Remove deprecated unstable attribute `#[simd]`

The `#[simd]` attribute has been deprecated since c8b6d5b2 back in 2015. Any nightly crates using it have had ample time to switch to `#[repr(simd)]`, and if they didn't they're likely broken by now anyway.

src/librustc/ty/mod.rs

@@ -1553,11 +1553,6 @@ pub fn new(tcx: TyCtxt, did: DefId) -> ReprOptions {
             }
         }
 
-        // FIXME(eddyb) This is deprecated and should be removed.
-        if tcx.has_attr(did, "simd") {
-            flags.insert(ReprFlags::IS_SIMD);
-        }
-
         // This is here instead of layout because the choice must make it into metadata.
         if !tcx.consider_optimizing(|| format!("Reorder fields of {:?}", tcx.item_path_str(did))) {
             flags.insert(ReprFlags::IS_LINEAR);

src/libsyntax/feature_gate.rs

@@ -131,7 +131,6 @@ pub fn new() -> Features {
     (active, link_llvm_intrinsics, "1.0.0", Some(29602)),
     (active, linkage, "1.0.0", Some(29603)),
     (active, quote, "1.0.0", Some(29601)),
-    (active, simd, "1.0.0", Some(27731)),
 
 
     // rustc internal
@@ -470,6 +469,8 @@ pub fn new() -> Features {
     (removed, unmarked_api, "1.0.0", None),
     (removed, pushpop_unsafe, "1.2.0", None),
     (removed, allocator, "1.0.0", None),
+    // Allows the `#[simd]` attribute -- removed in favor of `#[repr(simd)]`
+    (removed, simd, "1.0.0", Some(27731)),
 );
 
 declare_features! (
@@ -633,7 +634,6 @@ pub fn is_builtin_attr(attr: &ast::Attribute) -> bool {
     ("start", Normal, Ungated),
     ("test", Normal, Ungated),
     ("bench", Normal, Ungated),
-    ("simd", Normal, Ungated),
     ("repr", Normal, Ungated),
     ("path", Normal, Ungated),
     ("abi", Normal, Ungated),
@@ -1501,14 +1501,6 @@ fn visit_item(&mut self, i: &'a ast::Item) {
             }
 
             ast::ItemKind::Struct(..) => {
-                if let Some(attr) = attr::find_by_name(&i.attrs[..], "simd") {
-                    gate_feature_post!(&self, simd, attr.span,
-                                       "SIMD types are experimental and possibly buggy");
-                    self.context.parse_sess.span_diagnostic.span_warn(attr.span,
-                                                                      "the `#[simd]` attribute \
-                                                                       is deprecated, use \
-                                                                       `#[repr(simd)]` instead");
-                }
                 if let Some(attr) = attr::find_by_name(&i.attrs[..], "repr") {
                     for item in attr.meta_item_list().unwrap_or_else(Vec::new) {
                         if item.check_name("simd") {

src/test/ui/feature-gate/issue-43106-gating-of-builtin-attrs.rs

@@ -60,7 +60,6 @@
 #![start                     = "x4300"] //~ WARN unused attribute
 // see issue-43106-gating-of-test.rs for crate-level; but non crate-level is below at "4200"
 // see issue-43106-gating-of-bench.rs for crate-level; but non crate-level is below at "4100"
-#![simd                       = "4000"] //~ WARN unused attribute
 #![repr                       = "3900"] //~ WARN unused attribute
 #![path                       = "3800"] //~ WARN unused attribute
 #![abi                        = "3700"] //~ WARN unused attribute
@@ -328,24 +327,6 @@ mod inner { #![bench="4100"] }
     impl S { }
 }
 
-#[simd = "4000"]
-//~^ WARN unused attribute
-mod simd {
-    mod inner { #![simd="4000"] }
-    //~^ WARN unused attribute
-
-    #[simd = "4000"] fn f() { }
-    //~^ WARN unused attribute
-
-    struct S; // for `struct S` case, see feature-gate-repr-simd.rs
-
-    #[simd = "4000"] type T = S;
-    //~^ WARN unused attribute
-
-    #[simd = "4000"] impl S { }
-    //~^ WARN unused attribute
-}
-
 #[repr = "3900"]
 //~^ WARN unused attribute
 mod repr {