add tests, add base bone for the new lint

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::NOP_MATCH),
     LintId::of(matches::REDUNDANT_PATTERN_MATCHING),
     LintId::of(matches::SINGLE_MATCH),
     LintId::of(matches::WILDCARD_IN_OR_PATTERNS),

clippy_lints/src/lib.register_correctness.rs

@@ -37,6 +37,7 @@ store.register_group(true, "clippy::correctness", Some("clippy_correctness"), ve
     LintId::of(loops::NEVER_LOOP),
     LintId::of(loops::WHILE_IMMUTABLE_CONDITION),
     LintId::of(match_str_case_mismatch::MATCH_STR_CASE_MISMATCH),
+    LintId::of(matches::NOP_MATCH),
     LintId::of(mem_replace::MEM_REPLACE_WITH_UNINIT),
     LintId::of(methods::CLONE_DOUBLE_REF),
     LintId::of(methods::ITERATOR_STEP_BY_ZERO),

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::NOP_MATCH,
     matches::REDUNDANT_PATTERN_MATCHING,
     matches::REST_PAT_IN_FULLY_BOUND_STRUCTS,
     matches::SINGLE_MATCH,

clippy_lints/src/matches/mod.rs

@@ -21,6 +21,7 @@ mod redundant_pattern_match;
 mod rest_pat_in_fully_bound_struct;
 mod single_match;
 mod wild_in_or_pats;
+mod nop_match;
 
 declare_clippy_lint! {
     /// ### What it does
@@ -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 NOP_MATCH,
+    correctness,
+    "`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,
+    NOP_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);
+                    nop_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);
+            nop_match::check(cx, expr);
         }
     }
 

clippy_lints/src/matches/nop_match.rs

@@ -0,0 +1,34 @@
+#![allow(unused_variables)]
+use clippy_utils::diagnostics::span_lint_and_sugg;
+use rustc_lint::LateContext;
+use rustc_hir::{Arm, Expr};
+use rustc_errors::Applicability;
+use super::NOP_MATCH;
+
+pub(crate) fn check(cx: &LateContext<'_>, ex: &Expr<'_>) {
+    if false {
+        span_lint_and_sugg(
+            cx,
+            NOP_MATCH,
+            ex.span,
+            "this if-let expression is unnecessary",
+            "replace it with",
+            "".to_string(),
+            Applicability::MachineApplicable,
+        );
+    }
+}
+
+pub(crate) fn check_match(cx: &LateContext<'_>, ex: &Expr<'_>, arms: &[Arm<'_>]) {
+    if false {
+        span_lint_and_sugg(
+            cx,
+            NOP_MATCH,
+            ex.span,
+            "this match expression is unnecessary",
+            "replace it with",
+            "".to_string(),
+            Applicability::MachineApplicable,
+        );
+    }
+}