Allow simd_shuffle to accept vectors of any length

compiler/rustc_codegen_llvm/src/intrinsic.rs

@@ -918,12 +918,29 @@ fn generic_simd_intrinsic(
     }
 
     if let Some(stripped) = name_str.strip_prefix("simd_shuffle") {
-        let n: u64 = stripped.parse().unwrap_or_else(|_| {
-            span_bug!(span, "bad `simd_shuffle` instruction only caught in codegen?")
-        });
+        // If this intrinsic is the older "simd_shuffleN" form, simply parse the integer.
+        // If there is no suffix, use the index array length.
+        let n: u64 = if stripped.is_empty() {
+            // Make sure this is actually an array, since typeck only checks the length-suffixed
+            // version of this intrinsic.
+            match args[2].layout.ty.kind() {
+                ty::Array(ty, len) if matches!(ty.kind(), ty::Uint(ty::UintTy::U32)) => {
+                    len.try_eval_usize(bx.cx.tcx, ty::ParamEnv::reveal_all()).unwrap_or_else(|| {
+                        span_bug!(span, "could not evaluate shuffle index array length")
+                    })
+                }
+                _ => return_error!(
+                    "simd_shuffle index must be an array of `u32`, got `{}`",
+                    args[2].layout.ty
+                ),
+            }
+        } else {
+            stripped.parse().unwrap_or_else(|_| {
+                span_bug!(span, "bad `simd_shuffle` instruction only caught in codegen?")
+            })
+        };
 
         require_simd!(ret_ty, "return");
-
         let (out_len, out_ty) = ret_ty.simd_size_and_type(bx.tcx());
         require!(
             out_len == n,

compiler/rustc_error_codes/src/error_codes/E0439.md

@@ -1,8 +1,10 @@
+#### Note: this error code is no longer emitted by the compiler.
+
 The length of the platform-intrinsic function `simd_shuffle` wasn't specified.
 
 Erroneous code example:
 
-```compile_fail,E0439
+```ignore (no longer emitted)
 #![feature(platform_intrinsics)]
 
 extern "platform-intrinsic" {

compiler/rustc_span/src/symbol.rs

@@ -1210,6 +1210,7 @@ symbols! {
         simd_select_bitmask,
         simd_shl,
         simd_shr,
+        simd_shuffle,
         simd_sub,
         simd_trunc,
         simd_xor,

compiler/rustc_typeck/src/check/intrinsic.rs

@@ -2,7 +2,7 @@
 //! intrinsics that the compiler exposes.
 
 use crate::errors::{
-    SimdShuffleMissingLength, UnrecognizedAtomicOperation, UnrecognizedIntrinsicFunction,
+    UnrecognizedAtomicOperation, UnrecognizedIntrinsicFunction,
     WrongNumberOfGenericArgumentsToIntrinsic,
 };
 use crate::require_same_types;
@@ -468,6 +468,7 @@ pub fn check_platform_intrinsic_type(tcx: TyCtxt<'_>, it: &hir::ForeignItem<'_>)
         | sym::simd_reduce_max
         | sym::simd_reduce_min_nanless
         | sym::simd_reduce_max_nanless => (2, vec![param(0)], param(1)),
+        sym::simd_shuffle => (3, vec![param(0), param(0), param(1)], param(2)),
         name if name.as_str().starts_with("simd_shuffle") => {
             match name.as_str()["simd_shuffle".len()..].parse() {
                 Ok(n) => {
@@ -475,7 +476,9 @@ pub fn check_platform_intrinsic_type(tcx: TyCtxt<'_>, it: &hir::ForeignItem<'_>)
                     (2, params, param(1))
                 }
                 Err(_) => {
-                    tcx.sess.emit_err(SimdShuffleMissingLength { span: it.span, name });
+                    let msg =
+                        format!("unrecognized platform-specific intrinsic function: `{}`", name);
+                    tcx.sess.struct_span_err(it.span, &msg).emit();
                     return;
                 }
             }

compiler/rustc_typeck/src/errors.rs

@@ -121,14 +121,6 @@ pub struct AssocTypeBindingNotAllowed {
     pub span: Span,
 }
 
-#[derive(SessionDiagnostic)]
-#[error = "E0439"]
-pub struct SimdShuffleMissingLength {
-    #[message = "invalid `simd_shuffle`, needs length: `{name}`"]
-    pub span: Span,
-    pub name: Symbol,
-}
-
 #[derive(SessionDiagnostic)]
 #[error = "E0436"]
 pub struct FunctionalRecordUpdateOnNonStruct {