new lint [redundant_guards]

CHANGELOG.md

@@ -5190,6 +5190,7 @@ Released 2018-09-13
 [`redundant_else`]: https://rust-lang.github.io/rust-clippy/master/index.html#redundant_else
 [`redundant_feature_names`]: https://rust-lang.github.io/rust-clippy/master/index.html#redundant_feature_names
 [`redundant_field_names`]: https://rust-lang.github.io/rust-clippy/master/index.html#redundant_field_names
+[`redundant_guards`]: https://rust-lang.github.io/rust-clippy/master/index.html#redundant_guards
 [`redundant_locals`]: https://rust-lang.github.io/rust-clippy/master/index.html#redundant_locals
 [`redundant_pattern`]: https://rust-lang.github.io/rust-clippy/master/index.html#redundant_pattern
 [`redundant_pattern_matching`]: https://rust-lang.github.io/rust-clippy/master/index.html#redundant_pattern_matching

clippy_lints/src/declared_lints.rs

@@ -308,6 +308,7 @@ pub(crate) static LINTS: &[&crate::LintInfo] = &[
     crate::matches::MATCH_WILDCARD_FOR_SINGLE_VARIANTS_INFO,
     crate::matches::MATCH_WILD_ERR_ARM_INFO,
     crate::matches::NEEDLESS_MATCH_INFO,
+    crate::matches::REDUNDANT_GUARDS_INFO,
     crate::matches::REDUNDANT_PATTERN_MATCHING_INFO,
     crate::matches::REST_PAT_IN_FULLY_BOUND_STRUCTS_INFO,
     crate::matches::SIGNIFICANT_DROP_IN_SCRUTINEE_INFO,

clippy_lints/src/loops/utils.rs

@@ -76,7 +76,7 @@ impl<'a, 'tcx> Visitor<'tcx> for IncrementVisitor<'a, 'tcx> {
                     ExprKind::Assign(lhs, _, _) if lhs.hir_id == expr.hir_id => {
                         *state = IncrementVisitorVarState::DontWarn;
                     },
-                    ExprKind::AddrOf(BorrowKind::Ref, mutability, _) if mutability == Mutability::Mut => {
+                    ExprKind::AddrOf(BorrowKind::Ref, Mutability::Mut, _) => {
                         *state = IncrementVisitorVarState::DontWarn;
                     },
                     _ => (),
@@ -226,7 +226,7 @@ impl<'a, 'tcx> Visitor<'tcx> for InitializeVisitor<'a, 'tcx> {
                             InitializeVisitorState::DontWarn
                         }
                     },
-                    ExprKind::AddrOf(BorrowKind::Ref, mutability, _) if mutability == Mutability::Mut => {
+                    ExprKind::AddrOf(BorrowKind::Ref, Mutability::Mut, _) => {
                         self.state = InitializeVisitorState::DontWarn;
                     },
                     _ => (),

clippy_lints/src/matches/mod.rs

@@ -16,6 +16,7 @@ mod match_wild_enum;
 mod match_wild_err_arm;
 mod needless_match;
 mod overlapping_arms;
+mod redundant_guards;
 mod redundant_pattern_match;
 mod rest_pat_in_fully_bound_struct;
 mod significant_drop_in_scrutinee;
@@ -936,6 +937,36 @@ declare_clippy_lint! {
     "reimplementation of `filter`"
 }
 
+declare_clippy_lint! {
+    /// ### What it does
+    /// Checks for unnecessary guards in match expressions.
+    ///
+    /// ### Why is this bad?
+    /// It's more complex and much less readable. Making it part of the pattern can improve
+    /// exhaustiveness checking as well.
+    ///
+    /// ### Example
+    /// ```rust,ignore
+    /// match x {
+    ///     Some(x) if matches!(x, Some(1)) => ..,
+    ///     Some(x) if x == Some(2) => ..,
+    ///     _ => todo!(),
+    /// }
+    /// ```
+    /// Use instead:
+    /// ```rust,ignore
+    /// match x {
+    ///     Some(Some(1)) => ..,
+    ///     Some(Some(2)) => ..,
+    ///     _ => todo!(),
+    /// }
+    /// ```
+    #[clippy::version = "1.72.0"]
+    pub REDUNDANT_GUARDS,
+    complexity,
+    "checks for unnecessary guards in match expressions"
+}
+
 #[derive(Default)]
 pub struct Matches {
     msrv: Msrv,
@@ -978,6 +1009,7 @@ impl_lint_pass!(Matches => [
     TRY_ERR,
     MANUAL_MAP,
     MANUAL_FILTER,
+    REDUNDANT_GUARDS,
 ]);
 
 impl<'tcx> LateLintPass<'tcx> for Matches {
@@ -1025,6 +1057,7 @@ impl<'tcx> LateLintPass<'tcx> for Matches {
                     needless_match::check_match(cx, ex, arms, expr);
                     match_on_vec_items::check(cx, ex);
                     match_str_case_mismatch::check(cx, ex, arms);
+                    redundant_guards::check(cx, arms);
 
                     if !in_constant(cx, expr.hir_id) {
                         manual_unwrap_or::check(cx, expr, ex, arms);

clippy_lints/src/matches/redundant_guards.rs

@@ -0,0 +1,190 @@
+use clippy_utils::diagnostics::span_lint_and_then;
+use clippy_utils::path_to_local;
+use clippy_utils::source::snippet_with_applicability;
+use clippy_utils::visitors::{for_each_expr, is_local_used};
+use rustc_errors::Applicability;
+use rustc_hir::def::{DefKind, Res};
+use rustc_hir::{Arm, BinOpKind, Expr, ExprKind, Guard, MatchSource, Node, Pat, PatKind};
+use rustc_lint::LateContext;
+use rustc_span::Span;
+use std::ops::ControlFlow;
+
+use super::REDUNDANT_GUARDS;
+
+pub(super) fn check<'tcx>(cx: &LateContext<'tcx>, arms: &'tcx [Arm<'tcx>]) {
+    for outer_arm in arms {
+        let Some(guard) = outer_arm.guard else {
+            continue;
+        };
+
+        // `Some(x) if matches!(x, y)`
+        if let Guard::If(if_expr) = guard
+            && let ExprKind::Match(
+                scrutinee,
+                [
+                    arm,
+                    Arm {
+                        pat: Pat {
+                            kind: PatKind::Wild,
+                            ..
+                        },
+                        ..
+                    },
+                ],
+                MatchSource::Normal,
+            ) = if_expr.kind
+        {
+            emit_redundant_guards(
+                cx,
+                outer_arm,
+                if_expr.span,
+                scrutinee,
+                arm.pat.span,
+                arm.guard,
+            );
+        }
+        // `Some(x) if let Some(2) = x`
+        else if let Guard::IfLet(let_expr) = guard {
+            emit_redundant_guards(
+                cx,
+                outer_arm,
+                let_expr.span,
+                let_expr.init,
+                let_expr.pat.span,
+                None,
+            );
+        }
+        // `Some(x) if x == Some(2)`
+        else if let Guard::If(if_expr) = guard
+            && let ExprKind::Binary(bin_op, local, pat) = if_expr.kind
+            && matches!(bin_op.node, BinOpKind::Eq)
+            && expr_can_be_pat(cx, pat)
+            // Ensure they have the same type. If they don't, we'd need deref coercion which isn't
+            // possible (currently) in a pattern. In some cases, you can use something like
+            // `as_deref` or similar but in general, we shouldn't lint this as it'd create an
+            // extraordinary amount of FPs.
+            //
+            // This isn't necessary in the other two checks, as they must be a pattern already.
+            && cx.typeck_results().expr_ty(local) == cx.typeck_results().expr_ty(pat)
+        {
+            emit_redundant_guards(
+                cx,
+                outer_arm,
+                if_expr.span,
+                local,
+                pat.span,
+                None,
+            );
+        }
+    }
+}
+
+fn get_pat_binding<'tcx>(cx: &LateContext<'tcx>, guard_expr: &Expr<'_>, outer_arm: &Arm<'tcx>) -> Option<(Span, bool)> {
+    if let Some(local) = path_to_local(guard_expr) && !is_local_used(cx, outer_arm.body, local) {
+        let mut span = None;
+        let mut multiple_bindings = false;
+        // `each_binding` gives the `HirId` of the `Pat` itself, not the binding
+        outer_arm.pat.walk(|pat| {
+            if let PatKind::Binding(_, hir_id, _, _) = pat.kind
+                && hir_id == local
+                && span.replace(pat.span).is_some()
+            {
+                multiple_bindings = true;
+                return false;
+            }
+
+            true
+        });
+
+        // Ignore bindings from or patterns, like `First(x) | Second(x, _) | Third(x, _, _)`
+        if !multiple_bindings {
+            return span.map(|span| {
+                (
+                    span,
+                    !matches!(cx.tcx.hir().get_parent(local), Node::PatField(_)),
+                )
+            });
+        }
+    }
+
+    None
+}
+
+fn emit_redundant_guards<'tcx>(
+    cx: &LateContext<'tcx>,
+    outer_arm: &Arm<'tcx>,
+    guard_span: Span,
+    local: &Expr<'_>,
+    pat_span: Span,
+    inner_guard: Option<Guard<'_>>,
+) {
+    let mut app = Applicability::MaybeIncorrect;
+    let Some((pat_binding, can_use_shorthand)) = get_pat_binding(cx, local, outer_arm) else {
+        return;
+    };
+
+    span_lint_and_then(
+        cx,
+        REDUNDANT_GUARDS,
+        guard_span.source_callsite(),
+        "redundant guard",
+        |diag| {
+            let binding_replacement = snippet_with_applicability(cx, pat_span, "<binding_repl>", &mut app);
+            diag.multipart_suggestion_verbose(
+                "try",
+                vec![
+                    if can_use_shorthand {
+                        (pat_binding, binding_replacement.into_owned())
+                    } else {
+                        (pat_binding.shrink_to_hi(), format!(": {binding_replacement}"))
+                    },
+                    (
+                        guard_span.source_callsite().with_lo(outer_arm.pat.span.hi()),
+                        inner_guard.map_or_else(String::new, |guard| {
+                            let (prefix, span) = match guard {
+                                Guard::If(e) => ("if", e.span),
+                                Guard::IfLet(l) => ("if let", l.span),
+                            };
+
+                            format!(
+                                " {prefix} {}",
+                                snippet_with_applicability(cx, span, "<guard>", &mut app),
+                            )
+                        }),
+                    ),
+                ],
+                app,
+            );
+        },
+    );
+}
+
+/// Checks if the given `Expr` can also be represented as a `Pat`.
+fn expr_can_be_pat(cx: &LateContext<'_>, expr: &Expr<'_>) -> bool {
+    for_each_expr(expr, |expr| {
+        if match expr.kind {
+            ExprKind::ConstBlock(..) => cx.tcx.features().inline_const_pat,
+            ExprKind::Call(c, ..) if let ExprKind::Path(qpath) = c.kind => {
+                // Allow ctors
+                matches!(cx.qpath_res(&qpath, c.hir_id), Res::Def(DefKind::Ctor(..), ..))
+            },
+            ExprKind::Path(qpath) => {
+                matches!(
+                    cx.qpath_res(&qpath, expr.hir_id),
+                    Res::Def(DefKind::Struct | DefKind::Enum | DefKind::Ctor(..), ..),
+                )
+            },
+            ExprKind::AddrOf(..)
+            | ExprKind::Array(..)
+            | ExprKind::Tup(..)
+            | ExprKind::Struct(..)
+            | ExprKind::Lit(..) => true,
+            _ => false,
+        } {
+            return ControlFlow::Continue(());
+        }
+
+        ControlFlow::Break(())
+    })
+    .is_none()
+}

tests/ui/match_expr_like_matches_macro.fixed

@@ -5,7 +5,8 @@
     unreachable_patterns,
     dead_code,
     clippy::equatable_if_let,
-    clippy::needless_borrowed_reference
+    clippy::needless_borrowed_reference,
+    clippy::redundant_guards
 )]
 
 fn main() {

tests/ui/match_expr_like_matches_macro.rs

@@ -5,7 +5,8 @@
     unreachable_patterns,
     dead_code,
     clippy::equatable_if_let,
-    clippy::needless_borrowed_reference
+    clippy::needless_borrowed_reference,
+    clippy::redundant_guards
 )]
 
 fn main() {