Auto merge of #11818 - y21:more_redundant_guards, r=llogiq

[`redundant_guards`]: catch `is_empty`, `starts_with` and `ends_with` on slices and `str`s

Fixes #11807

Few things worth mentioning:
- Taking `snippet`s is now done at callsite, instead of passing a span and doing it in `emit_redundant_guards`. This is because we now need custom suggestion strings in certain places, like `""` for `str::is_empty`.
- This now uses `snippet` instead of `snippet_with_applicability`. I don't think this really makes any difference for `MaybeIncorrect`, though?
- This could also lint byte strings, as they're of type `&[u8; N]`, but that can be ugly so I decided to leave it out for now

changelog: [`redundant_guards`]: catch `str::is_empty`, `slice::is_empty`, `slice::starts_with` and `slice::ends_with`

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]) => {},
+        _ => {},
+    }
+}

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]) => {},
+        _ => {},
+    }
+}

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