Rollup merge of #122287 - RalfJung:simd-static-assert, r=pnkfelix

add test ensuring simd codegen checks don't run when a static assertion failed

stdarch relies on this to ensure that SIMD indices are in bounds.

I would love to know why this works, but I can't figure out where codegen decides to not codegen a function if a required-const does not evaluate. `@oli-obk` `@bjorn3` do you have any idea?

5 files changed, 15 insertions, 8 deletions

diff --git a/compiler/rustc_codegen_cranelift/src/constant.rs b/compiler/rustc_codegen_cranelift/src/constant.rs
index 18c5960ffc6..cec479218b7 100644
--- a/compiler/rustc_codegen_cranelift/src/constant.rs
+++ b/compiler/rustc_codegen_cranelift/src/constant.rs
@@ -71,7 +71,7 @@ pub(crate) fn eval_mir_constant<'tcx>(
     // This cannot fail because we checked all required_consts in advance.
     let val = cv
         .eval(fx.tcx, ty::ParamEnv::reveal_all(), Some(constant.span))
-        .expect("erroneous constant not captured by required_consts");
+        .expect("erroneous constant missed by mono item collection");
     (val, cv.ty())
 }
 
diff --git a/compiler/rustc_codegen_ssa/src/mir/constant.rs b/compiler/rustc_codegen_ssa/src/mir/constant.rs
index d532bd90426..ff899c50b3d 100644
--- a/compiler/rustc_codegen_ssa/src/mir/constant.rs
+++ b/compiler/rustc_codegen_ssa/src/mir/constant.rs
@@ -21,11 +21,11 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
     }
 
     pub fn eval_mir_constant(&self, constant: &mir::ConstOperand<'tcx>) -> mir::ConstValue<'tcx> {
-        // `MirUsedCollector` visited all constants before codegen began, so if we got here there
-        // can be no more constants that fail to evaluate.
+        // `MirUsedCollector` visited all required_consts before codegen began, so if we got here
+        // there can be no more constants that fail to evaluate.
         self.monomorphize(constant.const_)
             .eval(self.cx.tcx(), ty::ParamEnv::reveal_all(), Some(constant.span))
-            .expect("erroneous constant not captured by required_consts")
+            .expect("erroneous constant missed by mono item collection")
     }
 
     /// This is a convenience helper for `simd_shuffle_indices`. It has the precondition
diff --git a/compiler/rustc_codegen_ssa/src/mir/mod.rs b/compiler/rustc_codegen_ssa/src/mir/mod.rs
index bac10f31336..cd6afd2dbbf 100644
--- a/compiler/rustc_codegen_ssa/src/mir/mod.rs
+++ b/compiler/rustc_codegen_ssa/src/mir/mod.rs
@@ -211,7 +211,8 @@ pub fn codegen_mir<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
 
     // It may seem like we should iterate over `required_consts` to ensure they all successfully
     // evaluate; however, the `MirUsedCollector` already did that during the collection phase of
-    // monomorphization so we don't have to do it again.
+    // monomorphization, and if there is an error during collection then codegen never starts -- so
+    // we don't have to do it again.
 
     fx.per_local_var_debug_info = fx.compute_per_local_var_debug_info(&mut start_bx);
 
diff --git a/compiler/rustc_monomorphize/src/collector.rs b/compiler/rustc_monomorphize/src/collector.rs
index 33a446eb55a..2465f9fbfa8 100644
--- a/compiler/rustc_monomorphize/src/collector.rs
+++ b/compiler/rustc_monomorphize/src/collector.rs
@@ -818,13 +818,16 @@ impl<'a, 'tcx> MirVisitor<'tcx> for MirUsedCollector<'a, 'tcx> {
         self.super_rvalue(rvalue, location);
     }
 
-    /// This does not walk the constant, as it has been handled entirely here and trying
-    /// to walk it would attempt to evaluate the `ty::Const` inside, which doesn't necessarily
-    /// work, as some constants cannot be represented in the type system.
+    /// This does not walk the MIR of the constant as that is not needed for codegen, all we need is
+    /// to ensure that the constant evaluates successfully and walk the result.
     #[instrument(skip(self), level = "debug")]
     fn visit_constant(&mut self, constant: &mir::ConstOperand<'tcx>, location: Location) {
         let const_ = self.monomorphize(constant.const_);
         let param_env = ty::ParamEnv::reveal_all();
+        // Evaluate the constant. This makes const eval failure a collection-time error (rather than
+        // a codegen-time error). rustc stops after collection if there was an error, so this
+        // ensures codegen never has to worry about failing consts.
+        // (codegen relies on this and ICEs will happen if this is violated.)
         let val = match const_.eval(self.tcx, param_env, None) {
             Ok(v) => v,
             Err(ErrorHandled::Reported(..)) => return,
diff --git a/compiler/rustc_monomorphize/src/partitioning.rs b/compiler/rustc_monomorphize/src/partitioning.rs
index 296eb3120ee..15041b9cd41 100644
--- a/compiler/rustc_monomorphize/src/partitioning.rs
+++ b/compiler/rustc_monomorphize/src/partitioning.rs
@@ -1112,6 +1112,9 @@ fn collect_and_partition_mono_items(tcx: TyCtxt<'_>, (): ()) -> (&DefIdSet, &[Co
 
     let (items, usage_map) = collector::collect_crate_mono_items(tcx, collection_mode);
 
+    // If there was an error during collection (e.g. from one of the constants we evaluated),
+    // then we stop here. This way codegen does not have to worry about failing constants.
+    // (codegen relies on this and ICEs will happen if this is violated.)
     tcx.dcx().abort_if_errors();
 
     let (codegen_units, _) = tcx.sess.time("partition_and_assert_distinct_symbols", || {