rustc_lint/if_let_rescope.rs
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use std::iter::repeat;
use std::ops::ControlFlow;
use hir::intravisit::Visitor;
use rustc_ast::Recovered;
use rustc_errors::{
Applicability, Diag, EmissionGuarantee, SubdiagMessageOp, Subdiagnostic, SuggestionStyle,
};
use rustc_hir::{self as hir, HirIdSet};
use rustc_macros::LintDiagnostic;
use rustc_middle::ty::TyCtxt;
use rustc_session::lint::{FutureIncompatibilityReason, Level};
use rustc_session::{declare_lint, impl_lint_pass};
use rustc_span::Span;
use rustc_span::edition::Edition;
use crate::{LateContext, LateLintPass};
declare_lint! {
/// The `if_let_rescope` lint detects cases where a temporary value with
/// significant drop is generated on the right hand side of `if let`
/// and suggests a rewrite into `match` when possible.
///
/// ### Example
///
/// ```rust,edition2021
/// #![cfg_attr(not(bootstrap), feature(if_let_rescope))] // Simplify this in bootstrap bump.
/// #![warn(if_let_rescope)]
/// #![allow(unused_variables)]
///
/// struct Droppy;
/// impl Drop for Droppy {
/// fn drop(&mut self) {
/// // Custom destructor, including this `drop` implementation, is considered
/// // significant.
/// // Rust does not check whether this destructor emits side-effects that can
/// // lead to observable change in program semantics, when the drop order changes.
/// // Rust biases to be on the safe side, so that you can apply discretion whether
/// // this change indeed breaches any contract or specification that your code needs
/// // to honour.
/// println!("dropped");
/// }
/// }
/// impl Droppy {
/// fn get(&self) -> Option<u8> {
/// None
/// }
/// }
///
/// fn main() {
/// if let Some(value) = Droppy.get() {
/// // do something
/// } else {
/// // do something else
/// }
/// }
/// ```
///
/// {{produces}}
///
/// ### Explanation
///
/// With Edition 2024, temporaries generated while evaluating `if let`s
/// will be dropped before the `else` block.
/// This lint captures a possible change in runtime behaviour due to
/// a change in sequence of calls to significant `Drop::drop` destructors.
///
/// A significant [`Drop::drop`](https://doc.rust-lang.org/std/ops/trait.Drop.html)
/// destructor here refers to an explicit, arbitrary implementation of the `Drop` trait on the type
/// with exceptions including `Vec`, `Box`, `Rc`, `BTreeMap` and `HashMap`
/// that are marked by the compiler otherwise so long that the generic types have
/// no significant destructor recursively.
/// In other words, a type has a significant drop destructor when it has a `Drop` implementation
/// or its destructor invokes a significant destructor on a type.
/// Since we cannot completely reason about the change by just inspecting the existence of
/// a significant destructor, this lint remains only a suggestion and is set to `allow` by default.
///
/// Whenever possible, a rewrite into an equivalent `match` expression that
/// observe the same order of calls to such destructors is proposed by this lint.
/// Authors may take their own discretion whether the rewrite suggestion shall be
/// accepted, or rejected to continue the use of the `if let` expression.
pub IF_LET_RESCOPE,
Allow,
"`if let` assigns a shorter lifetime to temporary values being pattern-matched against in Edition 2024 and \
rewriting in `match` is an option to preserve the semantics up to Edition 2021",
@future_incompatible = FutureIncompatibleInfo {
reason: FutureIncompatibilityReason::EditionSemanticsChange(Edition::Edition2024),
reference: "issue #124085 <https://github.com/rust-lang/rust/issues/124085>",
};
}
/// Lint for potential change in program semantics of `if let`s
#[derive(Default)]
pub(crate) struct IfLetRescope {
skip: HirIdSet,
}
fn expr_parent_is_else(tcx: TyCtxt<'_>, hir_id: hir::HirId) -> bool {
let Some((_, hir::Node::Expr(expr))) = tcx.hir().parent_iter(hir_id).next() else {
return false;
};
let hir::ExprKind::If(_cond, _conseq, Some(alt)) = expr.kind else { return false };
alt.hir_id == hir_id
}
fn expr_parent_is_stmt(tcx: TyCtxt<'_>, hir_id: hir::HirId) -> bool {
let Some((_, hir::Node::Stmt(stmt))) = tcx.hir().parent_iter(hir_id).next() else {
return false;
};
let (hir::StmtKind::Semi(expr) | hir::StmtKind::Expr(expr)) = stmt.kind else { return false };
expr.hir_id == hir_id
}
fn match_head_needs_bracket(tcx: TyCtxt<'_>, expr: &hir::Expr<'_>) -> bool {
expr_parent_is_else(tcx, expr.hir_id) && matches!(expr.kind, hir::ExprKind::If(..))
}
impl IfLetRescope {
fn probe_if_cascade<'tcx>(&mut self, cx: &LateContext<'tcx>, mut expr: &'tcx hir::Expr<'tcx>) {
if self.skip.contains(&expr.hir_id) {
return;
}
let tcx = cx.tcx;
let source_map = tcx.sess.source_map();
let expr_end = match expr.kind {
hir::ExprKind::If(_cond, conseq, None) => conseq.span.shrink_to_hi(),
hir::ExprKind::If(_cond, _conseq, Some(alt)) => alt.span.shrink_to_hi(),
_ => return,
};
let mut add_bracket_to_match_head = match_head_needs_bracket(tcx, expr);
let mut significant_droppers = vec![];
let mut lifetime_ends = vec![];
let mut closing_brackets = 0;
let mut alt_heads = vec![];
let mut match_heads = vec![];
let mut consequent_heads = vec![];
let mut first_if_to_lint = None;
let mut first_if_to_rewrite = false;
let mut empty_alt = false;
while let hir::ExprKind::If(cond, conseq, alt) = expr.kind {
self.skip.insert(expr.hir_id);
// We are interested in `let` fragment of the condition.
// Otherwise, we probe into the `else` fragment.
if let hir::ExprKind::Let(&hir::LetExpr {
span,
pat,
init,
ty: ty_ascription,
recovered: Recovered::No,
}) = cond.kind
{
// Peel off round braces
let if_let_pat = source_map
.span_take_while(expr.span, |&ch| ch == '(' || ch.is_whitespace())
.between(init.span);
// The consequent fragment is always a block.
let before_conseq = conseq.span.shrink_to_lo();
let lifetime_end = source_map.end_point(conseq.span);
if let ControlFlow::Break(significant_dropper) =
(FindSignificantDropper { cx }).visit_expr(init)
{
first_if_to_lint = first_if_to_lint.or_else(|| Some((span, expr.hir_id)));
significant_droppers.push(significant_dropper);
lifetime_ends.push(lifetime_end);
if ty_ascription.is_some()
|| !expr.span.can_be_used_for_suggestions()
|| !pat.span.can_be_used_for_suggestions()
|| !if_let_pat.can_be_used_for_suggestions()
|| !before_conseq.can_be_used_for_suggestions()
{
// Our `match` rewrites does not support type ascription,
// so we just bail.
// Alternatively when the span comes from proc macro expansion,
// we will also bail.
// FIXME(#101728): change this when type ascription syntax is stabilized again
} else if let Ok(pat) = source_map.span_to_snippet(pat.span) {
let emit_suggestion = |alt_span| {
first_if_to_rewrite = true;
if add_bracket_to_match_head {
closing_brackets += 2;
match_heads.push(SingleArmMatchBegin::WithOpenBracket(if_let_pat));
} else {
// Sometimes, wrapping `match` into a block is undesirable,
// because the scrutinee temporary lifetime is shortened and
// the proposed fix will not work.
closing_brackets += 1;
match_heads
.push(SingleArmMatchBegin::WithoutOpenBracket(if_let_pat));
}
consequent_heads.push(ConsequentRewrite { span: before_conseq, pat });
if let Some(alt_span) = alt_span {
alt_heads.push(AltHead(alt_span));
}
};
if let Some(alt) = alt {
let alt_head = conseq.span.between(alt.span);
if alt_head.can_be_used_for_suggestions() {
// We lint only when the `else` span is user code, too.
emit_suggestion(Some(alt_head));
}
} else {
// This is the end of the `if .. else ..` cascade.
// We can stop here.
emit_suggestion(None);
empty_alt = true;
break;
}
}
}
}
// At this point, any `if let` fragment in the cascade is definitely preceeded by `else`,
// so a opening bracket is mandatory before each `match`.
add_bracket_to_match_head = true;
if let Some(alt) = alt {
expr = alt;
} else {
break;
}
}
if let Some((span, hir_id)) = first_if_to_lint {
tcx.emit_node_span_lint(IF_LET_RESCOPE, hir_id, span, IfLetRescopeLint {
significant_droppers,
lifetime_ends,
rewrite: first_if_to_rewrite.then_some(IfLetRescopeRewrite {
match_heads,
consequent_heads,
closing_brackets: ClosingBrackets {
span: expr_end,
count: closing_brackets,
empty_alt,
},
alt_heads,
}),
});
}
}
}
impl_lint_pass!(
IfLetRescope => [IF_LET_RESCOPE]
);
impl<'tcx> LateLintPass<'tcx> for IfLetRescope {
fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx hir::Expr<'tcx>) {
if expr.span.edition().at_least_rust_2024() || !cx.tcx.features().if_let_rescope {
return;
}
if let (Level::Allow, _) = cx.tcx.lint_level_at_node(IF_LET_RESCOPE, expr.hir_id) {
return;
}
if let hir::ExprKind::Loop(block, _label, hir::LoopSource::While, _span) = expr.kind
&& let Some(value) = block.expr
&& let hir::ExprKind::If(cond, _conseq, _alt) = value.kind
&& let hir::ExprKind::Let(..) = cond.kind
{
// Recall that `while let` is lowered into this:
// ```
// loop {
// if let .. { body } else { break; }
// }
// ```
// There is no observable change in drop order on the overall `if let` expression
// given that the `{ break; }` block is trivial so the edition change
// means nothing substantial to this `while` statement.
self.skip.insert(value.hir_id);
return;
}
if expr_parent_is_stmt(cx.tcx, expr.hir_id)
&& matches!(expr.kind, hir::ExprKind::If(_cond, _conseq, None))
{
// `if let` statement without an `else` branch has no observable change
// so we can skip linting it
return;
}
self.probe_if_cascade(cx, expr);
}
}
#[derive(LintDiagnostic)]
#[diag(lint_if_let_rescope)]
struct IfLetRescopeLint {
#[label]
significant_droppers: Vec<Span>,
#[help]
lifetime_ends: Vec<Span>,
#[subdiagnostic]
rewrite: Option<IfLetRescopeRewrite>,
}
// #[derive(Subdiagnostic)]
struct IfLetRescopeRewrite {
match_heads: Vec<SingleArmMatchBegin>,
consequent_heads: Vec<ConsequentRewrite>,
closing_brackets: ClosingBrackets,
alt_heads: Vec<AltHead>,
}
impl Subdiagnostic for IfLetRescopeRewrite {
fn add_to_diag_with<G: EmissionGuarantee, F: SubdiagMessageOp<G>>(
self,
diag: &mut Diag<'_, G>,
f: &F,
) {
let mut suggestions = vec![];
for match_head in self.match_heads {
match match_head {
SingleArmMatchBegin::WithOpenBracket(span) => {
suggestions.push((span, "{ match ".into()))
}
SingleArmMatchBegin::WithoutOpenBracket(span) => {
suggestions.push((span, "match ".into()))
}
}
}
for ConsequentRewrite { span, pat } in self.consequent_heads {
suggestions.push((span, format!("{{ {pat} => ")));
}
for AltHead(span) in self.alt_heads {
suggestions.push((span, " _ => ".into()));
}
let closing_brackets = self.closing_brackets;
suggestions.push((
closing_brackets.span,
closing_brackets
.empty_alt
.then_some(" _ => {}".chars())
.into_iter()
.flatten()
.chain(repeat('}').take(closing_brackets.count))
.collect(),
));
let msg = f(diag, crate::fluent_generated::lint_suggestion);
diag.multipart_suggestion_with_style(
msg,
suggestions,
Applicability::MachineApplicable,
SuggestionStyle::ShowCode,
);
}
}
struct AltHead(Span);
struct ConsequentRewrite {
span: Span,
pat: String,
}
struct ClosingBrackets {
span: Span,
count: usize,
empty_alt: bool,
}
enum SingleArmMatchBegin {
WithOpenBracket(Span),
WithoutOpenBracket(Span),
}
struct FindSignificantDropper<'tcx, 'a> {
cx: &'a LateContext<'tcx>,
}
impl<'tcx, 'a> Visitor<'tcx> for FindSignificantDropper<'tcx, 'a> {
type Result = ControlFlow<Span>;
fn visit_expr(&mut self, expr: &'tcx hir::Expr<'tcx>) -> Self::Result {
if self
.cx
.typeck_results()
.expr_ty(expr)
.has_significant_drop(self.cx.tcx, self.cx.param_env)
{
return ControlFlow::Break(expr.span);
}
match expr.kind {
hir::ExprKind::ConstBlock(_)
| hir::ExprKind::Lit(_)
| hir::ExprKind::Path(_)
| hir::ExprKind::Assign(_, _, _)
| hir::ExprKind::AssignOp(_, _, _)
| hir::ExprKind::Break(_, _)
| hir::ExprKind::Continue(_)
| hir::ExprKind::Ret(_)
| hir::ExprKind::Become(_)
| hir::ExprKind::InlineAsm(_)
| hir::ExprKind::OffsetOf(_, _)
| hir::ExprKind::Repeat(_, _)
| hir::ExprKind::Err(_)
| hir::ExprKind::Struct(_, _, _)
| hir::ExprKind::Closure(_)
| hir::ExprKind::Block(_, _)
| hir::ExprKind::DropTemps(_)
| hir::ExprKind::Loop(_, _, _, _) => ControlFlow::Continue(()),
hir::ExprKind::Tup(exprs) | hir::ExprKind::Array(exprs) => {
for expr in exprs {
self.visit_expr(expr)?;
}
ControlFlow::Continue(())
}
hir::ExprKind::Call(callee, args) => {
self.visit_expr(callee)?;
for expr in args {
self.visit_expr(expr)?;
}
ControlFlow::Continue(())
}
hir::ExprKind::MethodCall(_, receiver, args, _) => {
self.visit_expr(receiver)?;
for expr in args {
self.visit_expr(expr)?;
}
ControlFlow::Continue(())
}
hir::ExprKind::Index(left, right, _) | hir::ExprKind::Binary(_, left, right) => {
self.visit_expr(left)?;
self.visit_expr(right)
}
hir::ExprKind::Unary(_, expr)
| hir::ExprKind::Cast(expr, _)
| hir::ExprKind::Type(expr, _)
| hir::ExprKind::Yield(expr, _)
| hir::ExprKind::AddrOf(_, _, expr)
| hir::ExprKind::Match(expr, _, _)
| hir::ExprKind::Field(expr, _)
| hir::ExprKind::Let(&hir::LetExpr {
init: expr,
span: _,
pat: _,
ty: _,
recovered: Recovered::No,
}) => self.visit_expr(expr),
hir::ExprKind::Let(_) => ControlFlow::Continue(()),
hir::ExprKind::If(cond, _, _) => {
if let hir::ExprKind::Let(hir::LetExpr {
init,
span: _,
pat: _,
ty: _,
recovered: Recovered::No,
}) = cond.kind
{
self.visit_expr(init)?;
}
ControlFlow::Continue(())
}
}
}
}