fix empty needle corner case and add tests

4 files changed, 203 insertions, 2 deletions

diff --git a/clippy_lints/src/matches/redundant_guards.rs b/clippy_lints/src/matches/redundant_guards.rs
index 8d5907d6faf..365d178f548 100644
--- a/clippy_lints/src/matches/redundant_guards.rs
+++ b/clippy_lints/src/matches/redundant_guards.rs
@@ -127,10 +127,13 @@ pub(super) fn check<'tcx>(cx: &LateContext<'tcx>, arms: &'tcx [Arm<'tcx>]) {
             {
                 // `arr if arr.starts_with(&[123])` becomes [123, ..]
                 // `arr if arr.ends_with(&[123])` becomes [.., 123]
+                // `arr if arr.starts_with(&[])` becomes [..]  (why would anyone write this?)
 
                 let mut sugg = snippet(cx, needle.span, "<needle>").into_owned();
 
-                if path.ident.name == sym!(starts_with) {
+                if needles.is_empty() {
+                    sugg.insert_str(1, "..");
+                } else if path.ident.name == sym!(starts_with) {
                     sugg.insert_str(sugg.len() - 1, ", ..");
                 } else if path.ident.name == sym!(ends_with) {
                     sugg.insert_str(1, ".., ");
diff --git a/tests/ui/redundant_guards.fixed b/tests/ui/redundant_guards.fixed
index f8af9092725..aef26ef225c 100644
--- a/tests/ui/redundant_guards.fixed
+++ b/tests/ui/redundant_guards.fixed
@@ -193,3 +193,60 @@ mod issue11465 {
         }
     }
 }
+
+fn issue11807() {
+    #![allow(clippy::single_match)]
+
+    match Some(Some("")) {
+        Some(Some("")) => {},
+        _ => {},
+    }
+
+    match Some(Some(String::new())) {
+        // Do not lint: String deref-coerces to &str
+        Some(Some(x)) if x.is_empty() => {},
+        _ => {},
+    }
+
+    match Some(Some(&[] as &[i32])) {
+        Some(Some([])) => {},
+        _ => {},
+    }
+
+    match Some(Some([] as [i32; 0])) {
+        Some(Some([])) => {},
+        _ => {},
+    }
+
+    match Some(Some(Vec::<()>::new())) {
+        // Do not lint: Vec deref-coerces to &[T]
+        Some(Some(x)) if x.is_empty() => {},
+        _ => {},
+    }
+
+    match Some(Some(&[] as &[i32])) {
+        Some(Some([..])) => {},
+        _ => {},
+    }
+
+    match Some(Some(&[] as &[i32])) {
+        Some(Some([1, ..])) => {},
+        _ => {},
+    }
+
+    match Some(Some(&[] as &[i32])) {
+        Some(Some([1, 2, ..])) => {},
+        _ => {},
+    }
+
+    match Some(Some(&[] as &[i32])) {
+        Some(Some([.., 1, 2])) => {},
+        _ => {},
+    }
+
+    match Some(Some(Vec::<i32>::new())) {
+        // Do not lint: deref coercion
+        Some(Some(x)) if x.starts_with(&[1, 2]) => {},
+        _ => {},
+    }
+}
diff --git a/tests/ui/redundant_guards.rs b/tests/ui/redundant_guards.rs
index b46f8a6207e..5d476f5b040 100644
--- a/tests/ui/redundant_guards.rs
+++ b/tests/ui/redundant_guards.rs
@@ -193,3 +193,60 @@ mod issue11465 {
         }
     }
 }
+
+fn issue11807() {
+    #![allow(clippy::single_match)]
+
+    match Some(Some("")) {
+        Some(Some(x)) if x.is_empty() => {},
+        _ => {},
+    }
+
+    match Some(Some(String::new())) {
+        // Do not lint: String deref-coerces to &str
+        Some(Some(x)) if x.is_empty() => {},
+        _ => {},
+    }
+
+    match Some(Some(&[] as &[i32])) {
+        Some(Some(x)) if x.is_empty() => {},
+        _ => {},
+    }
+
+    match Some(Some([] as [i32; 0])) {
+        Some(Some(x)) if x.is_empty() => {},
+        _ => {},
+    }
+
+    match Some(Some(Vec::<()>::new())) {
+        // Do not lint: Vec deref-coerces to &[T]
+        Some(Some(x)) if x.is_empty() => {},
+        _ => {},
+    }
+
+    match Some(Some(&[] as &[i32])) {
+        Some(Some(x)) if x.starts_with(&[]) => {},
+        _ => {},
+    }
+
+    match Some(Some(&[] as &[i32])) {
+        Some(Some(x)) if x.starts_with(&[1]) => {},
+        _ => {},
+    }
+
+    match Some(Some(&[] as &[i32])) {
+        Some(Some(x)) if x.starts_with(&[1, 2]) => {},
+        _ => {},
+    }
+
+    match Some(Some(&[] as &[i32])) {
+        Some(Some(x)) if x.ends_with(&[1, 2]) => {},
+        _ => {},
+    }
+
+    match Some(Some(Vec::<i32>::new())) {
+        // Do not lint: deref coercion
+        Some(Some(x)) if x.starts_with(&[1, 2]) => {},
+        _ => {},
+    }
+}
diff --git a/tests/ui/redundant_guards.stderr b/tests/ui/redundant_guards.stderr
index b8d7834e358..f78d2a814f9 100644
--- a/tests/ui/redundant_guards.stderr
+++ b/tests/ui/redundant_guards.stderr
@@ -203,5 +203,89 @@ LL -             B { ref c, .. } if matches!(c, &1) => {},
 LL +             B { c: 1, .. } => {},
    |
 
-error: aborting due to 17 previous errors
+error: redundant guard
+  --> $DIR/redundant_guards.rs:201:26
+   |
+LL |         Some(Some(x)) if x.is_empty() => {},
+   |                          ^^^^^^^^^^^^
+   |
+help: try
+   |
+LL -         Some(Some(x)) if x.is_empty() => {},
+LL +         Some(Some("")) => {},
+   |
+
+error: redundant guard
+  --> $DIR/redundant_guards.rs:212:26
+   |
+LL |         Some(Some(x)) if x.is_empty() => {},
+   |                          ^^^^^^^^^^^^
+   |
+help: try
+   |
+LL -         Some(Some(x)) if x.is_empty() => {},
+LL +         Some(Some([])) => {},
+   |
+
+error: redundant guard
+  --> $DIR/redundant_guards.rs:217:26
+   |
+LL |         Some(Some(x)) if x.is_empty() => {},
+   |                          ^^^^^^^^^^^^
+   |
+help: try
+   |
+LL -         Some(Some(x)) if x.is_empty() => {},
+LL +         Some(Some([])) => {},
+   |
+
+error: redundant guard
+  --> $DIR/redundant_guards.rs:228:26
+   |
+LL |         Some(Some(x)) if x.starts_with(&[]) => {},
+   |                          ^^^^^^^^^^^^^^^^^^
+   |
+help: try
+   |
+LL -         Some(Some(x)) if x.starts_with(&[]) => {},
+LL +         Some(Some([..])) => {},
+   |
+
+error: redundant guard
+  --> $DIR/redundant_guards.rs:233:26
+   |
+LL |         Some(Some(x)) if x.starts_with(&[1]) => {},
+   |                          ^^^^^^^^^^^^^^^^^^^
+   |
+help: try
+   |
+LL -         Some(Some(x)) if x.starts_with(&[1]) => {},
+LL +         Some(Some([1, ..])) => {},
+   |
+
+error: redundant guard
+  --> $DIR/redundant_guards.rs:238:26
+   |
+LL |         Some(Some(x)) if x.starts_with(&[1, 2]) => {},
+   |                          ^^^^^^^^^^^^^^^^^^^^^^
+   |
+help: try
+   |
+LL -         Some(Some(x)) if x.starts_with(&[1, 2]) => {},
+LL +         Some(Some([1, 2, ..])) => {},
+   |
+
+error: redundant guard
+  --> $DIR/redundant_guards.rs:243:26
+   |
+LL |         Some(Some(x)) if x.ends_with(&[1, 2]) => {},
+   |                          ^^^^^^^^^^^^^^^^^^^^
+   |
+help: try
+   |
+LL -         Some(Some(x)) if x.ends_with(&[1, 2]) => {},
+LL +         Some(Some([.., 1, 2])) => {},
+   |
+
+error: aborting due to 24 previous errors