new lint that detects useless match expression

CHANGELOG.md

@@ -3327,6 +3327,7 @@ Released 2018-09-13
 [`needless_for_each`]: https://rust-lang.github.io/rust-clippy/master/index.html#needless_for_each
 [`needless_late_init`]: https://rust-lang.github.io/rust-clippy/master/index.html#needless_late_init
 [`needless_lifetimes`]: https://rust-lang.github.io/rust-clippy/master/index.html#needless_lifetimes
+[`needless_match`]: https://rust-lang.github.io/rust-clippy/master/index.html#needless_match
 [`needless_option_as_deref`]: https://rust-lang.github.io/rust-clippy/master/index.html#needless_option_as_deref
 [`needless_pass_by_value`]: https://rust-lang.github.io/rust-clippy/master/index.html#needless_pass_by_value
 [`needless_question_mark`]: https://rust-lang.github.io/rust-clippy/master/index.html#needless_question_mark

clippy_lints/src/lib.register_all.rs

@@ -134,6 +134,7 @@ store.register_group(true, "clippy::all", Some("clippy_all"), vec![
     LintId::of(matches::MATCH_OVERLAPPING_ARM),
     LintId::of(matches::MATCH_REF_PATS),
     LintId::of(matches::MATCH_SINGLE_BINDING),
+    LintId::of(matches::NEEDLESS_MATCH),
     LintId::of(matches::REDUNDANT_PATTERN_MATCHING),
     LintId::of(matches::SINGLE_MATCH),
     LintId::of(matches::WILDCARD_IN_OR_PATTERNS),

clippy_lints/src/lib.register_complexity.rs

@@ -30,6 +30,7 @@ store.register_group(true, "clippy::complexity", Some("clippy_complexity"), vec!
     LintId::of(map_unit_fn::RESULT_MAP_UNIT_FN),
     LintId::of(matches::MATCH_AS_REF),
     LintId::of(matches::MATCH_SINGLE_BINDING),
+    LintId::of(matches::NEEDLESS_MATCH),
     LintId::of(matches::WILDCARD_IN_OR_PATTERNS),
     LintId::of(methods::BIND_INSTEAD_OF_MAP),
     LintId::of(methods::CLONE_ON_COPY),

clippy_lints/src/lib.register_lints.rs

@@ -253,6 +253,7 @@ store.register_lints(&[
     matches::MATCH_SINGLE_BINDING,
     matches::MATCH_WILDCARD_FOR_SINGLE_VARIANTS,
     matches::MATCH_WILD_ERR_ARM,
+    matches::NEEDLESS_MATCH,
     matches::REDUNDANT_PATTERN_MATCHING,
     matches::REST_PAT_IN_FULLY_BOUND_STRUCTS,
     matches::SINGLE_MATCH,

clippy_lints/src/matches/mod.rs

@@ -16,6 +16,7 @@ mod match_same_arms;
 mod match_single_binding;
 mod match_wild_enum;
 mod match_wild_err_arm;
+mod needless_match;
 mod overlapping_arms;
 mod redundant_pattern_match;
 mod rest_pat_in_fully_bound_struct;
@@ -566,6 +567,49 @@ declare_clippy_lint! {
     "`match` with identical arm bodies"
 }
 
+declare_clippy_lint! {
+    /// ### What it does
+    /// Checks for unnecessary `match` or match-like `if let` returns for `Option` and `Result`
+    /// when function signatures are the same.
+    ///
+    /// ### Why is this bad?
+    /// This `match` block does nothing and might not be what the coder intended.
+    ///
+    /// ### Example
+    /// ```rust,ignore
+    /// fn foo() -> Result<(), i32> {
+    ///     match result {
+    ///         Ok(val) => Ok(val),
+    ///         Err(err) => Err(err),
+    ///     }
+    /// }
+    ///
+    /// fn bar() -> Option<i32> {
+    ///     if let Some(val) = option {
+    ///         Some(val)
+    ///     } else {
+    ///         None
+    ///     }
+    /// }
+    /// ```
+    ///
+    /// Could be replaced as
+    ///
+    /// ```rust,ignore
+    /// fn foo() -> Result<(), i32> {
+    ///     result
+    /// }
+    ///
+    /// fn bar() -> Option<i32> {
+    ///     option
+    /// }
+    /// ```
+    #[clippy::version = "1.61.0"]
+    pub NEEDLESS_MATCH,
+    complexity,
+    "`match` or match-like `if let` that are unnecessary"
+}
+
 #[derive(Default)]
 pub struct Matches {
     msrv: Option<RustcVersion>,
@@ -599,6 +643,7 @@ impl_lint_pass!(Matches => [
     REDUNDANT_PATTERN_MATCHING,
     MATCH_LIKE_MATCHES_MACRO,
     MATCH_SAME_ARMS,
+    NEEDLESS_MATCH,
 ]);
 
 impl<'tcx> LateLintPass<'tcx> for Matches {
@@ -622,6 +667,7 @@ impl<'tcx> LateLintPass<'tcx> for Matches {
                     overlapping_arms::check(cx, ex, arms);
                     match_wild_enum::check(cx, ex, arms);
                     match_as_ref::check(cx, ex, arms, expr);
+                    needless_match::check_match(cx, ex, arms);
 
                     if self.infallible_destructuring_match_linted {
                         self.infallible_destructuring_match_linted = false;
@@ -640,6 +686,7 @@ impl<'tcx> LateLintPass<'tcx> for Matches {
                 match_like_matches::check(cx, expr);
             }
             redundant_pattern_match::check(cx, expr);
+            needless_match::check(cx, expr);
         }
     }
 

clippy_lints/src/matches/needless_match.rs

@@ -0,0 +1,197 @@
+use super::NEEDLESS_MATCH;
+use clippy_utils::diagnostics::span_lint_and_sugg;
+use clippy_utils::source::snippet_with_applicability;
+use clippy_utils::ty::is_type_diagnostic_item;
+use clippy_utils::{eq_expr_value, get_parent_expr, higher, is_else_clause, is_lang_ctor, peel_blocks_with_stmt};
+use rustc_errors::Applicability;
+use rustc_hir::LangItem::OptionNone;
+use rustc_hir::{Arm, BindingAnnotation, Expr, ExprKind, Pat, PatKind, Path, PathSegment, QPath, UnOp};
+use rustc_lint::LateContext;
+use rustc_span::sym;
+
+pub(crate) fn check_match(cx: &LateContext<'_>, ex: &Expr<'_>, arms: &[Arm<'_>]) {
+    // This is for avoiding collision with `match_single_binding`.
+    if arms.len() < 2 {
+        return;
+    }
+
+    for arm in arms {
+        if let PatKind::Wild = arm.pat.kind {
+            let ret_expr = strip_return(arm.body);
+            if !eq_expr_value(cx, ex, ret_expr) {
+                return;
+            }
+        } else if !pat_same_as_expr(arm.pat, arm.body) {
+            return;
+        }
+    }
+
+    if let Some(match_expr) = get_parent_expr(cx, ex) {
+        let mut applicability = Applicability::MachineApplicable;
+        span_lint_and_sugg(
+            cx,
+            NEEDLESS_MATCH,
+            match_expr.span,
+            "this match expression is unnecessary",
+            "replace it with",
+            snippet_with_applicability(cx, ex.span, "..", &mut applicability).to_string(),
+            applicability,
+        );
+    }
+}
+
+/// Check for nop `if let` expression that assembled as unnecessary match
+///
+/// ```rust,ignore
+/// if let Some(a) = option {
+///     Some(a)
+/// } else {
+///     None
+/// }
+/// ```
+/// OR
+/// ```rust,ignore
+/// if let SomeEnum::A = some_enum {
+///     SomeEnum::A
+/// } else if let SomeEnum::B = some_enum {
+///     SomeEnum::B
+/// } else {
+///     some_enum
+/// }
+/// ```
+pub(crate) fn check(cx: &LateContext<'_>, ex: &Expr<'_>) {
+    if_chain! {
+        if let Some(ref if_let) = higher::IfLet::hir(cx, ex);
+        if !is_else_clause(cx.tcx, ex);
+        if check_if_let(cx, if_let);
+        then {
+            let mut applicability = Applicability::MachineApplicable;
+            span_lint_and_sugg(
+                cx,
+                NEEDLESS_MATCH,
+                ex.span,
+                "this if-let expression is unnecessary",
+                "replace it with",
+                snippet_with_applicability(cx, if_let.let_expr.span, "..", &mut applicability).to_string(),
+                applicability,
+            );
+        }
+    }
+}
+
+fn check_if_let(cx: &LateContext<'_>, if_let: &higher::IfLet<'_>) -> bool {
+    if let Some(if_else) = if_let.if_else {
+        if !pat_same_as_expr(if_let.let_pat, peel_blocks_with_stmt(if_let.if_then)) {
+            return false;
+        }
+
+        // Recurrsively check for each `else if let` phrase,
+        if let Some(ref nested_if_let) = higher::IfLet::hir(cx, if_else) {
+            return check_if_let(cx, nested_if_let);
+        }
+
+        if matches!(if_else.kind, ExprKind::Block(..)) {
+            let else_expr = peel_blocks_with_stmt(if_else);
+            let ret = strip_return(else_expr);
+            let let_expr_ty = cx.typeck_results().expr_ty(if_let.let_expr);
+            if is_type_diagnostic_item(cx, let_expr_ty, sym::Option) {
+                if let ExprKind::Path(ref qpath) = ret.kind {
+                    return is_lang_ctor(cx, qpath, OptionNone) || eq_expr_value(cx, if_let.let_expr, ret);
+                }
+            } else {
+                return eq_expr_value(cx, if_let.let_expr, ret);
+            }
+            return true;
+        }
+    }
+    false
+}
+
+/// Strip `return` keyword if the expression type is `ExprKind::Ret`.
+fn strip_return<'hir>(expr: &'hir Expr<'hir>) -> &'hir Expr<'hir> {
+    if let ExprKind::Ret(Some(ret)) = expr.kind {
+        ret
+    } else {
+        expr
+    }
+}
+
+fn pat_same_as_expr(pat: &Pat<'_>, expr: &Expr<'_>) -> bool {
+    let expr = strip_return(expr);
+    match (&pat.kind, &expr.kind) {
+        // Example: `Some(val) => Some(val)`
+        (
+            PatKind::TupleStruct(QPath::Resolved(_, path), [first_pat, ..], _),
+            ExprKind::Call(call_expr, [first_param, ..]),
+        ) => {
+            if let ExprKind::Path(QPath::Resolved(_, call_path)) = call_expr.kind {
+                if has_identical_segments(path.segments, call_path.segments)
+                    && has_same_non_ref_symbol(first_pat, first_param)
+                {
+                    return true;
+                }
+            }
+        },
+        // Example: `val => val`, or `ref val => *val`
+        (PatKind::Binding(annot, _, pat_ident, _), _) => {
+            let new_expr = if let (
+                BindingAnnotation::Ref | BindingAnnotation::RefMut,
+                ExprKind::Unary(UnOp::Deref, operand_expr),
+            ) = (annot, &expr.kind)
+            {
+                operand_expr
+            } else {
+                expr
+            };
+
+            if let ExprKind::Path(QPath::Resolved(
+                _,
+                Path {
+                    segments: [first_seg, ..],
+                    ..
+                },
+            )) = new_expr.kind
+            {
+                return pat_ident.name == first_seg.ident.name;
+            }
+        },
+        // Example: `Custom::TypeA => Custom::TypeB`, or `None => None`
+        (PatKind::Path(QPath::Resolved(_, p_path)), ExprKind::Path(QPath::Resolved(_, e_path))) => {
+            return has_identical_segments(p_path.segments, e_path.segments);
+        },
+        // Example: `5 => 5`
+        (PatKind::Lit(pat_lit_expr), ExprKind::Lit(expr_spanned)) => {
+            if let ExprKind::Lit(pat_spanned) = &pat_lit_expr.kind {
+                return pat_spanned.node == expr_spanned.node;
+            }
+        },
+        _ => {},
+    }
+
+    false
+}
+
+fn has_identical_segments(left_segs: &[PathSegment<'_>], right_segs: &[PathSegment<'_>]) -> bool {
+    if left_segs.len() != right_segs.len() {
+        return false;
+    }
+    for i in 0..left_segs.len() {
+        if left_segs[i].ident.name != right_segs[i].ident.name {
+            return false;
+        }
+    }
+    true
+}
+
+fn has_same_non_ref_symbol(pat: &Pat<'_>, expr: &Expr<'_>) -> bool {
+    if_chain! {
+        if let PatKind::Binding(annot, _, pat_ident, _) = pat.kind;
+        if !matches!(annot, BindingAnnotation::Ref | BindingAnnotation::RefMut);
+        if let ExprKind::Path(QPath::Resolved(_, Path {segments: [first_seg, ..], .. })) = expr.kind;
+        then {
+            return pat_ident.name == first_seg.ident.name;
+        }
+    }
+
+    false
+}

tests/ui/manual_map_option.fixed

@@ -148,6 +148,7 @@ fn main() {
 
     // #7077
     let s = &String::new();
+    #[allow(clippy::needless_match)]
     let _: Option<&str> = match Some(s) {
         Some(s) => Some(s),
         None => None,

tests/ui/manual_map_option.rs

@@ -214,6 +214,7 @@ fn main() {
 
     // #7077
     let s = &String::new();
+    #[allow(clippy::needless_match)]
     let _: Option<&str> = match Some(s) {
         Some(s) => Some(s),
         None => None,