Prepare removal of auto trait syntax in favor of new attribute #[rustc_auto_trait]

compiler/rustc_ast/src/ast.rs

@@ -2993,7 +2993,7 @@ pub enum ItemKind {
     Union(VariantData, Generics),
     /// A trait declaration (`trait`).
     ///
-    /// E.g., `trait Foo { .. }`, `trait Foo<T> { .. }` or `auto trait Foo {}`.
+    /// E.g., `trait Foo { .. }`, `trait Foo<T> { .. }`.
     Trait(Box<Trait>),
     /// Trait alias
     ///

compiler/rustc_ast_lowering/src/item.rs

@@ -149,7 +149,7 @@ impl<'a, 'hir> ItemLowerer<'a, 'hir> {
                 hir::ItemKind::Impl(impl_) => {
                     lctx.is_in_trait_impl = impl_.of_trait.is_some();
                 }
-                hir::ItemKind::Trait(_, _, generics, _, _) if lctx.tcx.features().effects => {
+                hir::ItemKind::Trait(_, generics, _, _) if lctx.tcx.features().effects => {
                     lctx.host_param_id = generics
                         .params
                         .iter()
@@ -443,7 +443,8 @@ impl<'hir> LoweringContext<'_, 'hir> {
                     items: new_impl_items,
                 }))
             }
-            ItemKind::Trait(box Trait { is_auto, unsafety, generics, bounds, items }) => {
+            // We intentionally ignore `is_auto` since `auto` is now meaningless.
+            ItemKind::Trait(box Trait { is_auto: _, unsafety, generics, bounds, items }) => {
                 // FIXME(const_trait_impl, effects, fee1-dead) this should be simplified if possible
                 let constness = attrs
                     .unwrap_or(&[])
@@ -467,7 +468,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                         (unsafety, items, bounds)
                     },
                 );
-                hir::ItemKind::Trait(*is_auto, unsafety, generics, bounds, items)
+                hir::ItemKind::Trait(unsafety, generics, bounds, items)
             }
             ItemKind::TraitAlias(generics, bounds) => {
                 let (generics, bounds) = self.lower_generics(

compiler/rustc_ast_passes/messages.ftl

@@ -184,8 +184,8 @@ ast_passes_nested_lifetimes = nested quantification of lifetimes
 
 ast_passes_nomangle_ascii = `#[no_mangle]` requires ASCII identifier
 
-ast_passes_obsolete_auto = `impl Trait for .. {"{}"}` is an obsolete syntax
-    .help = use `auto trait Trait {"{}"}` instead
+ast_passes_obsolete_auto_syntax = `{$syntax}` is an obsolete syntax for auto traits
+    .help = use `#[rustc_auto_trait] trait Trait {"{}"}` instead
 
 ast_passes_optional_const_exclusive = `~const` and `{$modifier}` are mutually exclusive
 

compiler/rustc_ast_passes/src/ast_validation.rs

@@ -810,7 +810,10 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
                         errors::VisibilityNotPermittedNote::TraitImpl,
                     );
                     if let TyKind::Err = self_ty.kind {
-                        this.err_handler().emit_err(errors::ObsoleteAuto { span: item.span });
+                        this.err_handler().emit_err(errors::ObsoleteAutoSyntax {
+                            span: item.span,
+                            syntax: "impl Trait for .. {}",
+                        });
                     }
                     if let (&Unsafe::Yes(span), &ImplPolarity::Negative(sp)) = (unsafety, polarity)
                     {
@@ -941,10 +944,11 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
                     }
                 }
             }
-            ItemKind::Trait(box Trait { is_auto, generics, bounds, items, .. }) => {
+            // We intentionally ignore `is_auto` since `auto` is now meaningless.
+            ItemKind::Trait(box Trait { is_auto: _, generics, bounds, items, .. }) => {
                 let is_const_trait = attr::contains_name(&item.attrs, sym::const_trait);
                 self.with_in_trait(is_const_trait, |this| {
-                    if *is_auto == IsAuto::Yes {
+                    if attr::contains_name(&item.attrs, sym::rustc_auto_trait) {
                         // Auto traits cannot have generics, super traits nor contain items.
                         this.deny_generic_params(generics, item.ident.span);
                         this.deny_super_traits(bounds, item.ident.span);

compiler/rustc_ast_passes/src/errors.rs

@@ -417,11 +417,12 @@ pub struct OutOfOrderParams<'a> {
 }
 
 #[derive(Diagnostic)]
-#[diag(ast_passes_obsolete_auto)]
+#[diag(ast_passes_obsolete_auto_syntax)]
 #[help]
-pub struct ObsoleteAuto {
+pub struct ObsoleteAutoSyntax {
     #[primary_span]
     pub span: Span,
+    pub syntax: &'static str,
 }
 
 #[derive(Diagnostic)]

compiler/rustc_ast_passes/src/feature_gate.rs

@@ -298,12 +298,11 @@ impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
             }
 
             ast::ItemKind::Trait(box ast::Trait { is_auto: ast::IsAuto::Yes, .. }) => {
-                gate_feature_post!(
-                    &self,
-                    auto_traits,
-                    i.span,
-                    "auto traits are experimental and possibly buggy"
-                );
+                // FIXME: Consider using a structured suggestion.
+                self.sess.emit_err(errors::ObsoleteAutoSyntax {
+                    span: i.span,
+                    syntax: "auto trait Trait {}",
+                });
             }
 
             ast::ItemKind::TraitAlias(..) => {

compiler/rustc_borrowck/src/diagnostics/mutability_errors.rs

@@ -673,8 +673,7 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
             true,
             td.as_local().and_then(|tld| match hir_map.find_by_def_id(tld) {
                 Some(Node::Item(hir::Item {
-                    kind: hir::ItemKind::Trait(_, _, _, _, items),
-                    ..
+                    kind: hir::ItemKind::Trait(_, _, _, items), ..
                 })) => {
                     let mut f_in_trait_opt = None;
                     for hir::TraitItemRef { id: fi, kind: k, .. } in *items {

compiler/rustc_codegen_cranelift/example/mini_core.rs

@@ -7,7 +7,6 @@
     decl_macro,
     rustc_attrs,
     transparent_unions,
-    auto_traits,
     thread_local
 )]
 #![no_core]
@@ -93,7 +92,8 @@ unsafe impl<'a, T: ?Sized> Sync for &'a T {}
 unsafe impl Sync for [u8; 16] {}
 
 #[lang = "freeze"]
-unsafe auto trait Freeze {}
+#[rustc_auto_trait]
+unsafe trait Freeze {}
 
 unsafe impl<T: ?Sized> Freeze for PhantomData<T> {}
 unsafe impl<T: ?Sized> Freeze for *const T {}
@@ -503,7 +503,8 @@ pub unsafe fn drop_in_place<T: ?Sized>(to_drop: *mut T) {
 }
 
 #[lang = "unpin"]
-pub auto trait Unpin {}
+#[rustc_auto_trait]
+pub trait Unpin {}
 
 #[lang = "deref"]
 pub trait Deref {

compiler/rustc_codegen_gcc/example/mini_core.rs

@@ -1,6 +1,6 @@
 #![feature(
     no_core, lang_items, intrinsics, unboxed_closures, type_ascription, extern_types,
-    decl_macro, rustc_attrs, transparent_unions, auto_traits,
+    decl_macro, rustc_attrs, transparent_unions,
     thread_local
 )]
 #![no_core]
@@ -89,7 +89,8 @@ unsafe impl<'a, T: ?Sized> Sync for &'a T {}
 unsafe impl Sync for [u8; 16] {}
 
 #[lang = "freeze"]
-unsafe auto trait Freeze {}
+#[rustc_auto_trait]
+unsafe trait Freeze {}
 
 unsafe impl<T: ?Sized> Freeze for PhantomData<T> {}
 unsafe impl<T: ?Sized> Freeze for *const T {}
@@ -452,7 +453,8 @@ pub unsafe fn drop_in_place<T: ?Sized>(to_drop: *mut T) {
 }
 
 #[lang = "unpin"]
-pub auto trait Unpin {}
+#[rustc_auto_trait]
+pub trait Unpin {}
 
 #[lang = "deref"]
 pub trait Deref {

compiler/rustc_codegen_gcc/tests/run/abort1.rs

@@ -3,7 +3,7 @@
 // Run-time:
 //   status: signal
 
-#![feature(auto_traits, lang_items, no_core, start, intrinsics)]
+#![feature(rustc_attrs, lang_items, no_core, start, intrinsics)]
 
 #![no_std]
 #![no_core]
@@ -27,7 +27,8 @@ trait Receiver {
 }
 
 #[lang = "freeze"]
-pub(crate) unsafe auto trait Freeze {}
+#[rustc_auto_trait]
+pub(crate) unsafe trait Freeze {}
 
 mod intrinsics {
     use super::Sized;

compiler/rustc_codegen_gcc/tests/run/abort2.rs

@@ -3,7 +3,7 @@
 // Run-time:
 //   status: signal
 
-#![feature(auto_traits, lang_items, no_core, start, intrinsics)]
+#![feature(rustc_attrs, lang_items, no_core, start, intrinsics)]
 
 #![no_std]
 #![no_core]
@@ -27,7 +27,8 @@ trait Receiver {
 }
 
 #[lang = "freeze"]
-pub(crate) unsafe auto trait Freeze {}
+#[rustc_auto_trait]
+pub(crate) unsafe trait Freeze {}
 
 mod intrinsics {
     use super::Sized;

compiler/rustc_codegen_gcc/tests/run/array.rs

@@ -7,7 +7,7 @@
 //     5
 //     10
 
-#![feature(arbitrary_self_types, auto_traits, lang_items, no_core, start, intrinsics)]
+#![feature(arbitrary_self_types, rustc_attrs, lang_items, no_core, start, intrinsics)]
 
 #![no_std]
 #![no_core]
@@ -36,7 +36,8 @@ trait Receiver {
 }
 
 #[lang = "freeze"]
-pub(crate) unsafe auto trait Freeze {}
+#[rustc_auto_trait]
+pub(crate) unsafe trait Freeze {}
 
 mod libc {
     #[link(name = "c")]

compiler/rustc_codegen_gcc/tests/run/assign.rs

@@ -6,7 +6,7 @@
 //     10
 
 #![allow(unused_attributes)]
-#![feature(auto_traits, lang_items, no_core, start, intrinsics, track_caller)]
+#![feature(rustc_attrs, lang_items, no_core, start, intrinsics, track_caller)]
 
 #![no_std]
 #![no_core]
@@ -35,7 +35,8 @@ trait Receiver {
 }
 
 #[lang = "freeze"]
-pub(crate) unsafe auto trait Freeze {}
+#[rustc_auto_trait]
+pub(crate) unsafe trait Freeze {}
 
 #[lang = "panic_location"]
 struct PanicLocation {

compiler/rustc_codegen_gcc/tests/run/closure.rs

@@ -8,7 +8,7 @@
 //     Int argument: 2
 //     Both args: 11
 
-#![feature(arbitrary_self_types, auto_traits, lang_items, no_core, start, intrinsics,
+#![feature(arbitrary_self_types, rustc_attrs, lang_items, no_core, start, intrinsics,
     unboxed_closures)]
 
 #![no_std]
@@ -38,7 +38,8 @@ trait Receiver {
 }
 
 #[lang = "freeze"]
-pub(crate) unsafe auto trait Freeze {}
+#[rustc_auto_trait]
+pub(crate) unsafe trait Freeze {}
 
 mod libc {
     #[link(name = "c")]

compiler/rustc_codegen_gcc/tests/run/condition.rs

@@ -5,7 +5,7 @@
 //   stdout: true
 //     1
 
-#![feature(arbitrary_self_types, auto_traits, lang_items, no_core, start, intrinsics)]
+#![feature(arbitrary_self_types, rustc_attrs, lang_items, no_core, start, intrinsics)]
 
 #![no_std]
 #![no_core]
@@ -39,7 +39,8 @@ trait Receiver {
 }
 
 #[lang = "freeze"]
-pub(crate) unsafe auto trait Freeze {}
+#[rustc_auto_trait]
+pub(crate) unsafe trait Freeze {}
 
 mod libc {
     #[link(name = "c")]

compiler/rustc_codegen_gcc/tests/run/empty_main.rs

@@ -3,7 +3,7 @@
 // Run-time:
 //   status: 0
 
-#![feature(auto_traits, lang_items, no_core, start)]
+#![feature(rustc_attrs, lang_items, no_core, start)]
 
 #![no_std]
 #![no_core]
@@ -27,7 +27,8 @@ trait Receiver {
 }
 
 #[lang = "freeze"]
-pub(crate) unsafe auto trait Freeze {}
+#[rustc_auto_trait]
+pub(crate) unsafe trait Freeze {}
 
 /*
  * Code

compiler/rustc_codegen_gcc/tests/run/exit.rs

@@ -3,7 +3,7 @@
 // Run-time:
 //   status: 2
 
-#![feature(auto_traits, lang_items, no_core, start, intrinsics)]
+#![feature(rustc_attrs, lang_items, no_core, start, intrinsics)]
 
 #![no_std]
 #![no_core]
@@ -34,7 +34,8 @@ trait Receiver {
 }
 
 #[lang = "freeze"]
-pub(crate) unsafe auto trait Freeze {}
+#[rustc_auto_trait]
+pub(crate) unsafe trait Freeze {}
 
 /*
  * Code

compiler/rustc_codegen_gcc/tests/run/exit_code.rs

@@ -3,7 +3,7 @@
 // Run-time:
 //   status: 1
 
-#![feature(auto_traits, lang_items, no_core, start)]
+#![feature(rustc_attrs, lang_items, no_core, start)]
 
 #![no_std]
 #![no_core]
@@ -27,7 +27,8 @@ trait Receiver {
 }
 
 #[lang = "freeze"]
-pub(crate) unsafe auto trait Freeze {}
+#[rustc_auto_trait]
+pub(crate) unsafe trait Freeze {}
 
 /*
  * Code

compiler/rustc_codegen_gcc/tests/run/fun_ptr.rs

@@ -4,7 +4,7 @@
 //   status: 0
 //   stdout: 1
 
-#![feature(arbitrary_self_types, auto_traits, lang_items, no_core, start, intrinsics)]
+#![feature(arbitrary_self_types, rustc_attrs, lang_items, no_core, start, intrinsics)]
 
 #![no_std]
 #![no_core]
@@ -33,7 +33,8 @@ trait Receiver {
 }
 
 #[lang = "freeze"]
-pub(crate) unsafe auto trait Freeze {}
+#[rustc_auto_trait]
+pub(crate) unsafe trait Freeze {}
 
 mod libc {
     #[link(name = "c")]

compiler/rustc_codegen_gcc/tests/run/int_overflow.rs

@@ -5,7 +5,7 @@
 //   status: signal
 
 #![allow(unused_attributes)]
-#![feature(auto_traits, lang_items, no_core, start, intrinsics)]
+#![feature(rustc_attrs, lang_items, no_core, start, intrinsics)]
 
 #![no_std]
 #![no_core]
@@ -34,7 +34,8 @@ trait Receiver {
 }
 
 #[lang = "freeze"]
-pub(crate) unsafe auto trait Freeze {}
+#[rustc_auto_trait]
+pub(crate) unsafe trait Freeze {}
 
 #[lang = "panic_location"]
 struct PanicLocation {