, and hence we would
-                // inherit the "allows unstable" from the
-                // backtrace.  But in fact this is not always the
-                // case. The actual source text that led to
-                // deriving can be `#[$attr]`, for example, where
-                // `$attr == deriving(Eq)`. In that case, the
-                // "#[structural_match]" would be considered to
-                // originate not from the deriving call but from
-                // text outside the deriving call, and hence would
-                // be forbidden from using unstable
-                // content.
-                //
-                // See tests src/run-pass/rfc1445 for
-                // examples. --nmatsakis
-                let span = Span { expn_id: cx.backtrace(), .. span };
-                assert!(cx.parse_sess.codemap().span_allows_unstable(span));
-                debug!("inserting structural_match with span {:?}", span);
-                let structural_match = intern_and_get_ident("structural_match");
-                item.attrs.push(cx.attribute(span,
-                                             cx.meta_word(span,
-                                                          structural_match)));
+            if let Some(eq_span) = eq_span {
+                if found_partial_eq {
+                    let structural_match = intern_and_get_ident("structural_match");
+                    item.attrs.push(cx.attribute(eq_span,
+                                                 cx.meta_word(eq_span,
+                                                              structural_match)));
+                }
             }
 
             item