Rollup merge of #146566 - cjgillot:mir-overlap-lint, r=saethlin

Lint more overlapping assignments in MIR.

In an effort to make bugs like https://github.com/rust-lang/rust/issues/146383 more easily discovered, this PR extends the "overlapping assignment" MIR lint.

I had to whitelist some rvalues, as they are actually allowed to alias, like `a = a + 1`.

2 files changed, 35 insertions, 27 deletions

diff --git a/compiler/rustc_mir_transform/src/dest_prop.rs b/compiler/rustc_mir_transform/src/dest_prop.rs
index abd1cd4e35a..74c22ff10c1 100644
--- a/compiler/rustc_mir_transform/src/dest_prop.rs
+++ b/compiler/rustc_mir_transform/src/dest_prop.rs
@@ -141,7 +141,7 @@ use rustc_data_structures::union_find::UnionFind;
 use rustc_index::bit_set::DenseBitSet;
 use rustc_index::interval::SparseIntervalMatrix;
 use rustc_index::{IndexVec, newtype_index};
-use rustc_middle::mir::visit::{MutVisitor, PlaceContext, Visitor};
+use rustc_middle::mir::visit::{MutVisitor, PlaceContext, VisitPlacesWith, Visitor};
 use rustc_middle::mir::*;
 use rustc_middle::ty::TyCtxt;
 use rustc_mir_dataflow::impls::{DefUse, MaybeLiveLocals};
@@ -503,22 +503,6 @@ impl TwoStepIndex {
     }
 }
 
-struct VisitPlacesWith<F>(F);
-
-impl<'tcx, F> Visitor<'tcx> for VisitPlacesWith<F>
-where
-    F: FnMut(Place<'tcx>, PlaceContext),
-{
-    fn visit_local(&mut self, local: Local, ctxt: PlaceContext, _: Location) {
-        (self.0)(local.into(), ctxt);
-    }
-
-    fn visit_place(&mut self, place: &Place<'tcx>, ctxt: PlaceContext, location: Location) {
-        (self.0)(*place, ctxt);
-        self.visit_projection(place.as_ref(), ctxt, location);
-    }
-}
-
 /// Add points depending on the result of the given dataflow analysis.
 fn save_as_intervals<'tcx>(
     elements: &DenseLocationMap,
diff --git a/compiler/rustc_mir_transform/src/lint.rs b/compiler/rustc_mir_transform/src/lint.rs
index f472c7cb493..2ab49645dc4 100644
--- a/compiler/rustc_mir_transform/src/lint.rs
+++ b/compiler/rustc_mir_transform/src/lint.rs
@@ -6,7 +6,7 @@ use std::borrow::Cow;
 
 use rustc_data_structures::fx::FxHashSet;
 use rustc_index::bit_set::DenseBitSet;
-use rustc_middle::mir::visit::{PlaceContext, Visitor};
+use rustc_middle::mir::visit::{PlaceContext, VisitPlacesWith, Visitor};
 use rustc_middle::mir::*;
 use rustc_middle::ty::TyCtxt;
 use rustc_mir_dataflow::impls::{MaybeStorageDead, MaybeStorageLive, always_storage_live_locals};
@@ -79,15 +79,39 @@ impl<'a, 'tcx> Visitor<'tcx> for Lint<'a, 'tcx> {
     fn visit_statement(&mut self, statement: &Statement<'tcx>, location: Location) {
         match &statement.kind {
             StatementKind::Assign(box (dest, rvalue)) => {
-                if let Rvalue::Use(Operand::Copy(src) | Operand::Move(src)) = rvalue {
-                    // The sides of an assignment must not alias. Currently this just checks whether
-                    // the places are identical.
-                    if dest == src {
-                        self.fail(
-                            location,
-                            "encountered `Assign` statement with overlapping memory",
-                        );
-                    }
+                let forbid_aliasing = match rvalue {
+                    Rvalue::Use(..)
+                    | Rvalue::CopyForDeref(..)
+                    | Rvalue::Repeat(..)
+                    | Rvalue::Aggregate(..)
+                    | Rvalue::Cast(..)
+                    | Rvalue::ShallowInitBox(..)
+                    | Rvalue::WrapUnsafeBinder(..) => true,
+                    Rvalue::ThreadLocalRef(..)
+                    | Rvalue::NullaryOp(..)
+                    | Rvalue::UnaryOp(..)
+                    | Rvalue::BinaryOp(..)
+                    | Rvalue::Ref(..)
+                    | Rvalue::RawPtr(..)
+                    | Rvalue::Discriminant(..) => false,
+                };
+                // The sides of an assignment must not alias.
+                if forbid_aliasing {
+                    VisitPlacesWith(|src: Place<'tcx>, _| {
+                        if *dest == src
+                            || (dest.local == src.local
+                                && !dest.is_indirect()
+                                && !src.is_indirect())
+                        {
+                            self.fail(
+                                location,
+                                format!(
+                                    "encountered `{statement:?}` statement with overlapping memory"
+                                ),
+                            );
+                        }
+                    })
+                    .visit_rvalue(rvalue, location);
                 }
             }
             StatementKind::StorageLive(local) => {