From dc639c944af8a47c5017317eebe2764087c8f776 Mon Sep 17 00:00:00 2001 From: Fabian Wolff Date: Wed, 14 Jul 2021 15:34:28 +0200 Subject: [PATCH] Warn about useless assignments of variables/fields to themselves --- compiler/rustc_passes/src/dead.rs | 54 +++++++++++++++++++ src/test/ui/issues/issue-3290.rs | 1 + src/test/ui/lint/dead-code/self-assign.rs | 50 +++++++++++++++++ src/test/ui/lint/dead-code/self-assign.stderr | 44 +++++++++++++++ .../ui/nll/issue-50461-used-mut-from-moves.rs | 1 + src/test/ui/self/self-re-assign.rs | 1 + 6 files changed, 151 insertions(+) create mode 100644 src/test/ui/lint/dead-code/self-assign.rs create mode 100644 src/test/ui/lint/dead-code/self-assign.stderr diff --git a/compiler/rustc_passes/src/dead.rs b/compiler/rustc_passes/src/dead.rs index bef56bbc287..713d572b93a 100644 --- a/compiler/rustc_passes/src/dead.rs +++ b/compiler/rustc_passes/src/dead.rs @@ -150,6 +150,59 @@ fn handle_assign(&mut self, expr: &'tcx hir::Expr<'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 @@ fn visit_expr(&mut self, expr: &'tcx hir::Expr<'tcx>) { } hir::ExprKind::Assign(ref left, ref right, ..) => { self.handle_assign(left); + self.check_for_self_assign(expr); self.visit_expr(right); return; } diff --git a/src/test/ui/issues/issue-3290.rs b/src/test/ui/issues/issue-3290.rs index 63e1b28a60a..840c7ff8355 100644 --- a/src/test/ui/issues/issue-3290.rs +++ b/src/test/ui/issues/issue-3290.rs @@ -1,5 +1,6 @@ // run-pass #![feature(box_syntax)] +#![allow(dead_code)] pub fn main() { let mut x: Box<_> = box 3; diff --git a/src/test/ui/lint/dead-code/self-assign.rs b/src/test/ui/lint/dead-code/self-assign.rs new file mode 100644 index 00000000000..b8bf7d860c4 --- /dev/null +++ b/src/test/ui/lint/dead-code/self-assign.rs @@ -0,0 +1,50 @@ +// Test that dead code warnings are issued for superfluous assignments of +// fields or variables to themselves (issue #75356). + +// check-pass +#![allow(unused_assignments)] +#![warn(dead_code)] + +fn main() { + let mut x = 0; + x = x; + //~^ WARNING: useless assignment of variable of type `i32` to itself + + x = (x); + //~^ WARNING: useless assignment of variable of type `i32` to itself + + x = {x}; + // block expressions don't count as self-assignments + + + struct S<'a> { f: &'a str } + let mut s = S { f: "abc" }; + s = s; + //~^ WARNING: useless assignment of variable of type `S` to itself + + s.f = s.f; + //~^ WARNING: useless assignment of field of type `&str` to itself + + + struct N0 { x: Box } + struct N1 { n: N0 } + struct N2(N1); + struct N3 { n: N2 }; + let mut n3 = N3 { n: N2(N1 { n: N0 { x: Box::new(42) } }) }; + n3.n.0.n.x = n3.n.0.n.x; + //~^ WARNING: useless assignment of field of type `Box` to itself + + let mut t = (1, ((2, 3, (4, 5)),)); + t.1.0.2.1 = t.1.0.2.1; + //~^ WARNING: useless assignment of field of type `i32` to itself + + + let mut y = 0; + macro_rules! assign_to_y { + ($cur:expr) => {{ + y = $cur; + }}; + } + assign_to_y!(y); + // self-assignments in macro expansions are not reported either +} diff --git a/src/test/ui/lint/dead-code/self-assign.stderr b/src/test/ui/lint/dead-code/self-assign.stderr new file mode 100644 index 00000000000..bb79c0ec72a --- /dev/null +++ b/src/test/ui/lint/dead-code/self-assign.stderr @@ -0,0 +1,44 @@ +warning: useless assignment of variable of type `i32` to itself + --> $DIR/self-assign.rs:10:5 + | +LL | x = x; + | ^^^^^ + | +note: the lint level is defined here + --> $DIR/self-assign.rs:6:9 + | +LL | #![warn(dead_code)] + | ^^^^^^^^^ + +warning: useless assignment of variable of type `i32` to itself + --> $DIR/self-assign.rs:13:5 + | +LL | x = (x); + | ^^^^^^^ + +warning: useless assignment of variable of type `S` to itself + --> $DIR/self-assign.rs:22:5 + | +LL | s = s; + | ^^^^^ + +warning: useless assignment of field of type `&str` to itself + --> $DIR/self-assign.rs:25:5 + | +LL | s.f = s.f; + | ^^^^^^^^^ + +warning: useless assignment of field of type `Box` to itself + --> $DIR/self-assign.rs:34:5 + | +LL | n3.n.0.n.x = n3.n.0.n.x; + | ^^^^^^^^^^^^^^^^^^^^^^^ + +warning: useless assignment of field of type `i32` to itself + --> $DIR/self-assign.rs:38:5 + | +LL | t.1.0.2.1 = t.1.0.2.1; + | ^^^^^^^^^^^^^^^^^^^^^ + +warning: 6 warnings emitted + diff --git a/src/test/ui/nll/issue-50461-used-mut-from-moves.rs b/src/test/ui/nll/issue-50461-used-mut-from-moves.rs index 69d7cdd83a6..2458b171e64 100644 --- a/src/test/ui/nll/issue-50461-used-mut-from-moves.rs +++ b/src/test/ui/nll/issue-50461-used-mut-from-moves.rs @@ -1,6 +1,7 @@ // run-pass #![deny(unused_mut)] +#![allow(dead_code)] struct Foo { pub value: i32 diff --git a/src/test/ui/self/self-re-assign.rs b/src/test/ui/self/self-re-assign.rs index a7b089ebff4..88e86146832 100644 --- a/src/test/ui/self/self-re-assign.rs +++ b/src/test/ui/self/self-re-assign.rs @@ -3,6 +3,7 @@ // that we do not glue_drop before we glue_take (#3290). #![feature(box_syntax)] +#![allow(dead_code)] use std::rc::Rc;