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Fix useless_let_if_seq FN when if is in the last expr of block #16063

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Merged
merged 2 commits into from
Nov 11, 2025
Merged

Fix useless_let_if_seq FN when if is in the last expr of block #16063

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8a1dde1
fix: `useless_let_if_seq` FN when `if` is in the last expr of block
profetia Nov 10, 2025
e1ac7da
fix: `useless_let_if_seq` wrongly unmangled macros
profetia Nov 10, 2025
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144 changes: 76 additions & 68 deletions clippy_lints/src/let_if_seq.rs
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Original file line number Diff line number Diff line change
@@ -1,6 +1,6 @@
use clippy_utils::diagnostics::span_lint_hir_and_then;
use clippy_utils::res::MaybeResPath;
use clippy_utils::source::snippet;
use clippy_utils::source::snippet_with_context;
use clippy_utils::visitors::is_local_used;
use rustc_errors::Applicability;
use rustc_hir as hir;
Expand Down Expand Up @@ -59,80 +59,88 @@ declare_lint_pass!(LetIfSeq => [USELESS_LET_IF_SEQ]);
impl<'tcx> LateLintPass<'tcx> for LetIfSeq {
fn check_block(&mut self, cx: &LateContext<'tcx>, block: &'tcx hir::Block<'_>) {
for [stmt, next] in block.stmts.array_windows::<2>() {
if let hir::StmtKind::Let(local) = stmt.kind
&& let hir::PatKind::Binding(mode, canonical_id, ident, None) = local.pat.kind
&& let hir::StmtKind::Expr(if_) = next.kind
&& let hir::ExprKind::If(cond, then, else_) = if_.kind
&& !is_local_used(cx, cond, canonical_id)
&& let hir::ExprKind::Block(then, _) = then.kind
&& let Some(value) = check_assign(cx, canonical_id, then)
&& !is_local_used(cx, value, canonical_id)
{
let span = stmt.span.to(if_.span);
if let hir::StmtKind::Expr(if_) = next.kind {
check_block_inner(cx, stmt, if_);
}
}

let has_interior_mutability = !cx
.typeck_results()
.node_type(canonical_id)
.is_freeze(cx.tcx, cx.typing_env());
if has_interior_mutability {
return;
}
if let Some(expr) = block.expr
&& let Some(stmt) = block.stmts.last()
{
check_block_inner(cx, stmt, expr);
}
}
}

fn check_block_inner<'tcx>(cx: &LateContext<'tcx>, stmt: &'tcx hir::Stmt<'tcx>, if_: &'tcx hir::Expr<'tcx>) {
if let hir::StmtKind::Let(local) = stmt.kind
&& let hir::PatKind::Binding(mode, canonical_id, ident, None) = local.pat.kind
&& let hir::ExprKind::If(cond, then, else_) = if_.kind
&& !is_local_used(cx, cond, canonical_id)
&& let hir::ExprKind::Block(then, _) = then.kind
&& let Some(value) = check_assign(cx, canonical_id, then)
&& !is_local_used(cx, value, canonical_id)
{
let span = stmt.span.to(if_.span);

let (default_multi_stmts, default) = if let Some(else_) = else_ {
if let hir::ExprKind::Block(else_, _) = else_.kind {
if let Some(default) = check_assign(cx, canonical_id, else_) {
(else_.stmts.len() > 1, default)
} else if let Some(default) = local.init {
(true, default)
} else {
continue;
}
} else {
continue;
}
let has_interior_mutability = !cx
.typeck_results()
.node_type(canonical_id)
.is_freeze(cx.tcx, cx.typing_env());
if has_interior_mutability {
return;
}

let (default_multi_stmts, default) = if let Some(else_) = else_ {
if let hir::ExprKind::Block(else_, _) = else_.kind {
if let Some(default) = check_assign(cx, canonical_id, else_) {
(else_.stmts.len() > 1, default)
} else if let Some(default) = local.init {
(false, default)
(true, default)
} else {
continue;
};
return;
}
} else {
return;
}
} else if let Some(default) = local.init {
(false, default)
} else {
return;
};

let mutability = match mode {
BindingMode(_, Mutability::Mut) => "<mut> ",
_ => "",
};
let mutability = match mode {
BindingMode(_, Mutability::Mut) => "<mut> ",
_ => "",
};

// FIXME: this should not suggest `mut` if we can detect that the variable is not
// use mutably after the `if`
// FIXME: this should not suggest `mut` if we can detect that the variable is not
// use mutably after the `if`

let sug = format!(
"let {mutability}{name} = if {cond} {{{then} {value} }} else {{{else} {default} }};",
name=ident.name,
cond=snippet(cx, cond.span, "_"),
then=if then.stmts.len() > 1 { " ..;" } else { "" },
else=if default_multi_stmts { " ..;" } else { "" },
value=snippet(cx, value.span, "<value>"),
default=snippet(cx, default.span, "<default>"),
);
span_lint_hir_and_then(
cx,
USELESS_LET_IF_SEQ,
local.hir_id,
span,
"`if _ { .. } else { .. }` is an expression",
|diag| {
diag.span_suggestion(
span,
"it is more idiomatic to write",
sug,
Applicability::HasPlaceholders,
);
if !mutability.is_empty() {
diag.note("you might not need `mut` at all");
}
},
);
}
}
let mut applicability = Applicability::HasPlaceholders;
let (cond_snip, _) = snippet_with_context(cx, cond.span, if_.span.ctxt(), "_", &mut applicability);
let (value_snip, _) = snippet_with_context(cx, value.span, if_.span.ctxt(), "<value>", &mut applicability);
let (default_snip, _) =
snippet_with_context(cx, default.span, if_.span.ctxt(), "<default>", &mut applicability);
let sug = format!(
"let {mutability}{name} = if {cond_snip} {{{then} {value_snip} }} else {{{else} {default_snip} }};",
name=ident.name,
then=if then.stmts.len() > 1 { " ..;" } else { "" },
else=if default_multi_stmts { " ..;" } else { "" },
);
span_lint_hir_and_then(
cx,
USELESS_LET_IF_SEQ,
local.hir_id,
span,
"`if _ { .. } else { .. }` is an expression",
|diag| {
diag.span_suggestion(span, "it is more idiomatic to write", sug, applicability);
if !mutability.is_empty() {
diag.note("you might not need `mut` at all");
}
},
);
}
}

Expand Down
31 changes: 31 additions & 0 deletions tests/ui/let_if_seq.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -139,3 +139,34 @@ fn main() {
}
println!("{}", val.get());
}

fn issue16062(bar: fn() -> bool) {
let foo;
//~^ useless_let_if_seq
if bar() {
foo = 42;
} else {
foo = 0;
}
}

fn issue16064(bar: fn() -> bool) {
macro_rules! mac {
($e:expr) => {
$e()
};
($base:expr, $lit:expr) => {
$lit * $base + 2
};
}

let foo;
//~^ useless_let_if_seq
if mac!(bar) {
foo = mac!(10, 4);
} else {
foo = 0;
}

let bar = 1;
}
26 changes: 25 additions & 1 deletion tests/ui/let_if_seq.stderr
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Original file line number Diff line number Diff line change
Expand Up @@ -52,5 +52,29 @@ LL | | }
|
= note: you might not need `mut` at all

error: aborting due to 4 previous errors
error: `if _ { .. } else { .. }` is an expression
--> tests/ui/let_if_seq.rs:144:5
|
LL | / let foo;
LL | |
LL | | if bar() {
LL | | foo = 42;
LL | | } else {
LL | | foo = 0;
LL | | }
| |_____^ help: it is more idiomatic to write: `let foo = if bar() { 42 } else { 0 };`

error: `if _ { .. } else { .. }` is an expression
--> tests/ui/let_if_seq.rs:163:5
|
LL | / let foo;
LL | |
LL | | if mac!(bar) {
LL | | foo = mac!(10, 4);
LL | | } else {
LL | | foo = 0;
LL | | }
| |_____^ help: it is more idiomatic to write: `let foo = if mac!(bar) { mac!(10, 4) } else { 0 };`

error: aborting due to 6 previous errors