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Auto merge of #87129 - FabianWolff:issue-75356, r=varkor

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Warn about useless assignments of variables/fields to themselves

This PR fixes #75356. Following `@varkor's` suggestion in https://github.com/rust-lang/rust/issues/75356#issuecomment-700339154, I have implemented this warning as part of the `dead_code` lint. Unlike the `-Wself-assign` implementation in [Clang](56e6d4742e/clang/lib/Sema/SemaExpr.cpp (L13875-L13909)), my implementation also warns about self-assignments of struct fields (`s.x = s.x`).

r? `@varkor`
This commit is contained in:
bors 2021-07-17 22:51:07 +00:00
commit eb0b95b55a
6 changed files with 151 additions and 0 deletions
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54
compiler/rustc_passes/src/dead.rs
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@ -150,6 +150,59 @@ impl<'tcx> MarkSymbolVisitor<'tcx> {
}
}
fn check_for_self_assign(&mut self, assign: &'tcx hir::Expr<'tcx>) {
fn check_for_self_assign_helper(
tcx: TyCtxt<'tcx>,
typeck_results: &'tcx ty::TypeckResults<'tcx>,
lhs: &'tcx hir::Expr<'tcx>,
rhs: &'tcx hir::Expr<'tcx>,
) -> bool {
match (&lhs.kind, &rhs.kind) {
(hir::ExprKind::Path(ref qpath_l), hir::ExprKind::Path(ref qpath_r)) => {
if let (Res::Local(id_l), Res::Local(id_r)) = (
typeck_results.qpath_res(qpath_l, lhs.hir_id),
typeck_results.qpath_res(qpath_r, rhs.hir_id),
) {
if id_l == id_r {
return true;
}
}
return false;
}
(hir::ExprKind::Field(lhs_l, ident_l), hir::ExprKind::Field(lhs_r, ident_r)) => {
if ident_l == ident_r {
return check_for_self_assign_helper(tcx, typeck_results, lhs_l, lhs_r);
}
return false;
}
_ => {
return false;
}
}
}
if let hir::ExprKind::Assign(lhs, rhs, _) = assign.kind {
if check_for_self_assign_helper(self.tcx, self.typeck_results(), lhs, rhs)
&& !assign.span.from_expansion()
{
let is_field_assign = matches!(lhs.kind, hir::ExprKind::Field(..));
self.tcx.struct_span_lint_hir(
lint::builtin::DEAD_CODE,
assign.hir_id,
assign.span,
|lint| {
lint.build(&format!(
"useless assignment of {} of type `{}` to itself",
if is_field_assign { "field" } else { "variable" },
self.typeck_results().expr_ty(lhs),
))
.emit();
},
)
}
}
}
fn handle_field_pattern_match(
&mut self,
lhs: &hir::Pat<'_>,
@ -287,6 +340,7 @@ impl<'tcx> Visitor<'tcx> for MarkSymbolVisitor<'tcx> {
}
hir::ExprKind::Assign(ref left, ref right, ..) => {
self.handle_assign(left);
self.check_for_self_assign(expr);
self.visit_expr(right);
return;
}