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manual_let_else: let/else is not divergent by default

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The divergent `else` block of a `let`/`else` statement does not make
the `let`/`else` statement itself divergent.
This commit is contained in:
Samuel Tardieu 2023-02-12 09:18:55 +01:00
parent 0e40f94a86
commit 7f15a11aa1
2 changed files with 105 additions and 52 deletions
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146
clippy_lints/src/manual_let_else.rs
View file

@ -4,11 +4,12 @@ use clippy_utils::msrvs::{self, Msrv};
use clippy_utils::peel_blocks;
use clippy_utils::source::snippet_with_context;
use clippy_utils::ty::is_type_diagnostic_item;
use clippy_utils::visitors::{for_each_expr, Descend};
use clippy_utils::visitors::{Descend, Visitable};
use if_chain::if_chain;
use rustc_data_structures::fx::FxHashSet;
use rustc_errors::Applicability;
use rustc_hir::{Expr, ExprKind, MatchSource, Pat, PatKind, QPath, Stmt, StmtKind};
use rustc_hir::intravisit::{walk_expr, Visitor};
use rustc_hir::{Expr, ExprKind, HirId, ItemId, Local, MatchSource, Pat, PatKind, QPath, Stmt, StmtKind, Ty};
use rustc_lint::{LateContext, LateLintPass, LintContext};
use rustc_middle::lint::in_external_macro;
use rustc_session::{declare_tool_lint, impl_lint_pass};
@ -162,61 +163,102 @@ fn emit_manual_let_else(cx: &LateContext<'_>, span: Span, expr: &Expr<'_>, pat:
);
}
fn expr_diverges(cx: &LateContext<'_>, expr: &'_ Expr<'_>) -> bool {
fn is_never(cx: &LateContext<'_>, expr: &'_ Expr<'_>) -> bool {
if let Some(ty) = cx.typeck_results().expr_ty_opt(expr) {
return ty.is_never();
}
false
/// Check whether an expression is divergent. May give false negatives.
fn expr_diverges(cx: &LateContext<'_>, expr: &Expr<'_>) -> bool {
struct V<'cx, 'tcx> {
cx: &'cx LateContext<'tcx>,
res: ControlFlow<(), Descend>,
}
// We can't just call is_never on expr and be done, because the type system
// sometimes coerces the ! type to something different before we can get
// our hands on it. So instead, we do a manual search. We do fall back to
// is_never in some places when there is no better alternative.
for_each_expr(expr, |ex| {
match ex.kind {
ExprKind::Continue(_) | ExprKind::Break(_, _) | ExprKind::Ret(_) => ControlFlow::Break(()),
ExprKind::Call(call, _) => {
if is_never(cx, ex) || is_never(cx, call) {
return ControlFlow::Break(());
impl<'tcx> Visitor<'tcx> for V<'_, '_> {
fn visit_expr(&mut self, e: &'tcx Expr<'tcx>) {
fn is_never(cx: &LateContext<'_>, expr: &'_ Expr<'_>) -> bool {
if let Some(ty) = cx.typeck_results().expr_ty_opt(expr) {
return ty.is_never();
}
ControlFlow::Continue(Descend::Yes)
},
ExprKind::MethodCall(..) => {
if is_never(cx, ex) {
return ControlFlow::Break(());
}
ControlFlow::Continue(Descend::Yes)
},
ExprKind::If(if_expr, if_then, if_else) => {
let else_diverges = if_else.map_or(false, |ex| expr_diverges(cx, ex));
let diverges = expr_diverges(cx, if_expr) || (else_diverges && expr_diverges(cx, if_then));
if diverges {
return ControlFlow::Break(());
}
ControlFlow::Continue(Descend::No)
},
ExprKind::Match(match_expr, match_arms, _) => {
let diverges = expr_diverges(cx, match_expr)
|| match_arms.iter().all(|arm| {
let guard_diverges = arm.guard.as_ref().map_or(false, |g| expr_diverges(cx, g.body()));
guard_diverges || expr_diverges(cx, arm.body)
});
if diverges {
return ControlFlow::Break(());
}
ControlFlow::Continue(Descend::No)
},
false
}
// Don't continue into loops or labeled blocks, as they are breakable,
// and we'd have to start checking labels.
ExprKind::Block(_, Some(_)) | ExprKind::Loop(..) => ControlFlow::Continue(Descend::No),
if self.res.is_break() {
return;
}
// Default: descend
_ => ControlFlow::Continue(Descend::Yes),
// We can't just call is_never on expr and be done, because the type system
// sometimes coerces the ! type to something different before we can get
// our hands on it. So instead, we do a manual search. We do fall back to
// is_never in some places when there is no better alternative.
self.res = match e.kind {
ExprKind::Continue(_) | ExprKind::Break(_, _) | ExprKind::Ret(_) => ControlFlow::Break(()),
ExprKind::Call(call, _) => {
if is_never(self.cx, e) || is_never(self.cx, call) {
ControlFlow::Break(())
} else {
ControlFlow::Continue(Descend::Yes)
}
},
ExprKind::MethodCall(..) => {
if is_never(self.cx, e) {
ControlFlow::Break(())
} else {
ControlFlow::Continue(Descend::Yes)
}
},
ExprKind::If(if_expr, if_then, if_else) => {
let else_diverges = if_else.map_or(false, |ex| expr_diverges(self.cx, ex));
let diverges =
expr_diverges(self.cx, if_expr) || (else_diverges && expr_diverges(self.cx, if_then));
if diverges {
ControlFlow::Break(())
} else {
ControlFlow::Continue(Descend::No)
}
},
ExprKind::Match(match_expr, match_arms, _) => {
let diverges = expr_diverges(self.cx, match_expr)
|| match_arms.iter().all(|arm| {
let guard_diverges = arm.guard.as_ref().map_or(false, |g| expr_diverges(self.cx, g.body()));
guard_diverges || expr_diverges(self.cx, arm.body)
});
if diverges {
ControlFlow::Break(())
} else {
ControlFlow::Continue(Descend::No)
}
},
// Don't continue into loops or labeled blocks, as they are breakable,
// and we'd have to start checking labels.
ExprKind::Block(_, Some(_)) | ExprKind::Loop(..) => ControlFlow::Continue(Descend::No),
// Default: descend
_ => ControlFlow::Continue(Descend::Yes),
};
if let ControlFlow::Continue(Descend::Yes) = self.res {
walk_expr(self, e);
}
}
})
.is_some()
fn visit_local(&mut self, local: &'tcx Local<'_>) {
// Don't visit the else block of a let/else statement as it will not make
// the statement divergent even though the else block is divergent.
if let Some(init) = local.init {
self.visit_expr(init);
}
}
// Avoid unnecessary `walk_*` calls.
fn visit_ty(&mut self, _: &'tcx Ty<'tcx>) {}
fn visit_pat(&mut self, _: &'tcx Pat<'tcx>) {}
fn visit_qpath(&mut self, _: &'tcx QPath<'tcx>, _: HirId, _: Span) {}
// Avoid monomorphising all `visit_*` functions.
fn visit_nested_item(&mut self, _: ItemId) {}
}
let mut v = V {
cx,
res: ControlFlow::Continue(Descend::Yes),
};
expr.visit(&mut v);
v.res.is_break()
}
fn pat_allowed_for_else(cx: &LateContext<'_>, pat: &'_ Pat<'_>, check_types: bool) -> bool {

11
tests/ui/manual_let_else.rs
View file

@ -248,4 +248,15 @@ fn not_fire() {
Some(value) => value,
_ => macro_call!(),
};
// Issue 10296
// The let/else block in the else part is not divergent despite the presence of return
let _x = if let Some(x) = Some(1) {
x
} else {
let Some(_z) = Some(3) else {
return
};
1
};
}