Auto merge of #46550 - jseyfried:cleanup_builtin_hygiene, r=nrc

macros: hygienize use of `core`/`std` in builtin macros

Today, if a builtin macro wants to access an item from `core` or `std` (depending `#![no_std]`), it generates `::core::path::to::item` or `::std::path::to::item` respectively (c.f. `fn std_path()` in `libsyntax/ext/base.rs`).

This PR refactors the builtin macros to instead always emit `$crate::path::to::item` here. That is, the def site of builtin macros is taken to be in `extern crate core;` or `extern crate std;`. Since builtin macros are macros 1.0 (i.e. mostly unhygienic), changing the def site can only effect the resolution of `$crate`.

r? @nrc

src/test/pretty/issue-4264.pp

@@ -40,7 +40,7 @@ pub fn bar() ({
 
 
                   ((::fmt::format as
-                       for<'r> fn(std::fmt::Arguments<'r>) -> std::string::String {std::fmt::format})(((<::std::fmt::Arguments>::new_v1
+                       for<'r> fn(std::fmt::Arguments<'r>) -> std::string::String {std::fmt::format})(((<::fmt::Arguments>::new_v1
                                                                                                            as
                                                                                                            fn(&[&str], &[std::fmt::ArgumentV1<'_>]) -> std::fmt::Arguments<'_> {std::fmt::Arguments<'_>::new_v1})((&([("test"
                                                                                                                                                                                                                           as

src/test/run-pass-fulldeps/auxiliary/custom_derive_partial_eq.rs

@@ -69,7 +69,7 @@ fn expand_deriving_partial_eq(cx: &mut ExtCtxt, span: Span, mitem: &MetaItem, it
     let trait_def = TraitDef {
         span: span,
         attributes: Vec::new(),
-        path: deriving::generic::ty::Path::new(vec!["std", "cmp", "PartialEq"]),
+        path: deriving::generic::ty::Path::new(vec!["cmp", "PartialEq"]),
         additional_bounds: Vec::new(),
         generics: LifetimeBounds::empty(),
         is_unsafe: false,

src/test/run-pass-fulldeps/auxiliary/custom_derive_plugin.rs

@@ -50,7 +50,7 @@ fn expand(cx: &mut ExtCtxt,
     let trait_def = TraitDef {
         span: span,
         attributes: vec![],
-        path: Path::new(vec!["TotalSum"]),
+        path: Path::new_local("TotalSum"),
         additional_bounds: vec![],
         generics: LifetimeBounds::empty(),
         associated_types: vec![],

src/test/run-pass-fulldeps/auxiliary/custom_derive_plugin_attr.rs

@@ -46,7 +46,7 @@ fn expand(cx: &mut ExtCtxt,
     let trait_def = TraitDef {
         span: span,
         attributes: vec![],
-        path: Path::new(vec!["TotalSum"]),
+        path: Path::new_local("TotalSum"),
         additional_bounds: vec![],
         generics: LifetimeBounds::empty(),
         associated_types: vec![],

src/test/run-pass-fulldeps/derive-no-std-not-supported.rs

@@ -8,22 +8,22 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+#![feature(rustc_private)]
 #![no_std]
 
 extern crate serialize as rustc_serialize;
 
-#[derive(RustcEncodable)]  //~ ERROR this trait cannot be derived
+#[derive(RustcEncodable)]
 struct Bar {
     x: u32,
 }
 
-#[derive(RustcDecodable)]  //~ ERROR this trait cannot be derived
+#[derive(RustcDecodable)]
 struct Baz {
     x: u32,
 }
 
 fn main() {
-    Foo { x: 0 };
     Bar { x: 0 };
     Baz { x: 0 };
 }