From e006c77d611e20692beb3f319b0fec8cb6f09dc3 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Matthias=20Kr=C3=BCger?= <matthias.krueger@famsik.de>
Date: Sun, 28 Mar 2021 00:04:44 +0100
Subject: [PATCH] redundant_pattern_matching: look inside Refs

look inside refs and detect if let &None = ...

Fixes https://github.com/rust-lang/rust-clippy/issues/5396

changelog:  redundant_pattern_matching: look inside Refs to fix FNs with "if let &None = .. "
---
 clippy_lints/src/matches.rs    |  8 +++++++-
 tests/ui/match_ref_pats.stderr | 16 +++++++++++++++-
 2 files changed, 22 insertions(+), 2 deletions(-)

diff --git a/clippy_lints/src/matches.rs b/clippy_lints/src/matches.rs
index 3680429fed7..425124b78f4 100644
--- a/clippy_lints/src/matches.rs
+++ b/clippy_lints/src/matches.rs
@@ -1723,7 +1723,13 @@ mod redundant_pattern_match {
         arms: &[Arm<'_>],
         keyword: &'static str,
     ) {
-        let good_method = match arms[0].pat.kind {
+        // also look inside refs
+        let mut kind = &arms[0].pat.kind;
+        // if we have &None for example, peel it so we can detect "if let None = x"
+        if let PatKind::Ref(inner, _mutability) = kind {
+            kind = &inner.kind;
+        }
+        let good_method = match kind {
             PatKind::TupleStruct(ref path, ref patterns, _) if patterns.len() == 1 => {
                 if let PatKind::Wild = patterns[0].kind {
                     if match_qpath(path, &paths::RESULT_OK) {
diff --git a/tests/ui/match_ref_pats.stderr b/tests/ui/match_ref_pats.stderr
index 52cb4a14b72..67474e65cde 100644
--- a/tests/ui/match_ref_pats.stderr
+++ b/tests/ui/match_ref_pats.stderr
@@ -46,6 +46,14 @@ LL |         Some(v) => println!("{:?}", v),
 LL |         None => println!("none"),
    |
 
+error: redundant pattern matching, consider using `is_none()`
+  --> $DIR/match_ref_pats.rs:35:12
+   |
+LL |     if let &None = a {
+   |     -------^^^^^---- help: try this: `if a.is_none()`
+   |
+   = note: `-D clippy::redundant-pattern-matching` implied by `-D warnings`
+
 error: you don't need to add `&` to all patterns
   --> $DIR/match_ref_pats.rs:35:5
    |
@@ -59,6 +67,12 @@ help: instead of prefixing all patterns with `&`, you can dereference the expres
 LL |     if let None = *a {
    |            ^^^^   ^^
 
+error: redundant pattern matching, consider using `is_none()`
+  --> $DIR/match_ref_pats.rs:40:12
+   |
+LL |     if let &None = &b {
+   |     -------^^^^^----- help: try this: `if b.is_none()`
+
 error: you don't need to add `&` to both the expression and the patterns
   --> $DIR/match_ref_pats.rs:40:5
    |
@@ -87,5 +101,5 @@ LL |         match *foo_variant!(0) {
 LL |             Foo::A => println!("A"),
    |
 
-error: aborting due to 6 previous errors
+error: aborting due to 8 previous errors