Stabilize #[repr(transparent)]

Tracking issue FCP: #43036 (comment)
Reference PR: rust-lang/reference#353

Author: SimonSapin

src/doc/unstable-book/src/language-features/repr-transparent.md

@@ -105,7 +105,7 @@
 #![feature(pin)]
 #![feature(ptr_internals)]
 #![feature(ptr_offset_from)]
-#![feature(repr_transparent)]
+#![cfg_attr(stage0, feature(repr_transparent))]
 #![feature(rustc_attrs)]
 #![feature(specialization)]
 #![feature(staged_api)]

src/liballoc/lib.rs

@@ -100,7 +100,7 @@
 #![feature(optin_builtin_traits)]
 #![feature(prelude_import)]
 #![feature(repr_simd, platform_intrinsics)]
-#![feature(repr_transparent)]
+#![cfg_attr(stage0, feature(repr_transparent))]
 #![feature(rustc_attrs)]
 #![feature(rustc_const_unstable)]
 #![feature(simd_ffi)]

src/libcore/lib.rs

@@ -1958,8 +1958,6 @@ representation hints.
 Erroneous code example:
 
 ```compile_fail,E0692
-#![feature(repr_transparent)]
-
 #[repr(transparent, C)] // error: incompatible representation hints
 struct Grams(f32);
 ```
@@ -1969,17 +1967,13 @@ another type, so adding more representation hints is contradictory. Remove
 either the `transparent` hint or the other hints, like this:
 
 ```
-#![feature(repr_transparent)]
-
 #[repr(transparent)]
 struct Grams(f32);
 ```
 
 Alternatively, move the other attributes to the contained type:
 
 ```
-#![feature(repr_transparent)]
-
 #[repr(C)]
 struct Foo {
     x: i32,
@@ -1994,8 +1988,6 @@ Note that introducing another `struct` just to have a place for the other
 attributes may have unintended side effects on the representation:
 
 ```
-#![feature(repr_transparent)]
-
 #[repr(transparent)]
 struct Grams(f32);
 

src/librustc/diagnostics.rs

@@ -4581,8 +4581,6 @@ on fields that were not guaranteed to be zero-sized.
 Erroneous code example:
 
 ```compile_fail,E0690
-#![feature(repr_transparent)]
-
 #[repr(transparent)]
 struct LengthWithUnit<U> { // error: transparent struct needs exactly one
     value: f32,            //        non-zero-sized field, but has 2
@@ -4602,8 +4600,6 @@ To combine `repr(transparent)` with type parameters, `PhantomData` may be
 useful:
 
 ```
-#![feature(repr_transparent)]
-
 use std::marker::PhantomData;
 
 #[repr(transparent)]
@@ -4621,7 +4617,7 @@ field that requires non-trivial alignment.
 Erroneous code example:
 
 ```compile_fail,E0691
-#![feature(repr_transparent, repr_align, attr_literals)]
+#![feature(repr_align, attr_literals)]
 
 #[repr(align(32))]
 struct ForceAlign32;
@@ -4640,8 +4636,6 @@ requirement.
 Consider removing the over-aligned zero-sized field:
 
 ```
-#![feature(repr_transparent)]
-
 #[repr(transparent)]
 struct Wrapper(f32);
 ```
@@ -4650,7 +4644,7 @@ Alternatively, `PhantomData<T>` has alignment 1 for all `T`, so you can use it
 if you need to keep the field for some reason:
 
 ```
-#![feature(repr_transparent, repr_align, attr_literals)]
+#![feature(repr_align, attr_literals)]
 
 use std::marker::PhantomData;
 

src/librustc_typeck/diagnostics.rs

@@ -399,9 +399,6 @@ declare_features! (
     // `extern` in paths
     (active, extern_in_paths, "1.23.0", Some(44660), None),
 
-    // Allows `#[repr(transparent)]` attribute on newtype structs
-    (active, repr_transparent, "1.25.0", Some(43036), None),
-
     // Use `?` as the Kleene "at most one" operator
     (active, macro_at_most_once_rep, "1.25.0", Some(48075), None),
 
@@ -615,6 +612,8 @@ declare_features! (
     (accepted, termination_trait_test, "1.27.0", Some(48854), None),
     // The #[global_allocator] attribute
     (accepted, global_allocator, "1.28.0", Some(27389), None),
+    // Allows `#[repr(transparent)]` attribute on newtype structs
+    (accepted, repr_transparent, "1.28.0", Some(43036), None),
 );
 
 // If you change this, please modify src/doc/unstable-book as well. You must
@@ -1595,11 +1594,6 @@ impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
                             gate_feature_post!(&self, repr_simd, attr.span,
                                                "SIMD types are experimental and possibly buggy");
                         }
-                        if item.check_name("transparent") {
-                            gate_feature_post!(&self, repr_transparent, attr.span,
-                                               "the `#[repr(transparent)]` attribute \
-                                               is experimental");
-                        }
                         if let Some((name, _)) = item.name_value_literal() {
                             if name == "packed" {
                                 gate_feature_post!(&self, repr_packed, attr.span,

src/libsyntax/feature_gate.rs

@@ -18,7 +18,6 @@
 // See repr-transparent.rs
 
 #![crate_type="lib"]
-#![feature(repr_transparent)]
 
 
 #[repr(C)]

src/test/codegen/repr-transparent-aggregates-1.rs

@@ -22,7 +22,6 @@
 // See repr-transparent.rs
 
 #![crate_type="lib"]
-#![feature(repr_transparent)]
 
 
 #[repr(C)]

src/test/codegen/repr-transparent-aggregates-2.rs

@@ -14,7 +14,6 @@
 // See repr-transparent.rs
 
 #![crate_type="lib"]
-#![feature(repr_transparent)]
 
 
 #[repr(C)]

src/test/codegen/repr-transparent-aggregates-3.rs

@@ -13,7 +13,6 @@
 // compile-flags: -C no-prepopulate-passes
 
 #![crate_type="lib"]
-#![feature(repr_transparent)]
 
 #[repr(C)]
 pub struct Rgb8 { r: u8, g: u8, b: u8 }