Auto merge of #121225 - RalfJung:simd-extract-insert-const-idx, r=oli-obk,Amanieu

require simd_insert, simd_extract indices to be constants

As discussed in #77477 (see in particular [here](#77477 (comment))). This PR doesn't touch codegen yet -- the first step is to ensure that the indices are always constants; the second step is to then make use of this fact in backends.

Blocked on rust-lang/stdarch#1530 propagating to the rustc repo.

Author: bors

Diff for: compiler/rustc_borrowck/messages.ftl

@@ -163,7 +163,13 @@ borrowck_returned_lifetime_wrong =
 borrowck_returned_ref_escaped =
     returns a reference to a captured variable which escapes the closure body
 
-borrowck_simd_shuffle_last_const = last argument of `simd_shuffle` is required to be a `const` item
+borrowck_simd_intrinsic_arg_const =
+    {$arg ->
+        [1] 1st
+        [2] 2nd
+        [3] 3rd
+        *[other] {$arg}th
+    } argument of `{$intrinsic}` is required to be a `const` item
 
 borrowck_suggest_create_freash_reborrow =
     consider reborrowing the `Pin` instead of moving it

Diff for: compiler/rustc_borrowck/src/session_diagnostics.rs

@@ -454,8 +454,10 @@ pub(crate) enum TypeNoCopy<'a, 'tcx> {
 }
 
 #[derive(Diagnostic)]
-#[diag(borrowck_simd_shuffle_last_const)]
-pub(crate) struct SimdShuffleLastConst {
+#[diag(borrowck_simd_intrinsic_arg_const)]
+pub(crate) struct SimdIntrinsicArgConst {
     #[primary_span]
     pub span: Span,
+    pub arg: usize,
+    pub intrinsic: String,
 }

Diff for: compiler/rustc_borrowck/src/type_check/mod.rs

@@ -49,7 +49,7 @@ use rustc_mir_dataflow::impls::MaybeInitializedPlaces;
 use rustc_mir_dataflow::move_paths::MoveData;
 use rustc_mir_dataflow::ResultsCursor;
 
-use crate::session_diagnostics::{MoveUnsized, SimdShuffleLastConst};
+use crate::session_diagnostics::{MoveUnsized, SimdIntrinsicArgConst};
 use crate::{
     borrow_set::BorrowSet,
     constraints::{OutlivesConstraint, OutlivesConstraintSet},
@@ -1664,9 +1664,22 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
 
         let func_ty = func.ty(body, self.infcx.tcx);
         if let ty::FnDef(def_id, _) = *func_ty.kind() {
-            if let Some(sym::simd_shuffle) = self.tcx().intrinsic(def_id) {
-                if !matches!(args[2], Spanned { node: Operand::Constant(_), .. }) {
-                    self.tcx().dcx().emit_err(SimdShuffleLastConst { span: term.source_info.span });
+            // Some of the SIMD intrinsics are special: they need a particular argument to be a constant.
+            // (Eventually this should use const-generics, but those are not up for the task yet:
+            // https://github.com/rust-lang/rust/issues/85229.)
+            if let Some(name @ (sym::simd_shuffle | sym::simd_insert | sym::simd_extract)) =
+                self.tcx().intrinsic(def_id)
+            {
+                let idx = match name {
+                    sym::simd_shuffle => 2,
+                    _ => 1,
+                };
+                if !matches!(args[idx], Spanned { node: Operand::Constant(_), .. }) {
+                    self.tcx().dcx().emit_err(SimdIntrinsicArgConst {
+                        span: term.source_info.span,
+                        arg: idx + 1,
+                        intrinsic: name.to_string(),
+                    });
                 }
             }
         }

Diff for: compiler/rustc_codegen_ssa/src/mir/block.rs

@@ -864,8 +864,7 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                     .map(|(i, arg)| {
                         // The indices passed to simd_shuffle in the
                         // third argument must be constant. This is
-                        // checked by const-qualification, which also
-                        // promotes any complex rvalues to constants.
+                        // checked by the type-checker.
                         if i == 2 && intrinsic == sym::simd_shuffle {
                             if let mir::Operand::Constant(constant) = &arg.node {
                                 let (llval, ty) = self.simd_shuffle_indices(bx, constant);

Diff for: tests/codegen/simd-intrinsic/simd-intrinsic-generic-extract-insert.rs

@@ -33,12 +33,7 @@ extern "platform-intrinsic" {
 pub fn main() {
     let mut t = T::<i32x4>([0; 4]);
     unsafe {
-        for i in 0_i32..4 {
-            t = simd_insert(t, i as u32, i);
-        }
-        for i in 0_i32..4 {
-            assert_eq!(i, simd_extract(t, i as u32));
-            //~^ ERROR: use of moved value: `t`
-        }
+        t = simd_insert(t, 3, 3);
+        assert_eq!(3, simd_extract(t, 3));
     }
 }

Diff for: tests/ui/simd/array-trait.rs

@@ -23,18 +23,6 @@ LL | pub struct T<S: Simd>([S::Lane; S::SIZE]);
    = help: try adding a `where` bound using this expression: `where [(); S::SIZE]:`
    = note: this error originates in the derive macro `Clone` (in Nightly builds, run with -Z macro-backtrace for more info)
 
-error[E0382]: use of moved value: `t`
-  --> $DIR/array-trait.rs:40:40
-   |
-LL |     let mut t = T::<i32x4>([0; 4]);
-   |         ----- move occurs because `t` has type `T<i32x4>`, which does not implement the `Copy` trait
-...
-LL |         for i in 0_i32..4 {
-   |         ----------------- inside of this loop
-LL |             assert_eq!(i, simd_extract(t, i as u32));
-   |                                        ^ value moved here, in previous iteration of loop
-
-error: aborting due to 4 previous errors
+error: aborting due to 3 previous errors
 
-Some errors have detailed explanations: E0077, E0382.
-For more information about an error, try `rustc --explain E0077`.
+For more information about this error, try `rustc --explain E0077`.

Diff for: tests/ui/simd/array-trait.stderr

@@ -22,21 +22,13 @@ pub fn main() {
     let mut s = S([0; 4]);
 
     unsafe {
-        for i in 0_i32..4 {
-            s = simd_insert(s, i as u32, i);
-        }
-        for i in 0_i32..4 {
-            assert_eq!(i, simd_extract(s, i as u32));
-        }
+        s = simd_insert(s, 3, 3);
+        assert_eq!(3, simd_extract(s, 3));
     }
 
     let mut t = T::<4>([0; 4]);
     unsafe {
-        for i in 0_i32..4 {
-            t = simd_insert(t, i as u32, i);
-        }
-        for i in 0_i32..4 {
-            assert_eq!(i, simd_extract(t, i as u32));
-        }
+        t = simd_insert(t, 3, 3);
+        assert_eq!(3, simd_extract(t, 3));
     }
 }