Add matches_if_let lint for if matches! conditions by Taym95 · Pull Request #16739 · rust-lang/rust-clippy

Taym95 · 2026-03-20T20:57:42Z

changelog: [matches_if_let]: add a new restriction lint that suggests replacing if matches!(...) with if let when the rewrite is safe, including guarded cases when if let chains are available.

Followed lint naming conventions
Added passing UI tests (including committed .stderr file)
cargo test passes locally
Executed cargo dev update_lints
Added lint documentation
Run cargo dev fmt

Signed-off-by: Taym Haddadi <haddadi.taym@gmail.com>

rustbot · 2026-03-20T20:57:48Z

r? @Jarcho

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github-actions · 2026-03-20T21:11:35Z

Lintcheck changes for 94a3813

Lint	Added	Removed	Changed
`clippy::matches_if_let`	42	0	0

This comment will be updated if you push new changes

Jarcho

The logic for the lint seems to be fine other than the one note about checking the parent SyntaxContext. Can you also switch this to using if chains rather than a bunch of returns.

View changes since this review

clippy_lints/src/matches/if_matches.rs

Jarcho · 2026-03-23T10:22:38Z

clippy_lints/src/matches/if_matches.rs

+    if !is_matches_expansion(first_arm, second_arm) {
+        return;
+    }


This isn't needed. We already know this is from the matches macro.

Jarcho · 2026-03-23T10:23:27Z

clippy_lints/src/matches/if_matches.rs

+    cx: &LateContext<'tcx>,
+    expr: &'tcx Expr<'tcx>,
+    scrutinee: &'tcx Expr<'tcx>,
+    arms: &'tcx [Arm<'tcx>],


This should be taking the single arm of interest.

Jarcho · 2026-03-23T10:25:28Z

clippy_lints/src/matches/if_matches.rs

+    let Some(if_expr) = get_parent_expr(cx, expr) else {
+        return;
+    };
+
+    let ExprKind::If(cond, then, _) = if_expr.kind else {
+        return;
+    };


This will catch if matches!(), but not if x && matches!().

This won't block the PR, but this should be handled.

Thanks, I tried to fix it part of this RR, please have another look.

Jarcho · 2026-03-23T10:33:47Z

clippy_lints/src/matches/if_matches.rs

+    let mut uses_binding_name_in_body = false;
+    first_arm.pat.each_binding_or_first(&mut |_, _, _, ident| {
+        uses_binding_name_in_body |= contains_name(ident.name, then, cx);
+    });


This should be collecting the binding IDs then walking the body once. SsoHashSet would be appropriate here.

Jarcho · 2026-03-23T10:37:59Z

clippy_lints/src/matches/if_matches.rs

+    let pat = snippet_with_context(cx, first_arm.pat.span, ctxt, "..", &mut app).0;
+    let scrutinee = Sugg::hir_with_context(cx, scrutinee, ctxt, "..", &mut app)
+        .maybe_paren()
+        .into_string();


Sugg implements Display. No need to call into_string.

Jarcho · 2026-03-23T10:38:22Z

clippy_lints/src/matches/if_matches.rs

+    let suggestion = if let Some(guard) = first_arm.guard {
+        let guard = Sugg::hir_with_context(cx, guard, ctxt, "..", &mut app)
+            .maybe_paren()
+            .into_string();


Sugg implements Display. No need to call into_string.

Jarcho · 2026-03-23T10:41:01Z

clippy_lints/src/matches/if_matches.rs

+        MATCHES_IF_LET,
+        matches_span,
+        "this `matches!` can be written as an `if let`",
+        "consider using `if let`",


We use the action to take here so just "use `if let`".

Jarcho · 2026-03-23T10:44:09Z

clippy_lints/src/matches/if_matches.rs

+        cx,
+        MATCHES_IF_LET,
+        matches_span,
+        "this `matches!` can be written as an `if let`",


This would be better as something like "`matches!` used as an 'if' condition". All matches can technically be replaced.

Jarcho · 2026-03-23T10:52:36Z

clippy_lints/src/matches/if_matches.rs

+        return;
+    };
+
+    if cond.hir_id != expr.hir_id || if_expr.span.from_expansion() {


This should be checking that the if is from the same context as the matches call.

rustbot · 2026-03-23T10:54:57Z

Reminder, once the PR becomes ready for a review, use @rustbot ready.

Jarcho · 2026-03-23T11:24:18Z

@rustbot label lint-nominated

rustbot · 2026-03-23T11:24:24Z

This lint has been nominated for inclusion.

A FCP topic has been created on Zulip.

Signed-off-by: Taym Haddadi <haddadi.taym@gmail.com>

Jarcho

Again, please use let chains for the main check function. There's no need to have multiple returns spread throughout the function.

View changes since this review

Jarcho · 2026-03-26T06:00:12Z

clippy_lints/src/matches/if_matches.rs

+                if op.node == BinOpKind::And && (left.hir_id == child_id || right.hir_id == child_id) =>
+            {
+                child_id = parent_id;
+                needs_if_let_chain = true;
+            },
+            ExprKind::DropTemps(inner) if inner.hir_id == child_id => child_id = parent_id,


The checks for the child ID on both arms are always true. No need to check.

Jarcho · 2026-03-26T06:11:59Z

clippy_lints/src/matches/if_matches.rs

+    }
+
+    let mut visitor = BodyUsesBindingName { cx, binding_names };
+    visitor.visit_expr(then).is_break()


This also needs to visit every expression in the condition in the if after the matches.

Jarcho · 2026-03-26T06:16:12Z

clippy_lints/src/matches/if_matches.rs

+            ExprKind::Binary(op, left, right)
+                if op.node == BinOpKind::And && (left.hir_id == child_id || right.hir_id == child_id) =>
+            {
+                child_id = parent_id;
+                needs_if_let_chain = true;
+            },


You'll need to record the rhs expression every time you come from the lhs for the binding use check.

Add matches_if_let lint for if matches! conditions

c041841

Signed-off-by: Taym Haddadi <haddadi.taym@gmail.com>

rustbot assigned Jarcho Mar 20, 2026

rustbot added needs-fcp PRs that add, remove, or rename lints and need an FCP S-waiting-on-review Status: Awaiting review from the assignee but also interested parties labels Mar 20, 2026

Taym95 force-pushed the matches_if_let branch from b78d20c to c041841 Compare March 20, 2026 21:00

Jarcho requested changes Mar 23, 2026

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rustbot added S-waiting-on-author Status: This is awaiting some action from the author. (Use `@rustbot ready` to update this status) and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties labels Mar 23, 2026

rustbot added the lint-nominated Create an FCP-thread on Zulip for this PR label Mar 23, 2026

Address review comments

94a3813

Signed-off-by: Taym Haddadi <haddadi.taym@gmail.com>

Jarcho requested changes Mar 26, 2026

View reviewed changes

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Taym95 commented Mar 20, 2026

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rustbot commented Mar 23, 2026

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