Auto merge of #107270 - cjgillot:remove-zst, r=oli-obk

Replace ZST operands and debuginfo by constants.

This is work that ConstProp will not have to do.
Split from https://github.com/rust-lang/rust/pull/107267

compiler/rustc_mir_transform/src/remove_zsts.rs

@@ -1,7 +1,9 @@
-//! Removes assignments to ZST places.
+//! Removes operations on ZST places, and convert ZST operands to constants.
 
 use crate::MirPass;
-use rustc_middle::mir::{Body, StatementKind};
+use rustc_middle::mir::interpret::ConstValue;
+use rustc_middle::mir::visit::*;
+use rustc_middle::mir::*;
 use rustc_middle::ty::{self, Ty, TyCtxt};
 
 pub struct RemoveZsts;
@@ -16,38 +18,24 @@ impl<'tcx> MirPass<'tcx> for RemoveZsts {
         if tcx.type_of(body.source.def_id()).subst_identity().is_generator() {
             return;
         }
-        let param_env = tcx.param_env(body.source.def_id());
-        let basic_blocks = body.basic_blocks.as_mut_preserves_cfg();
+        let param_env = tcx.param_env_reveal_all_normalized(body.source.def_id());
         let local_decls = &body.local_decls;
-        for block in basic_blocks {
-            for statement in block.statements.iter_mut() {
-                if let StatementKind::Assign(box (place, _)) | StatementKind::Deinit(box place) =
-                    statement.kind
-                {
-                    let place_ty = place.ty(local_decls, tcx).ty;
-                    if !maybe_zst(place_ty) {
-                        continue;
-                    }
-                    let Ok(layout) = tcx.layout_of(param_env.and(place_ty)) else {
-                        continue;
-                    };
-                    if !layout.is_zst() {
-                        continue;
-                    }
-                    if tcx.consider_optimizing(|| {
-                        format!(
-                            "RemoveZsts - Place: {:?} SourceInfo: {:?}",
-                            place, statement.source_info
-                        )
-                    }) {
-                        statement.make_nop();
-                    }
-                }
-            }
+        let mut replacer = Replacer { tcx, param_env, local_decls };
+        for var_debug_info in &mut body.var_debug_info {
+            replacer.visit_var_debug_info(var_debug_info);
+        }
+        for (bb, data) in body.basic_blocks.as_mut_preserves_cfg().iter_enumerated_mut() {
+            replacer.visit_basic_block_data(bb, data);
         }
     }
 }
 
+struct Replacer<'a, 'tcx> {
+    tcx: TyCtxt<'tcx>,
+    param_env: ty::ParamEnv<'tcx>,
+    local_decls: &'a LocalDecls<'tcx>,
+}
+
 /// A cheap, approximate check to avoid unnecessary `layout_of` calls.
 fn maybe_zst(ty: Ty<'_>) -> bool {
     match ty.kind() {
@@ -63,3 +51,93 @@ fn maybe_zst(ty: Ty<'_>) -> bool {
         _ => false,
     }
 }
+
+impl<'tcx> Replacer<'_, 'tcx> {
+    fn known_to_be_zst(&self, ty: Ty<'tcx>) -> bool {
+        if !maybe_zst(ty) {
+            return false;
+        }
+        let Ok(layout) = self.tcx.layout_of(self.param_env.and(ty)) else {
+            return false;
+        };
+        layout.is_zst()
+    }
+
+    fn make_zst(&self, ty: Ty<'tcx>) -> Constant<'tcx> {
+        debug_assert!(self.known_to_be_zst(ty));
+        Constant {
+            span: rustc_span::DUMMY_SP,
+            user_ty: None,
+            literal: ConstantKind::Val(ConstValue::ZeroSized, ty),
+        }
+    }
+}
+
+impl<'tcx> MutVisitor<'tcx> for Replacer<'_, 'tcx> {
+    fn tcx(&self) -> TyCtxt<'tcx> {
+        self.tcx
+    }
+
+    fn visit_var_debug_info(&mut self, var_debug_info: &mut VarDebugInfo<'tcx>) {
+        match var_debug_info.value {
+            VarDebugInfoContents::Const(_) => {}
+            VarDebugInfoContents::Place(place) => {
+                let place_ty = place.ty(self.local_decls, self.tcx).ty;
+                if self.known_to_be_zst(place_ty) {
+                    var_debug_info.value = VarDebugInfoContents::Const(self.make_zst(place_ty))
+                }
+            }
+            VarDebugInfoContents::Composite { ty, fragments: _ } => {
+                if self.known_to_be_zst(ty) {
+                    var_debug_info.value = VarDebugInfoContents::Const(self.make_zst(ty))
+                }
+            }
+        }
+    }
+
+    fn visit_operand(&mut self, operand: &mut Operand<'tcx>, loc: Location) {
+        if let Operand::Constant(_) = operand {
+            return;
+        }
+        let op_ty = operand.ty(self.local_decls, self.tcx);
+        if self.known_to_be_zst(op_ty)
+            && self.tcx.consider_optimizing(|| {
+                format!("RemoveZsts - Operand: {:?} Location: {:?}", operand, loc)
+            })
+        {
+            *operand = Operand::Constant(Box::new(self.make_zst(op_ty)))
+        }
+    }
+
+    fn visit_statement(&mut self, statement: &mut Statement<'tcx>, loc: Location) {
+        let place_for_ty = match statement.kind {
+            StatementKind::Assign(box (place, ref rvalue)) => {
+                rvalue.is_safe_to_remove().then_some(place)
+            }
+            StatementKind::Deinit(box place)
+            | StatementKind::SetDiscriminant { box place, variant_index: _ }
+            | StatementKind::AscribeUserType(box (place, _), _)
+            | StatementKind::Retag(_, box place)
+            | StatementKind::PlaceMention(box place)
+            | StatementKind::FakeRead(box (_, place)) => Some(place),
+            StatementKind::StorageLive(local) | StatementKind::StorageDead(local) => {
+                Some(local.into())
+            }
+            StatementKind::Coverage(_)
+            | StatementKind::Intrinsic(_)
+            | StatementKind::Nop
+            | StatementKind::ConstEvalCounter => None,
+        };
+        if let Some(place_for_ty) = place_for_ty
+            && let ty = place_for_ty.ty(self.local_decls, self.tcx).ty
+            && self.known_to_be_zst(ty)
+            && self.tcx.consider_optimizing(|| {
+                format!("RemoveZsts - Place: {:?} SourceInfo: {:?}", place_for_ty, statement.source_info)
+            })
+        {
+            statement.make_nop();
+        } else {
+            self.super_statement(statement, loc);
+        }
+    }
+}