Removing false positive for the match_single_binding lint

* Applying suggested changes from the PR

clippy_lints/src/matches.rs

@@ -4,8 +4,8 @@ use crate::utils::usage::is_unused;
 use crate::utils::{
     expr_block, get_arg_name, get_parent_expr, in_macro, indent_of, is_allowed, is_expn_of, is_refutable,
     is_type_diagnostic_item, is_wild, match_qpath, match_type, match_var, meets_msrv, multispan_sugg, remove_blocks,
-    snippet, snippet_block, snippet_with_applicability, span_lint_and_help, span_lint_and_note, span_lint_and_sugg,
+    snippet, snippet_block, snippet_opt, snippet_with_applicability, span_lint_and_help, span_lint_and_note,
-    span_lint_and_then,
+    span_lint_and_sugg, span_lint_and_then,
 };
 use crate::utils::{paths, search_same, SpanlessEq, SpanlessHash};
 use if_chain::if_chain;
@@ -1237,6 +1237,24 @@ fn check_match_single_binding<'a>(cx: &LateContext<'a>, ex: &Expr<'a>, arms: &[A
     if in_macro(expr.span) || arms.len() != 1 || is_refutable(cx, arms[0].pat) {
         return;
     }
+
+    // HACK:
+    // This is a hack to deal with arms that are excluded by macros like `#[cfg]`. It is only used here
+    // to prevent false positives as there is currently no better way to detect if code was excluded by
+    // a macro. See PR #6435
+    if_chain! {
+        if let Some(match_snippet) = snippet_opt(cx, expr.span);
+        if let Some(arm_snippet) = snippet_opt(cx, arms[0].span);
+        if let Some(ex_snippet) = snippet_opt(cx, ex.span);
+        let rest_snippet = match_snippet.replace(&arm_snippet, "").replace(&ex_snippet, "");
+        if rest_snippet.contains("=>");
+        then {
+            // The code it self contains another thick arrow "=>"
+            // -> Either another arm or a comment
+            return;
+        }
+    }
+
     let matched_vars = ex.span;
     let bind_names = arms[0].pat.span;
     let match_body = remove_blocks(&arms[0].body);

tests/ui/match_single_binding.fixed

@@ -87,4 +87,32 @@ fn main() {
             unwrapped
         })
         .collect::<Vec<u8>>();
+    // Ok
+    let x = 1;
+    match x {
+        #[cfg(disabled_feature)]
+        0 => println!("Disabled branch"),
+        _ => println!("Enabled branch"),
+    }
+    // Lint
+    let x = 1;
+    let y = 1;
+    println!("Single branch");
+    // Ok
+    let x = 1;
+    let y = 1;
+    match match y {
+        0 => 1,
+        _ => 2,
+    } {
+        #[cfg(disabled_feature)]
+        0 => println!("Array index start"),
+        _ => println!("Not an array index start"),
+    }
+    // False negative
+    let x = 1;
+    match x {
+        // =>
+        _ => println!("Not an array index start"),
+    }
 }

tests/ui/match_single_binding.rs

@@ -99,4 +99,37 @@ fn main() {
             unwrapped => unwrapped,
         })
         .collect::<Vec<u8>>();
+    // Ok
+    let x = 1;
+    match x {
+        #[cfg(disabled_feature)]
+        0 => println!("Disabled branch"),
+        _ => println!("Enabled branch"),
+    }
+    // Lint
+    let x = 1;
+    let y = 1;
+    match match y {
+        0 => 1,
+        _ => 2,
+    } {
+        _ => println!("Single branch"),
+    }
+    // Ok
+    let x = 1;
+    let y = 1;
+    match match y {
+        0 => 1,
+        _ => 2,
+    } {
+        #[cfg(disabled_feature)]
+        0 => println!("Array index start"),
+        _ => println!("Not an array index start"),
+    }
+    // False negative
+    let x = 1;
+    match x {
+        // =>
+        _ => println!("Not an array index start"),
+    }
 }

tests/ui/match_single_binding.stderr

@@ -167,5 +167,16 @@ LL |             unwrapped
 LL |         })
    |
 
-error: aborting due to 11 previous errors
+error: this match could be replaced by its body itself
+  --> $DIR/match_single_binding.rs:112:5
+   |
+LL | /     match match y {
+LL | |         0 => 1,
+LL | |         _ => 2,
+LL | |     } {
+LL | |         _ => println!("Single branch"),
+LL | |     }
+   | |_____^ help: consider using the match body instead: `println!("Single branch");`
+
+error: aborting due to 12 previous errors