fix(explicit_counter_loop): suggest .take(n) for for _ in 0..n counter loops

fix(explicit_counter_loop): suggest `.take(n)` for `for _ in 0..n` counter loops

clippy_lints/src/loops/explicit_counter_loop.rs

@@ -2,6 +2,7 @@
 
 use super::{EXPLICIT_COUNTER_LOOP, IncrementVisitor, InitializeVisitor, make_iterator_snippet};
 use clippy_utils::diagnostics::span_lint_and_then;
+use clippy_utils::higher::Range;
 use clippy_utils::source::snippet_with_applicability;
 use clippy_utils::sugg::{EMPTY, Sugg};
 use clippy_utils::{get_enclosing_block, is_integer_const, is_integer_literal_untyped};
@@ -83,6 +84,26 @@ pub(super) fn check<'tcx>(
                     snippet
                 });
 
+                // If the loop variable is unused and the range is `0..n`, suggest `(init..).take(n)`.
+                if pat_snippet == "_"
+                    && let Some(range) = Range::hir(cx, arg)
+                    && range.limits == RangeLimits::HalfOpen
+                    && range.start.is_some_and(|start| is_integer_const(cx, start, 0))
+                    && let Some(end) = range.end
+                {
+                    let end = snippet_with_applicability(cx, end.span, "..", &mut applicability);
+                    diag.span_suggestion(
+                        span,
+                        "consider using",
+                        format!(
+                            "{loop_label}for {name} in ({}).take({end})",
+                            initializer.range(&EMPTY, RangeLimits::HalfOpen)
+                        ),
+                        applicability,
+                    );
+                    return;
+                }
+
                 diag.span_suggestion(
                     span,
                     "consider using",

tests/ui/explicit_counter_loop.rs

@@ -332,3 +332,34 @@ fn add_assign(&mut self, rhs: u8) {
         priority += 1;
     }
 }
+
+pub fn issue_16642() {
+    let mut base = 100;
+    const MAX: usize = 10;
+    for _ in 0..MAX {
+        //~^ explicit_counter_loop
+        base += 1;
+    }
+
+    let mut base = 100;
+
+    let nums = vec![1, 2, 3, 4];
+    for _ in nums {
+        //~^ explicit_counter_loop
+        base += 1;
+    }
+
+    // inclusive range: should not suggest .take()
+    let mut base = 100;
+    for _ in 0..=MAX {
+        //~^ explicit_counter_loop
+        base += 1;
+    }
+
+    // non-zero start: should not suggest .take(), falls through to zip
+    let mut base = 100;
+    for _ in 5..MAX {
+        //~^ explicit_counter_loop
+        base += 1;
+    }
+}

tests/ui/explicit_counter_loop.stderr

@@ -81,5 +81,29 @@ error: the variable `j` is used as a loop counter
 LL |     for item in &v {
    |     ^^^^^^^^^^^^^^ help: consider using: `for (j, item) in (s + 1..).zip(v.iter())`
 
-error: aborting due to 13 previous errors
+error: the variable `base` is used as a loop counter
+  --> tests/ui/explicit_counter_loop.rs:339:5
+   |
+LL |     for _ in 0..MAX {
+   |     ^^^^^^^^^^^^^^^ help: consider using: `for base in (100..).take(MAX)`
+
+error: the variable `base` is used as a loop counter
+  --> tests/ui/explicit_counter_loop.rs:347:5
+   |
+LL |     for _ in nums {
+   |     ^^^^^^^^^^^^^ help: consider using: `for (base, _) in (100..).zip(nums.into_iter())`
+
+error: the variable `base` is used as a loop counter
+  --> tests/ui/explicit_counter_loop.rs:354:5
+   |
+LL |     for _ in 0..=MAX {
+   |     ^^^^^^^^^^^^^^^^ help: consider using: `for (base, _) in (100..).zip((0..=MAX))`
+
+error: the variable `base` is used as a loop counter
+  --> tests/ui/explicit_counter_loop.rs:361:5
+   |
+LL |     for _ in 5..MAX {
+   |     ^^^^^^^^^^^^^^^ help: consider using: `for (base, _) in (100..).zip((5..MAX))`
+
+error: aborting due to 17 previous errors