Auto merge of #7292 - Jarcho:suspicious_splitn, r=flip1995

Add lint `suspicious_splitn`

fixes: #7245
changelog: Add lint `suspicious_splitn`

tests/ui/single_char_pattern.fixed

@@ -25,8 +25,8 @@ fn main() {
     x.rsplit('x');
     x.split_terminator('x');
     x.rsplit_terminator('x');
-    x.splitn(0, 'x');
-    x.rsplitn(0, 'x');
+    x.splitn(2, 'x');
+    x.rsplitn(2, 'x');
     x.matches('x');
     x.rmatches('x');
     x.match_indices('x');

tests/ui/single_char_pattern.rs

@@ -25,8 +25,8 @@ fn main() {
     x.rsplit("x");
     x.split_terminator("x");
     x.rsplit_terminator("x");
-    x.splitn(0, "x");
-    x.rsplitn(0, "x");
+    x.splitn(2, "x");
+    x.rsplitn(2, "x");
     x.matches("x");
     x.rmatches("x");
     x.match_indices("x");

tests/ui/single_char_pattern.stderr

@@ -75,13 +75,13 @@ LL |     x.rsplit_terminator("x");
 error: single-character string constant used as pattern
   --> $DIR/single_char_pattern.rs:28:17
    |
-LL |     x.splitn(0, "x");
+LL |     x.splitn(2, "x");
    |                 ^^^ help: try using a `char` instead: `'x'`
 
 error: single-character string constant used as pattern
   --> $DIR/single_char_pattern.rs:29:18
    |
-LL |     x.rsplitn(0, "x");
+LL |     x.rsplitn(2, "x");
    |                  ^^^ help: try using a `char` instead: `'x'`
 
 error: single-character string constant used as pattern

tests/ui/suspicious_splitn.rs

@@ -0,0 +1,20 @@
+#![warn(clippy::suspicious_splitn)]
+
+fn main() {
+    let _ = "a,b,c".splitn(3, ',');
+    let _ = [0, 1, 2, 1, 3].splitn(3, |&x| x == 1);
+    let _ = "".splitn(0, ',');
+    let _ = [].splitn(0, |&x: &u32| x == 1);
+
+    let _ = "a,b".splitn(0, ',');
+    let _ = "a,b".rsplitn(0, ',');
+    let _ = "a,b".splitn(1, ',');
+    let _ = [0, 1, 2].splitn(0, |&x| x == 1);
+    let _ = [0, 1, 2].splitn_mut(0, |&x| x == 1);
+    let _ = [0, 1, 2].splitn(1, |&x| x == 1);
+    let _ = [0, 1, 2].rsplitn_mut(1, |&x| x == 1);
+
+    const X: usize = 0;
+    let _ = "a,b".splitn(X + 1, ',');
+    let _ = "a,b".splitn(X, ',');
+}

tests/ui/suspicious_splitn.stderr

@@ -0,0 +1,75 @@
+error: `splitn` called with `0` splits
+  --> $DIR/suspicious_splitn.rs:9:13
+   |
+LL |     let _ = "a,b".splitn(0, ',');
+   |             ^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: `-D clippy::suspicious-splitn` implied by `-D warnings`
+   = note: the resulting iterator will always return `None`
+
+error: `rsplitn` called with `0` splits
+  --> $DIR/suspicious_splitn.rs:10:13
+   |
+LL |     let _ = "a,b".rsplitn(0, ',');
+   |             ^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: the resulting iterator will always return `None`
+
+error: `splitn` called with `1` split
+  --> $DIR/suspicious_splitn.rs:11:13
+   |
+LL |     let _ = "a,b".splitn(1, ',');
+   |             ^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: the resulting iterator will always return the entire string followed by `None`
+
+error: `splitn` called with `0` splits
+  --> $DIR/suspicious_splitn.rs:12:13
+   |
+LL |     let _ = [0, 1, 2].splitn(0, |&x| x == 1);
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: the resulting iterator will always return `None`
+
+error: `splitn_mut` called with `0` splits
+  --> $DIR/suspicious_splitn.rs:13:13
+   |
+LL |     let _ = [0, 1, 2].splitn_mut(0, |&x| x == 1);
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: the resulting iterator will always return `None`
+
+error: `splitn` called with `1` split
+  --> $DIR/suspicious_splitn.rs:14:13
+   |
+LL |     let _ = [0, 1, 2].splitn(1, |&x| x == 1);
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: the resulting iterator will always return the entire slice followed by `None`
+
+error: `rsplitn_mut` called with `1` split
+  --> $DIR/suspicious_splitn.rs:15:13
+   |
+LL |     let _ = [0, 1, 2].rsplitn_mut(1, |&x| x == 1);
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: the resulting iterator will always return the entire slice followed by `None`
+
+error: `splitn` called with `1` split
+  --> $DIR/suspicious_splitn.rs:18:13
+   |
+LL |     let _ = "a,b".splitn(X + 1, ',');
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: the resulting iterator will always return the entire string followed by `None`
+
+error: `splitn` called with `0` splits
+  --> $DIR/suspicious_splitn.rs:19:13
+   |
+LL |     let _ = "a,b".splitn(X, ',');
+   |             ^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: the resulting iterator will always return `None`
+
+error: aborting due to 9 previous errors
+