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Auto merge of #107251 - dingxiangfei2009:let-chain-rescope, r=jieyouxu

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Rescope temp lifetime in if-let into IfElse with migration lint

Tracking issue #124085

This PR shortens the temporary lifetime to cover only the pattern matching and consequent branch of a `if let`.

At the expression location, means that the lifetime is shortened from previously the deepest enclosing block or statement in Edition 2021. This warrants an Edition change.

Coming with the Edition change, this patch also implements an edition lint to warn about the change and a safe rewrite suggestion to preserve the 2021 semantics in most cases.

Related to #103108.
Related crater runs: https://github.com/rust-lang/rust/pull/129466.
This commit is contained in:
bors 2024-09-13 03:47:30 +00:00
commit a5efa01895
29 changed files with 1393 additions and 35 deletions
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430
compiler/rustc_lint/src/if_let_rescope.rs Normal file
View file

@ -0,0 +1,430 @@
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::edition::Edition;
use rustc_span::Span;
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
/// #![feature(if_let_rescope)]
/// #![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 = expr.span.shrink_to_hi();
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
{
let if_let_pat = expr.span.shrink_to_lo().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()
{
// 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 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.into());
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(())
}
}
}
}

3
compiler/rustc_lint/src/lib.rs
View file

@ -56,6 +56,7 @@ mod expect;
mod for_loops_over_fallibles;
mod foreign_modules;
pub mod hidden_unicode_codepoints;
mod if_let_rescope;
mod impl_trait_overcaptures;
mod internal;
mod invalid_from_utf8;
@ -94,6 +95,7 @@ use drop_forget_useless::*;
use enum_intrinsics_non_enums::EnumIntrinsicsNonEnums;
use for_loops_over_fallibles::*;
use hidden_unicode_codepoints::*;
use if_let_rescope::IfLetRescope;
use impl_trait_overcaptures::ImplTraitOvercaptures;
use internal::*;
use invalid_from_utf8::*;
@ -243,6 +245,7 @@ late_lint_methods!(
NonLocalDefinitions: NonLocalDefinitions::default(),
ImplTraitOvercaptures: ImplTraitOvercaptures,
TailExprDropOrder: TailExprDropOrder,
IfLetRescope: IfLetRescope::default(),
]
]
);