From d765fb8fafba4f6a25e34c7568d7bc8e3be8226d Mon Sep 17 00:00:00 2001 From: Ralf Jung Date: Sun, 10 Mar 2024 12:39:53 +0100 Subject: [PATCH] add comments explaining where post-mono const eval errors abort compilation --- compiler/rustc_codegen_cranelift/src/constant.rs | 2 +- compiler/rustc_codegen_ssa/src/mir/constant.rs | 6 +++--- compiler/rustc_codegen_ssa/src/mir/mod.rs | 3 ++- compiler/rustc_monomorphize/src/collector.rs | 9 ++++++--- compiler/rustc_monomorphize/src/partitioning.rs | 3 +++ 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 a6fcf1fd38c..89fd57b6d96 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 32f823a5ac6..573a8f68347 100644 --- a/compiler/rustc_monomorphize/src/collector.rs +++ b/compiler/rustc_monomorphize/src/collector.rs @@ -814,13 +814,16 @@ 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 8bebc30e435..c9ebc732460 100644 --- a/compiler/rustc_monomorphize/src/partitioning.rs +++ b/compiler/rustc_monomorphize/src/partitioning.rs @@ -1114,6 +1114,9 @@ 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", || {