add basic code to check nop match blocks

modify `manual_map_option` uitest because one test case has confliction.

clippy_lints/src/matches/mod.rs

@@ -16,12 +16,12 @@
 mod match_single_binding;
 mod match_wild_enum;
 mod match_wild_err_arm;
+mod nop_match;
 mod overlapping_arms;
 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

clippy_lints/src/matches/nop_match.rs

@@ -1,9 +1,11 @@
 #![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;
+use clippy_utils::diagnostics::span_lint_and_sugg;
+use clippy_utils::source::snippet_with_applicability;
+use clippy_utils::{eq_expr_value, get_parent_expr};
+use rustc_errors::Applicability;
+use rustc_hir::{Arm, BindingAnnotation, Expr, ExprKind, Pat, PatKind, PathSegment, QPath};
+use rustc_lint::LateContext;
 
 pub(crate) fn check(cx: &LateContext<'_>, ex: &Expr<'_>) {
     if false {
@@ -20,15 +22,95 @@ pub(crate) fn check(cx: &LateContext<'_>, ex: &Expr<'_>) {
 }
 
 pub(crate) fn check_match(cx: &LateContext<'_>, ex: &Expr<'_>, arms: &[Arm<'_>]) {
-    if false {
+    // 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,
             NOP_MATCH,
-            ex.span,
+            match_expr.span,
             "this match expression is unnecessary",
             "replace it with",
-            "".to_string(),
-            Applicability::MachineApplicable,
+            snippet_with_applicability(cx, ex.span, "..", &mut applicability).to_string(),
+            applicability,
         );
     }
 }
+
+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) {
+        (
+            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 is_identical_segments(path.segments, call_path.segments)
+                    && has_same_non_ref_symbol(first_pat, first_param)
+                {
+                    return true;
+                }
+            }
+        },
+        (PatKind::Path(QPath::Resolved(_, p_path)), ExprKind::Path(QPath::Resolved(_, e_path))) => {
+            return is_identical_segments(p_path.segments, e_path.segments);
+        },
+        (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 is_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)) = expr.kind;
+        if let Some(first_seg) = path.segments.first();
+        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::nop_match)]
     let _: Option<&str> = match Some(s) {
         Some(s) => Some(s),
         None => None,

tests/ui/manual_map_option.rs

@@ -214,6 +214,7 @@ const fn f4() {
 
     // #7077
     let s = &String::new();
+    #[allow(clippy::nop_match)]
     let _: Option<&str> = match Some(s) {
         Some(s) => Some(s),
         None => None,

tests/ui/nop_match.fixed

@@ -4,26 +4,47 @@
 #![allow(clippy::question_mark)]
 #![allow(dead_code)]
 
-fn option_match() -> Option<i32> {
-    match Some(1) {
-        Some(a) => Some(a),
-        None => None
+fn func_ret_err<T>(err: T) -> Result<(), T> {
+    Err(err)
+}
+
+enum SampleEnum {
+    A,
+    B,
+    C,
+}
+
+fn useless_prim_type_match(x: i32) -> i32 {
+    x
+}
+
+fn useless_custom_type_match(se: SampleEnum) -> SampleEnum {
+    se
+}
+
+// Don't trigger
+fn mingled_custom_type(se: SampleEnum) -> SampleEnum {
+    match se {
+        SampleEnum::A => SampleEnum::B,
+        SampleEnum::B => SampleEnum::C,
+        SampleEnum::C => SampleEnum::A,
     }
 }
 
+fn option_match() -> Option<i32> {
+    Some(1)
+}
+
 fn result_match() -> Result<i32, i32> {
-    match Ok(1) {
-        Ok(a) => Ok(a),
-        Err(err) => Err(err)
-    }
+    Ok(1)
+}
+
+fn result_match_func_call() {
+    let _ = func_ret_err(0_i32);
 }
 
 fn option_check() -> Option<i32> {
-    if let Some(a) = Some(1) {
-        Some(a)
-    } else {
-        None
-    }
+    if let Some(a) = Some(1) { Some(a) } else { None }
 }
 
 fn option_check_no_else() -> Option<i32> {
@@ -33,10 +54,6 @@ fn option_check_no_else() -> Option<i32> {
     None
 }
 
-fn func_ret_err<T>(err: T) -> Result<(), T> {
-    Err(err)
-}
-
 fn result_check_no_else() -> Result<(), i32> {
     if let Err(e) = func_ret_err(0_i32) {
         return Err(e);
@@ -54,30 +71,18 @@ fn result_check_a() -> Result<(), i32> {
 
 // Don't trigger
 fn result_check_b() -> Result<(), i32> {
-    if let Err(e) = Ok(1) {
-        Err(e)
-    } else {
-        Ok(())
-    }
+    if let Err(e) = Ok(1) { Err(e) } else { Ok(()) }
 }
 
 fn result_check_c() -> Result<(), i32> {
     let example = Ok(());
-    if let Err(e) = example {
-        Err(e)
-    } else {
-        example
-    }
+    if let Err(e) = example { Err(e) } else { example }
 }
 
 // Don't trigger
 fn result_check_d() -> Result<(), i32> {
     let example = Ok(1);
-    if let Err(e) = example {
-        Err(e)
-    } else {
-        Ok(())
-    }
+    if let Err(e) = example { Err(e) } else { Ok(()) }
 }
 
-fn main() { }
+fn main() {}

tests/ui/nop_match.rs

@@ -4,26 +4,65 @@
 #![allow(clippy::question_mark)]
 #![allow(dead_code)]
 
+fn func_ret_err<T>(err: T) -> Result<(), T> {
+    Err(err)
+}
+
+enum SampleEnum {
+    A,
+    B,
+    C,
+}
+
+fn useless_prim_type_match(x: i32) -> i32 {
+    match x {
+        0 => 0,
+        1 => 1,
+        2 => 2,
+        _ => x,
+    }
+}
+
+fn useless_custom_type_match(se: SampleEnum) -> SampleEnum {
+    match se {
+        SampleEnum::A => SampleEnum::A,
+        SampleEnum::B => SampleEnum::B,
+        SampleEnum::C => SampleEnum::C,
+    }
+}
+
+// Don't trigger
+fn mingled_custom_type(se: SampleEnum) -> SampleEnum {
+    match se {
+        SampleEnum::A => SampleEnum::B,
+        SampleEnum::B => SampleEnum::C,
+        SampleEnum::C => SampleEnum::A,
+    }
+}
+
 fn option_match() -> Option<i32> {
     match Some(1) {
         Some(a) => Some(a),
-        None => None
+        None => None,
     }
 }
 
 fn result_match() -> Result<i32, i32> {
     match Ok(1) {
         Ok(a) => Ok(a),
-        Err(err) => Err(err)
+        Err(err) => Err(err),
     }
 }
 
+fn result_match_func_call() {
+    let _ = match func_ret_err(0_i32) {
+        Ok(a) => Ok(a),
+        Err(err) => Err(err),
+    };
+}
+
 fn option_check() -> Option<i32> {
-    if let Some(a) = Some(1) {
-        Some(a)
-    } else {
-        None
-    }
+    if let Some(a) = Some(1) { Some(a) } else { None }
 }
 
 fn option_check_no_else() -> Option<i32> {
@@ -33,10 +72,6 @@ fn option_check_no_else() -> Option<i32> {
     None
 }
 
-fn func_ret_err<T>(err: T) -> Result<(), T> {
-    Err(err)
-}
-
 fn result_check_no_else() -> Result<(), i32> {
     if let Err(e) = func_ret_err(0_i32) {
         return Err(e);
@@ -54,30 +89,18 @@ fn result_check_a() -> Result<(), i32> {
 
 // Don't trigger
 fn result_check_b() -> Result<(), i32> {
-    if let Err(e) = Ok(1) {
-        Err(e)
-    } else {
-        Ok(())
-    }
+    if let Err(e) = Ok(1) { Err(e) } else { Ok(()) }
 }
 
 fn result_check_c() -> Result<(), i32> {
     let example = Ok(());
-    if let Err(e) = example {
-        Err(e)
-    } else {
-        example
-    }
+    if let Err(e) = example { Err(e) } else { example }
 }
 
 // Don't trigger
 fn result_check_d() -> Result<(), i32> {
     let example = Ok(1);
-    if let Err(e) = example {
-        Err(e)
-    } else {
-        Ok(())
-    }
+    if let Err(e) = example { Err(e) } else { Ok(()) }
 }
 
-fn main() { }
+fn main() {}

tests/ui/nop_match.stderr

@@ -0,0 +1,53 @@
+error: this match expression is unnecessary
+  --> $DIR/nop_match.rs:18:5
+   |
+LL | /     match x {
+LL | |         0 => 0,
+LL | |         1 => 1,
+LL | |         2 => 2,
+LL | |         _ => x,
+LL | |     }
+   | |_____^ help: replace it with: `x`
+   |
+   = note: `-D clippy::nop-match` implied by `-D warnings`
+
+error: this match expression is unnecessary
+  --> $DIR/nop_match.rs:27:5
+   |
+LL | /     match se {
+LL | |         SampleEnum::A => SampleEnum::A,
+LL | |         SampleEnum::B => SampleEnum::B,
+LL | |         SampleEnum::C => SampleEnum::C,
+LL | |     }
+   | |_____^ help: replace it with: `se`
+
+error: this match expression is unnecessary
+  --> $DIR/nop_match.rs:44:5
+   |
+LL | /     match Some(1) {
+LL | |         Some(a) => Some(a),
+LL | |         None => None,
+LL | |     }
+   | |_____^ help: replace it with: `Some(1)`
+
+error: this match expression is unnecessary
+  --> $DIR/nop_match.rs:51:5
+   |
+LL | /     match Ok(1) {
+LL | |         Ok(a) => Ok(a),
+LL | |         Err(err) => Err(err),
+LL | |     }
+   | |_____^ help: replace it with: `Ok(1)`
+
+error: this match expression is unnecessary
+  --> $DIR/nop_match.rs:58:13
+   |
+LL |       let _ = match func_ret_err(0_i32) {
+   |  _____________^
+LL | |         Ok(a) => Ok(a),
+LL | |         Err(err) => Err(err),
+LL | |     };
+   | |_____^ help: replace it with: `func_ret_err(0_i32)`
+
+error: aborting due to 5 previous errors
+