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Auto merge of #12172 - samueltardieu:issue-12166, r=Alexendoo
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no_effect_underscore_binding: _ prefixed variables can be used

Prefixing a variable with a `_` does not mean that it will not be used. If such a variable is used later, do not warn about the fact that its initialization does not have a side effect as this is fine.

changelog: [`no_effect_underscore_binding`]: warn only if variable is unused

Fix #12166
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@bors
bors committed Jan 21, 2024
2 parents fe3e682 + 6267b6c commit 7386856
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Showing 3 changed files with 109 additions and 75 deletions.
2 changes: 1 addition & 1 deletion clippy_lints/src/lib.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -719,7 +719,7 @@ pub fn register_lints(store: &mut rustc_lint::LintStore, conf: &'static Conf) {
store.register_late_pass(|_| Box::new(needless_update::NeedlessUpdate));
store.register_late_pass(|_| Box::new(needless_borrowed_ref::NeedlessBorrowedRef));
store.register_late_pass(|_| Box::new(borrow_deref_ref::BorrowDerefRef));
store.register_late_pass(|_| Box::new(no_effect::NoEffect));
store.register_late_pass(|_| Box::<no_effect::NoEffect>::default());
store.register_late_pass(|_| Box::new(temporary_assignment::TemporaryAssignment));
store.register_late_pass(move |_| Box::new(transmute::Transmute::new(msrv())));
store.register_late_pass(move |_| {
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180 changes: 106 additions & 74 deletions clippy_lints/src/no_effect.rs
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Original file line number Diff line number Diff line change
@@ -1,16 +1,18 @@
use clippy_utils::diagnostics::{span_lint_hir, span_lint_hir_and_then};
use clippy_utils::source::snippet_opt;
use clippy_utils::ty::has_drop;
use clippy_utils::{any_parent_is_automatically_derived, get_parent_node, is_lint_allowed, peel_blocks};
use clippy_utils::{any_parent_is_automatically_derived, get_parent_node, is_lint_allowed, path_to_local, peel_blocks};
use rustc_errors::Applicability;
use rustc_hir::def::{DefKind, Res};
use rustc_hir::{
is_range_literal, BinOpKind, BlockCheckMode, Expr, ExprKind, ItemKind, Node, PatKind, Stmt, StmtKind, UnsafeSource,
is_range_literal, BinOpKind, BlockCheckMode, Expr, ExprKind, HirId, HirIdMap, ItemKind, Node, PatKind, Stmt,
StmtKind, UnsafeSource,
};
use rustc_infer::infer::TyCtxtInferExt as _;
use rustc_lint::{LateContext, LateLintPass, LintContext};
use rustc_middle::lint::in_external_macro;
use rustc_session::declare_lint_pass;
use rustc_session::impl_lint_pass;
use rustc_span::Span;
use std::ops::Deref;

declare_clippy_lint! {
Expand Down Expand Up @@ -74,95 +76,125 @@ declare_clippy_lint! {
"outer expressions with no effect"
}

declare_lint_pass!(NoEffect => [NO_EFFECT, UNNECESSARY_OPERATION, NO_EFFECT_UNDERSCORE_BINDING]);
#[derive(Default)]
pub struct NoEffect {
underscore_bindings: HirIdMap<Span>,
local_bindings: Vec<Vec<HirId>>,
}

impl_lint_pass!(NoEffect => [NO_EFFECT, UNNECESSARY_OPERATION, NO_EFFECT_UNDERSCORE_BINDING]);

impl<'tcx> LateLintPass<'tcx> for NoEffect {
fn check_stmt(&mut self, cx: &LateContext<'tcx>, stmt: &'tcx Stmt<'_>) {
if check_no_effect(cx, stmt) {
if self.check_no_effect(cx, stmt) {
return;
}
check_unnecessary_operation(cx, stmt);
}
}

fn check_no_effect(cx: &LateContext<'_>, stmt: &Stmt<'_>) -> bool {
if let StmtKind::Semi(expr) = stmt.kind {
// move `expr.span.from_expansion()` ahead
if expr.span.from_expansion() {
return false;
fn check_block(&mut self, _: &LateContext<'tcx>, _: &'tcx rustc_hir::Block<'tcx>) {
self.local_bindings.push(Vec::default());
}

fn check_block_post(&mut self, cx: &LateContext<'tcx>, _: &'tcx rustc_hir::Block<'tcx>) {
for hir_id in self.local_bindings.pop().unwrap() {
if let Some(span) = self.underscore_bindings.remove(&hir_id) {
span_lint_hir(
cx,
NO_EFFECT_UNDERSCORE_BINDING,
hir_id,
span,
"binding to `_` prefixed variable with no side-effect",
);
}
}
let expr = peel_blocks(expr);
}

if is_operator_overridden(cx, expr) {
// Return `true`, to prevent `check_unnecessary_operation` from
// linting on this statement as well.
return true;
fn check_expr(&mut self, _: &LateContext<'tcx>, expr: &'tcx rustc_hir::Expr<'tcx>) {
if let Some(def_id) = path_to_local(expr) {
self.underscore_bindings.remove(&def_id);
}
if has_no_effect(cx, expr) {
span_lint_hir_and_then(
cx,
NO_EFFECT,
expr.hir_id,
stmt.span,
"statement with no effect",
|diag| {
for parent in cx.tcx.hir().parent_iter(stmt.hir_id) {
if let Node::Item(item) = parent.1
&& let ItemKind::Fn(..) = item.kind
&& let Some(Node::Block(block)) = get_parent_node(cx.tcx, stmt.hir_id)
&& let [.., final_stmt] = block.stmts
&& final_stmt.hir_id == stmt.hir_id
{
let expr_ty = cx.typeck_results().expr_ty(expr);
let mut ret_ty = cx
.tcx
.fn_sig(item.owner_id)
.instantiate_identity()
.output()
.skip_binder();
}
}

// Remove `impl Future<Output = T>` to get `T`
if cx.tcx.ty_is_opaque_future(ret_ty)
&& let Some(true_ret_ty) = cx.tcx.infer_ctxt().build().get_impl_future_output_ty(ret_ty)
impl NoEffect {
fn check_no_effect(&mut self, cx: &LateContext<'_>, stmt: &Stmt<'_>) -> bool {
if let StmtKind::Semi(expr) = stmt.kind {
// move `expr.span.from_expansion()` ahead
if expr.span.from_expansion() {
return false;
}
let expr = peel_blocks(expr);

if is_operator_overridden(cx, expr) {
// Return `true`, to prevent `check_unnecessary_operation` from
// linting on this statement as well.
return true;
}
if has_no_effect(cx, expr) {
span_lint_hir_and_then(
cx,
NO_EFFECT,
expr.hir_id,
stmt.span,
"statement with no effect",
|diag| {
for parent in cx.tcx.hir().parent_iter(stmt.hir_id) {
if let Node::Item(item) = parent.1
&& let ItemKind::Fn(..) = item.kind
&& let Some(Node::Block(block)) = get_parent_node(cx.tcx, stmt.hir_id)
&& let [.., final_stmt] = block.stmts
&& final_stmt.hir_id == stmt.hir_id
{
ret_ty = true_ret_ty;
}
let expr_ty = cx.typeck_results().expr_ty(expr);
let mut ret_ty = cx
.tcx
.fn_sig(item.owner_id)
.instantiate_identity()
.output()
.skip_binder();

// Remove `impl Future<Output = T>` to get `T`
if cx.tcx.ty_is_opaque_future(ret_ty)
&& let Some(true_ret_ty) =
cx.tcx.infer_ctxt().build().get_impl_future_output_ty(ret_ty)
{
ret_ty = true_ret_ty;
}

if !ret_ty.is_unit() && ret_ty == expr_ty {
diag.span_suggestion(
stmt.span.shrink_to_lo(),
"did you mean to return it?",
"return ",
Applicability::MaybeIncorrect,
);
if !ret_ty.is_unit() && ret_ty == expr_ty {
diag.span_suggestion(
stmt.span.shrink_to_lo(),
"did you mean to return it?",
"return ",
Applicability::MaybeIncorrect,
);
}
}
}
}
},
);
return true;
}
} else if let StmtKind::Local(local) = stmt.kind {
if !is_lint_allowed(cx, NO_EFFECT_UNDERSCORE_BINDING, local.hir_id)
&& let Some(init) = local.init
&& local.els.is_none()
&& !local.pat.span.from_expansion()
&& has_no_effect(cx, init)
&& let PatKind::Binding(_, _, ident, _) = local.pat.kind
&& ident.name.to_ident_string().starts_with('_')
&& !any_parent_is_automatically_derived(cx.tcx, local.hir_id)
{
span_lint_hir(
cx,
NO_EFFECT_UNDERSCORE_BINDING,
init.hir_id,
ident.span,
"binding to `_` prefixed variable with no side-effect",
);
return true;
},
);
return true;
}
} else if let StmtKind::Local(local) = stmt.kind {
if !is_lint_allowed(cx, NO_EFFECT_UNDERSCORE_BINDING, local.hir_id)
&& let Some(init) = local.init
&& local.els.is_none()
&& !local.pat.span.from_expansion()
&& has_no_effect(cx, init)
&& let PatKind::Binding(_, hir_id, ident, _) = local.pat.kind
&& ident.name.to_ident_string().starts_with('_')
&& !any_parent_is_automatically_derived(cx.tcx, local.hir_id)
{
if let Some(l) = self.local_bindings.last_mut() {
l.push(hir_id);
self.underscore_bindings.insert(hir_id, ident.span);
}
return true;
}
}
false
}
false
}

fn is_operator_overridden(cx: &LateContext<'_>, expr: &Expr<'_>) -> bool {
Expand Down
2 changes: 2 additions & 0 deletions tests/ui/no_effect.rs
View file
Original file line number Diff line number Diff line change
Expand Up @@ -181,6 +181,8 @@ fn main() {
//~^ ERROR: binding to `_` prefixed variable with no side-effect
let _cat = [2, 4, 6, 8][2];
//~^ ERROR: binding to `_` prefixed variable with no side-effect
let _issue_12166 = 42;
let underscore_variable_above_can_be_used_dont_lint = _issue_12166;

#[allow(clippy::no_effect)]
0;
Expand Down

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