Auto merge of #40346 - jseyfried:path_and_tokenstream_attr, r=nrc

`TokenStream`-based attributes, paths in attribute and derive macro invocations

This PR
 - refactors `Attribute` to use  `Path` and `TokenStream` instead of `MetaItem`.
 - supports macro invocation paths for attribute procedural macros.
   - e.g. `#[::foo::attr_macro] struct S;`, `#[cfg_attr(all(), foo::attr_macro)] struct S;`
 - supports macro invocation paths for derive procedural macros.
   - e.g. `#[derive(foo::Bar, super::Baz)] struct S;`
 - supports arbitrary tokens as arguments to attribute procedural macros.
   - e.g. `#[foo::attr_macro arbitrary + tokens] struct S;`
 - supports using arbitrary tokens in "inert attributes" with derive procedural macros.
   - e.g. `#[derive(Foo)] struct S(#[inert arbitrary + tokens] i32);`
where `#[proc_macro_derive(Foo, attributes(inert))]`

r? @nrc

Author: bors

src/librustc/hir/check_attr.rs

@@ -120,11 +120,12 @@ impl<'a> CheckAttrVisitor<'a> {
     }
 
     fn check_attribute(&self, attr: &ast::Attribute, target: Target) {
-        let name: &str = &attr.name().as_str();
-        match name {
-            "inline" => self.check_inline(attr, target),
-            "repr" => self.check_repr(attr, target),
-            _ => (),
+        if let Some(name) = attr.name() {
+            match &*name.as_str() {
+                "inline" => self.check_inline(attr, target),
+                "repr" => self.check_repr(attr, target),
+                _ => (),
+            }
         }
     }
 }

src/librustc/hir/lowering.rs

@@ -1296,7 +1296,7 @@ impl<'a> LoweringContext<'a> {
         let attrs = self.lower_attrs(&i.attrs);
         let mut vis = self.lower_visibility(&i.vis);
         if let ItemKind::MacroDef(ref tts) = i.node {
-            if i.attrs.iter().any(|attr| attr.name() == "macro_export") {
+            if i.attrs.iter().any(|attr| attr.path == "macro_export") {
                 self.exported_macros.push(hir::MacroDef {
                     name: name, attrs: attrs, id: i.id, span: i.span, body: tts.clone().into(),
                 });