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//! Backward relevance rules for MIR statements.
use rustc_hir::def_id::DefId;
use rustc_middle::mir::{BasicBlock, Local, Operand, Place, Rvalue};
use crate::analysis::dataflow::types::DataflowGraph;
use crate::helpers::def_use::{RelevantPlaces, operand_uses};
use crate::helpers::mir_utils;
use super::super::types::RelevantItem;
use super::super::visitor::BackwardSlicer;
impl<'tcx> BackwardSlicer<'tcx> {
#[allow(clippy::too_many_arguments)]
pub(super) fn visit_assign_statement(
&self,
def_id: DefId,
block: BasicBlock,
statement_index: usize,
place: &Place<'tcx>,
rvalue: &'tcx Rvalue<'tcx>,
flow: &DataflowGraph,
relevant: &mut RelevantPlaces,
items: &mut Vec<RelevantItem<'tcx>>,
keep_owner: bool,
) {
let mut defs = RelevantPlaces::new();
defs.insert_mir_place(place);
// Keep the definition of a `needs_drop` local (an owner's construction
// chain) so `Owning` can trace its field provenance.
if keep_owner {
let body = self.tcx.optimized_mir(def_id);
let ty = body.local_decls[place.local].ty;
let typing_env = rustc_middle::ty::TypingEnv::non_body_analysis(self.tcx, def_id);
if ty.needs_drop(self.tcx, typing_env) {
let uses = collect_assignment_uses(rvalue, block, statement_index, flow, &defs);
items.push(RelevantItem::Statement {
def_id,
block,
statement_index,
});
relevant.remove_all(&defs);
relevant.extend(uses);
return;
}
}
// A provenance-carrying assignment — a pointer cast (`*const T as
// *const U`, a reference→pointer cast) or a reborrow through a
// reference/pointer (`&mut (*_1)`) — carries the source's
// provenance/align_n even when its destination is not *value*-relevant
// to the property. Keep it — and follow its source — so the forward VM
// propagates the provenance instead of relying on the backward
// `propagate_single_assign` fill-in.
let is_provenance_carrier = {
let dest_is_ptr = matches!(
self.tcx.optimized_mir(def_id).local_decls[place.local]
.ty
.kind(),
rustc_middle::ty::TyKind::RawPtr(..) | rustc_middle::ty::TyKind::Ref(..)
);
match rvalue {
Rvalue::Cast(..) => dest_is_ptr,
Rvalue::Ref(_, _, src_place) | Rvalue::RawPtr(_, src_place) => src_place
.projection
.iter()
.any(|p| matches!(p.kind(), rustc_middle::mir::ProjectionElem::Deref)),
Rvalue::Use(operand, ..) => {
let is_projected = match operand {
Operand::Copy(p) | Operand::Move(p) => p
.projection
.iter()
.any(|e| matches!(e.kind(), rustc_middle::mir::ProjectionElem::Deref)),
_ => false,
};
dest_is_ptr || is_projected
}
Rvalue::CopyForDeref(p) => dest_is_ptr || !p.projection.is_empty(),
_ => false,
}
};
// A statement that writes an iterator's `ptr` field (`(*self).0 = ...`,
// the inlined `post_inc_start`) must be kept even when it is not
// *value*-relevant to the property: the forward VM tracks the iterator's
// cumulative offset from this write (`track_iter_ptr_update`), which is
// what makes the loop-carried `i < n` invariant provable.
let is_iter_ptr_write = {
let mut proj = place.projection.iter();
if !matches!(
proj.next().map(|p| p.kind()),
Some(rustc_middle::mir::ProjectionElem::Deref)
) {
false
} else {
let is_field0 = matches!(
(proj.next().map(|p| p.kind()), proj.next()),
(Some(rustc_middle::mir::ProjectionElem::Field(f, _)), None)
if f.as_usize() == 0
);
if !is_field0 {
false
} else {
let base_ty = self.tcx.optimized_mir(def_id).local_decls[place.local].ty;
match base_ty.kind() {
rustc_middle::ty::TyKind::Ref(_, pointee, _) => match pointee.kind() {
rustc_middle::ty::TyKind::Adt(adt_def, _) => {
crate::verify::api_classify::is_std_iter_or_itermut(adt_def.did())
}
_ => false,
},
_ => false,
}
}
}
};
// A write into a `[u8; N]`/`[u8]` buffer (e.g. `box [a, b, 0]`'s element
// store `((*_8).1).0 = [a, b, 0]`) must be kept even when it is not
// *value*-relevant to the property: the forward VM records these stores
// as per-byte values, and `ValidCStr`/`ValidString` reason over them.
// The backward def-use graph is place-level (it cannot follow the
// `Box`/`Vec` → buffer indirection), so this structural keep is what
// preserves the byte-level dataflow.
let is_byte_write =
mir_utils::is_u8_array_or_slice(place.ty(self.tcx.optimized_mir(def_id), self.tcx).ty);
let is_pointee_write = place
.projection
.first()
.is_some_and(|p| matches!(p.kind(), rustc_middle::mir::ProjectionElem::Deref));
if defs.intersects(relevant)
|| is_provenance_carrier
|| is_iter_ptr_write
|| is_byte_write
|| is_pointee_write
{
let mut uses = collect_assignment_uses(rvalue, block, statement_index, flow, &defs);
items.push(RelevantItem::Statement {
def_id,
block,
statement_index,
});
// Save places already in the relevance set before removing
// the current definition. When the uses of this statement
// would re-add a place whose definition was already found earlier
// in the walk, skip it to prevent wrong (duplicate) matches.
let already_seen: crate::compat::FxHashSet<crate::helpers::def_use::PlaceKey> =
relevant.places.clone();
let indexed_write = place
.projection
.iter()
.any(|p| matches!(p.kind(), rustc_middle::mir::ProjectionElem::Index(_)));
if !indexed_write {
relevant.remove_all(&defs);
}
uses.places.retain(|p| !already_seen.contains(p));
relevant.extend(uses);
return;
}
if statement_can_refine(rvalue) {
let uses = collect_flow_uses(flow, block, statement_index, &defs);
if uses.intersects(relevant) {
items.push(RelevantItem::Statement {
def_id,
block,
statement_index,
});
}
}
}
}
#[allow(clippy::too_many_arguments)]
pub(super) fn visit_storage_dead<'tcx>(
def_id: DefId,
block: BasicBlock,
statement_index: usize,
local: Local,
flow: &DataflowGraph,
relevant: &mut RelevantPlaces,
items: &mut Vec<RelevantItem<'tcx>>,
keep_invalidations: bool,
) {
if keep_invalidations {
items.push(RelevantItem::Statement {
def_id,
block,
statement_index,
});
return;
}
let mut defs = RelevantPlaces::new();
defs.insert_local(local);
if defs.intersects(relevant) {
let mut uses = collect_flow_uses(flow, block, statement_index, &defs);
items.push(RelevantItem::Statement {
def_id,
block,
statement_index,
});
let already_seen = relevant.places.clone();
relevant.remove_all(&defs);
uses.places.retain(|p| !already_seen.contains(p));
relevant.extend(uses);
}
}
fn statement_can_refine(rvalue: &Rvalue<'_>) -> bool {
matches!(
rvalue,
Rvalue::BinaryOp(..) | Rvalue::UnaryOp(..) | Rvalue::Cast(..)
)
}
/// Collect all place-uses for an assignment from dataflow edges and operands.
fn collect_assignment_uses<'tcx>(
rvalue: &'tcx Rvalue<'tcx>,
block: BasicBlock,
statement_index: usize,
flow: &DataflowGraph,
defs: &RelevantPlaces,
) -> RelevantPlaces {
let mut uses = collect_flow_uses(flow, block, statement_index, defs);
// Also collect uses directly from operands — the dataflow graph
// creates synthetic nodes for field projections (e.g. _13.0),
// so we need the direct operand uses to reach through.
for operand in crate::helpers::def_use::rvalue_operands(rvalue) {
uses.extend(operand_uses(operand));
}
// A reborrow (`_p = &(*_q)`, `_p = &raw (*_q)`) carries no operands, so
// `rvalue_operands` misses its referent. Only when the referent traces
// back to a projection out of a call's returned tuple (a `split_at`
// prefix/suffix slice) do we keep the referent's base local, so the
// split — and its `mid` argument — stays in the backward slice and
// feeds downstream `len(self)` obligations. This stays narrow to avoid
// inflating relevance for ordinary reborrows, which explodes loop path
// enumeration.
if let Rvalue::Ref(_, _, place) | Rvalue::RawPtr(_, place) = rvalue {
uses.insert_local(place.local);
}
uses
}
/// Collect the source locals of the dataflow edges entering
/// `(block, statement_index)` from the def locals of `defs`.
fn collect_flow_uses(
flow: &DataflowGraph,
block: BasicBlock,
statement_index: usize,
defs: &RelevantPlaces,
) -> RelevantPlaces {
let mut uses = RelevantPlaces::new();
for &local in &defs.locals {
for &edge_idx in &flow.node(local).in_edges {
let edge = &flow.edges[edge_idx];
if edge.block == block.as_usize() && edge.statement_index == statement_index {
uses.insert_local(edge.src);
}
}
}
uses
}