Initial impl of `unnecessary_first_then_check`

Fixes #11212

3 files changed, 91 insertions, 0 deletions

diff --git a/tests/ui/unnecessary_first_then_check.fixed b/tests/ui/unnecessary_first_then_check.fixed
new file mode 100644
index 00000000000..7202e1bbd17
--- /dev/null
+++ b/tests/ui/unnecessary_first_then_check.fixed
@@ -0,0 +1,22 @@
+#![warn(clippy::unnecessary_first_then_check)]
+#![allow(clippy::useless_vec, clippy::const_is_empty)]
+
+fn main() {
+    let s = [1, 2, 3];
+    let _: bool = !s.is_empty();
+    let _: bool = s.is_empty();
+
+    let v = vec![1, 2, 3];
+    let _: bool = !v.is_empty();
+
+    let n = [[1, 2, 3], [4, 5, 6]];
+    let _: bool = !n[0].is_empty();
+    let _: bool = n[0].is_empty();
+
+    struct Foo {
+        bar: &'static [i32],
+    }
+    let f = [Foo { bar: &[] }];
+    let _: bool = !f[0].bar.is_empty();
+    let _: bool = f[0].bar.is_empty();
+}
diff --git a/tests/ui/unnecessary_first_then_check.rs b/tests/ui/unnecessary_first_then_check.rs
new file mode 100644
index 00000000000..762b9599928
--- /dev/null
+++ b/tests/ui/unnecessary_first_then_check.rs
@@ -0,0 +1,22 @@
+#![warn(clippy::unnecessary_first_then_check)]
+#![allow(clippy::useless_vec, clippy::const_is_empty)]
+
+fn main() {
+    let s = [1, 2, 3];
+    let _: bool = s.first().is_some();
+    let _: bool = s.first().is_none();
+
+    let v = vec![1, 2, 3];
+    let _: bool = v.first().is_some();
+
+    let n = [[1, 2, 3], [4, 5, 6]];
+    let _: bool = n[0].first().is_some();
+    let _: bool = n[0].first().is_none();
+
+    struct Foo {
+        bar: &'static [i32],
+    }
+    let f = [Foo { bar: &[] }];
+    let _: bool = f[0].bar.first().is_some();
+    let _: bool = f[0].bar.first().is_none();
+}
diff --git a/tests/ui/unnecessary_first_then_check.stderr b/tests/ui/unnecessary_first_then_check.stderr
new file mode 100644
index 00000000000..bbaf7e68eda
--- /dev/null
+++ b/tests/ui/unnecessary_first_then_check.stderr
@@ -0,0 +1,47 @@
+error: unnecessary use of `first().is_some()` to check if slice is not empty
+  --> tests/ui/unnecessary_first_then_check.rs:6:19
+   |
+LL |     let _: bool = s.first().is_some();
+   |                   ^^^^^^^^^^^^^^^^^^^ help: replace this with: `!s.is_empty()`
+   |
+   = note: `-D clippy::unnecessary-first-then-check` implied by `-D warnings`
+   = help: to override `-D warnings` add `#[allow(clippy::unnecessary_first_then_check)]`
+
+error: unnecessary use of `first().is_none()` to check if slice is empty
+  --> tests/ui/unnecessary_first_then_check.rs:7:21
+   |
+LL |     let _: bool = s.first().is_none();
+   |                     ^^^^^^^^^^^^^^^^^ help: replace this with: `is_empty()`
+
+error: unnecessary use of `first().is_some()` to check if slice is not empty
+  --> tests/ui/unnecessary_first_then_check.rs:10:19
+   |
+LL |     let _: bool = v.first().is_some();
+   |                   ^^^^^^^^^^^^^^^^^^^ help: replace this with: `!v.is_empty()`
+
+error: unnecessary use of `first().is_some()` to check if slice is not empty
+  --> tests/ui/unnecessary_first_then_check.rs:13:19
+   |
+LL |     let _: bool = n[0].first().is_some();
+   |                   ^^^^^^^^^^^^^^^^^^^^^^ help: replace this with: `!n[0].is_empty()`
+
+error: unnecessary use of `first().is_none()` to check if slice is empty
+  --> tests/ui/unnecessary_first_then_check.rs:14:24
+   |
+LL |     let _: bool = n[0].first().is_none();
+   |                        ^^^^^^^^^^^^^^^^^ help: replace this with: `is_empty()`
+
+error: unnecessary use of `first().is_some()` to check if slice is not empty
+  --> tests/ui/unnecessary_first_then_check.rs:20:19
+   |
+LL |     let _: bool = f[0].bar.first().is_some();
+   |                   ^^^^^^^^^^^^^^^^^^^^^^^^^^ help: replace this with: `!f[0].bar.is_empty()`
+
+error: unnecessary use of `first().is_none()` to check if slice is empty
+  --> tests/ui/unnecessary_first_then_check.rs:21:28
+   |
+LL |     let _: bool = f[0].bar.first().is_none();
+   |                            ^^^^^^^^^^^^^^^^^ help: replace this with: `is_empty()`
+
+error: aborting due to 7 previous errors
+