Rollup merge of rust-lang#116277 - RalfJung:post-mono, r=oli-obk

dont call mir.post_mono_checks in codegen

It seems like all tests are still passing when I remove this... let's see what CI says.

Author: workingjubilee

compiler/rustc_codegen_cranelift/src/base.rs

@@ -250,17 +250,6 @@ pub(crate) fn verify_func(
 }
 
 fn codegen_fn_body(fx: &mut FunctionCx<'_, '_, '_>, start_block: Block) {
-    if let Err(err) =
-        fx.mir.post_mono_checks(fx.tcx, ty::ParamEnv::reveal_all(), |c| Ok(fx.monomorphize(c)))
-    {
-        err.emit_err(fx.tcx);
-        fx.bcx.append_block_params_for_function_params(fx.block_map[START_BLOCK]);
-        fx.bcx.switch_to_block(fx.block_map[START_BLOCK]);
-        // compilation should have been aborted
-        fx.bcx.ins().trap(TrapCode::UnreachableCodeReached);
-        return;
-    }
-
     let arg_uninhabited = fx
         .mir
         .args_iter()

compiler/rustc_codegen_ssa/src/mir/constant.rs

@@ -21,6 +21,8 @@ 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.
         self.monomorphize(constant.const_)
             .eval(self.cx.tcx(), ty::ParamEnv::reveal_all(), Some(constant.span))
             .expect("erroneous constant not captured by required_consts")

compiler/rustc_codegen_ssa/src/mir/mod.rs

@@ -209,18 +209,11 @@ pub fn codegen_mir<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
         caller_location: None,
     };
 
-    fx.per_local_var_debug_info = fx.compute_per_local_var_debug_info(&mut start_bx);
+    // 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.
 
-    // Rust post-monomorphization checks; we later rely on them.
-    if let Err(err) =
-        mir.post_mono_checks(cx.tcx(), ty::ParamEnv::reveal_all(), |c| Ok(fx.monomorphize(c)))
-    {
-        err.emit_err(cx.tcx());
-        // This IR shouldn't ever be emitted, but let's try to guard against any of this code
-        // ever running.
-        start_bx.abort();
-        return;
-    }
+    fx.per_local_var_debug_info = fx.compute_per_local_var_debug_info(&mut start_bx);
 
     let memory_locals = analyze::non_ssa_locals(&fx);
 

compiler/rustc_const_eval/src/interpret/eval_context.rs

@@ -750,12 +750,14 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
 
         // Make sure all the constants required by this frame evaluate successfully (post-monomorphization check).
         if M::POST_MONO_CHECKS {
-            // `ctfe_query` does some error message decoration that we want to be in effect here.
-            self.ctfe_query(None, |tcx| {
-                body.post_mono_checks(*tcx, self.param_env, |c| {
-                    self.subst_from_current_frame_and_normalize_erasing_regions(c)
-                })
-            })?;
+            for &const_ in &body.required_consts {
+                let c =
+                    self.subst_from_current_frame_and_normalize_erasing_regions(const_.const_)?;
+                c.eval(*self.tcx, self.param_env, Some(const_.span)).map_err(|err| {
+                    err.emit_note(*self.tcx);
+                    err
+                })?;
+            }
         }
 
         // done
@@ -1054,14 +1056,14 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
         Ok(())
     }
 
-    /// Call a query that can return `ErrorHandled`. If `span` is `Some`, point to that span when an error occurs.
+    /// Call a query that can return `ErrorHandled`. Should be used for statics and other globals.
+    /// (`mir::Const`/`ty::Const` have `eval` methods that can be used directly instead.)
     pub fn ctfe_query<T>(
         &self,
-        span: Option<Span>,
         query: impl FnOnce(TyCtxtAt<'tcx>) -> Result<T, ErrorHandled>,
     ) -> Result<T, ErrorHandled> {
         // Use a precise span for better cycle errors.
-        query(self.tcx.at(span.unwrap_or_else(|| self.cur_span()))).map_err(|err| {
+        query(self.tcx.at(self.cur_span())).map_err(|err| {
             err.emit_note(*self.tcx);
             err
         })
@@ -1082,7 +1084,7 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
         } else {
             self.param_env
         };
-        let val = self.ctfe_query(None, |tcx| tcx.eval_to_allocation_raw(param_env.and(gid)))?;
+        let val = self.ctfe_query(|tcx| tcx.eval_to_allocation_raw(param_env.and(gid)))?;
         self.raw_const_to_mplace(val)
     }
 
@@ -1092,7 +1094,12 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
         span: Option<Span>,
         layout: Option<TyAndLayout<'tcx>>,
     ) -> InterpResult<'tcx, OpTy<'tcx, M::Provenance>> {
-        let const_val = self.ctfe_query(span, |tcx| val.eval(*tcx, self.param_env, span))?;
+        let const_val = val.eval(*self.tcx, self.param_env, span).map_err(|err| {
+            // FIXME: somehow this is reachable even when POST_MONO_CHECKS is on.
+            // Are we not always populating `required_consts`?
+            err.emit_note(*self.tcx);
+            err
+        })?;
         self.const_val_to_op(const_val, val.ty(), layout)
     }
 

compiler/rustc_const_eval/src/interpret/intrinsics.rs

@@ -164,7 +164,7 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
                     sym::type_name => Ty::new_static_str(self.tcx.tcx),
                     _ => bug!(),
                 };
-                let val = self.ctfe_query(None, |tcx| {
+                let val = self.ctfe_query(|tcx| {
                     tcx.const_eval_global_id(self.param_env, gid, Some(tcx.span))
                 })?;
                 let val = self.const_val_to_op(val, ty, Some(dest.layout))?;

compiler/rustc_const_eval/src/interpret/memory.rs

@@ -536,7 +536,7 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
                 }
 
                 // We don't give a span -- statics don't need that, they cannot be generic or associated.
-                let val = self.ctfe_query(None, |tcx| tcx.eval_static_initializer(def_id))?;
+                let val = self.ctfe_query(|tcx| tcx.eval_static_initializer(def_id))?;
                 (val, Some(def_id))
             }
         };

compiler/rustc_middle/src/mir/interpret/error.rs

@@ -43,21 +43,6 @@ impl ErrorHandled {
         }
     }
 
-    pub fn emit_err(&self, tcx: TyCtxt<'_>) -> ErrorGuaranteed {
-        match self {
-            &ErrorHandled::Reported(err, span) => {
-                if !err.is_tainted_by_errors && !span.is_dummy() {
-                    tcx.sess.emit_err(error::ErroneousConstant { span });
-                }
-                err.error
-            }
-            &ErrorHandled::TooGeneric(span) => tcx.sess.delay_span_bug(
-                span,
-                "encountered TooGeneric error when monomorphic data was expected",
-            ),
-        }
-    }
-
     pub fn emit_note(&self, tcx: TyCtxt<'_>) {
         match self {
             &ErrorHandled::Reported(err, span) => {

compiler/rustc_middle/src/mir/mod.rs

@@ -2,7 +2,7 @@
 //!
 //! [rustc dev guide]: https://rustc-dev-guide.rust-lang.org/mir/index.html
 
-use crate::mir::interpret::{AllocRange, ConstAllocation, ErrorHandled, Scalar};
+use crate::mir::interpret::{AllocRange, ConstAllocation, Scalar};
 use crate::mir::visit::MirVisitable;
 use crate::ty::codec::{TyDecoder, TyEncoder};
 use crate::ty::fold::{FallibleTypeFolder, TypeFoldable};
@@ -568,34 +568,6 @@ impl<'tcx> Body<'tcx> {
     pub fn is_custom_mir(&self) -> bool {
         self.injection_phase.is_some()
     }
-
-    /// *Must* be called once the full substitution for this body is known, to ensure that the body
-    /// is indeed fit for code generation or consumption more generally.
-    ///
-    /// Sadly there's no nice way to represent an "arbitrary normalizer", so we take one for
-    /// constants specifically. (`Option<GenericArgsRef>` could be used for that, but the fact
-    /// that `Instance::args_for_mir_body` is private and instead instance exposes normalization
-    /// functions makes it seem like exposing the generic args is not the intended strategy.)
-    ///
-    /// Also sadly, CTFE doesn't even know whether it runs on MIR that is already polymorphic or still monomorphic,
-    /// so we cannot just immediately ICE on TooGeneric.
-    ///
-    /// Returns Ok(()) if everything went fine, and `Err` if a problem occurred and got reported.
-    pub fn post_mono_checks(
-        &self,
-        tcx: TyCtxt<'tcx>,
-        param_env: ty::ParamEnv<'tcx>,
-        normalize_const: impl Fn(Const<'tcx>) -> Result<Const<'tcx>, ErrorHandled>,
-    ) -> Result<(), ErrorHandled> {
-        // For now, the only thing we have to check is is to ensure that all the constants used in
-        // the body successfully evaluate.
-        for &const_ in &self.required_consts {
-            let c = normalize_const(const_.const_)?;
-            c.eval(tcx, param_env, Some(const_.span))?;
-        }
-
-        Ok(())
-    }
 }
 
 #[derive(Copy, Clone, PartialEq, Eq, Debug, TyEncodable, TyDecodable, HashStable)]

compiler/rustc_middle/src/mir/visit.rs

@@ -184,6 +184,8 @@ macro_rules! make_mir_visitor {
 
             visit_place_fns!($($mutability)?);
 
+            /// This is called for every constant in the MIR body and every `required_consts`
+            /// (i.e., including consts that have been dead-code-eliminated).
             fn visit_constant(
                 &mut self,
                 constant: & $($mutability)? ConstOperand<'tcx>,

compiler/rustc_monomorphize/src/collector.rs

@@ -1426,6 +1426,8 @@ fn collect_used_items<'tcx>(
         );
     }
 
+    // Here we rely on the visitor also visiting `required_consts`, so that we evaluate them
+    // and abort compilation if any of them errors.
     MirUsedCollector {
         tcx,
         body: &body,