add `manual_slice_fill` lint (#14082)

close #13856

changelog: [`manual_slice_fill`]: new lint

14 files changed, 433 insertions, 35 deletions

diff --git a/CHANGELOG.md b/CHANGELOG.md
index bae091efff0..047f0db0d33 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -5773,6 +5773,7 @@ Released 2018-09-13
 [`manual_retain`]: https://rust-lang.github.io/rust-clippy/master/index.html#manual_retain
 [`manual_rotate`]: https://rust-lang.github.io/rust-clippy/master/index.html#manual_rotate
 [`manual_saturating_arithmetic`]: https://rust-lang.github.io/rust-clippy/master/index.html#manual_saturating_arithmetic
+[`manual_slice_fill`]: https://rust-lang.github.io/rust-clippy/master/index.html#manual_slice_fill
 [`manual_slice_size_calculation`]: https://rust-lang.github.io/rust-clippy/master/index.html#manual_slice_size_calculation
 [`manual_split_once`]: https://rust-lang.github.io/rust-clippy/master/index.html#manual_split_once
 [`manual_str_repeat`]: https://rust-lang.github.io/rust-clippy/master/index.html#manual_str_repeat
diff --git a/book/src/lint_configuration.md b/book/src/lint_configuration.md
index b8f9fff9c61..dab2630a56f 100644
--- a/book/src/lint_configuration.md
+++ b/book/src/lint_configuration.md
@@ -752,6 +752,7 @@ The minimum rust version that the project supports. Defaults to the `rust-versio
 * [`manual_rem_euclid`](https://rust-lang.github.io/rust-clippy/master/index.html#manual_rem_euclid)
 * [`manual_repeat_n`](https://rust-lang.github.io/rust-clippy/master/index.html#manual_repeat_n)
 * [`manual_retain`](https://rust-lang.github.io/rust-clippy/master/index.html#manual_retain)
+* [`manual_slice_fill`](https://rust-lang.github.io/rust-clippy/master/index.html#manual_slice_fill)
 * [`manual_split_once`](https://rust-lang.github.io/rust-clippy/master/index.html#manual_split_once)
 * [`manual_str_repeat`](https://rust-lang.github.io/rust-clippy/master/index.html#manual_str_repeat)
 * [`manual_strip`](https://rust-lang.github.io/rust-clippy/master/index.html#manual_strip)
diff --git a/clippy_config/src/conf.rs b/clippy_config/src/conf.rs
index 552141476f3..a1591188bee 100644
--- a/clippy_config/src/conf.rs
+++ b/clippy_config/src/conf.rs
@@ -621,6 +621,7 @@ define_Conf! {
         manual_rem_euclid,
         manual_repeat_n,
         manual_retain,
+        manual_slice_fill,
         manual_split_once,
         manual_str_repeat,
         manual_strip,
diff --git a/clippy_lints/src/declared_lints.rs b/clippy_lints/src/declared_lints.rs
index af473b0c2ed..6c016473c8a 100644
--- a/clippy_lints/src/declared_lints.rs
+++ b/clippy_lints/src/declared_lints.rs
@@ -292,6 +292,7 @@ pub static LINTS: &[&crate::LintInfo] = &[
     crate::loops::MANUAL_FIND_INFO,
     crate::loops::MANUAL_FLATTEN_INFO,
     crate::loops::MANUAL_MEMCPY_INFO,
+    crate::loops::MANUAL_SLICE_FILL_INFO,
     crate::loops::MANUAL_WHILE_LET_SOME_INFO,
     crate::loops::MISSING_SPIN_LOOP_INFO,
     crate::loops::MUT_RANGE_BOUND_INFO,
diff --git a/clippy_lints/src/loops/manual_slice_fill.rs b/clippy_lints/src/loops/manual_slice_fill.rs
new file mode 100644
index 00000000000..7c656423579
--- /dev/null
+++ b/clippy_lints/src/loops/manual_slice_fill.rs
@@ -0,0 +1,111 @@
+use clippy_utils::diagnostics::span_lint_and_sugg;
+use clippy_utils::eager_or_lazy::switch_to_eager_eval;
+use clippy_utils::macros::span_is_local;
+use clippy_utils::msrvs::{self, Msrv};
+use clippy_utils::source::{HasSession, snippet_with_applicability};
+use clippy_utils::ty::implements_trait;
+use clippy_utils::{higher, peel_blocks_with_stmt, span_contains_comment};
+use rustc_ast::ast::LitKind;
+use rustc_ast::{RangeLimits, UnOp};
+use rustc_data_structures::packed::Pu128;
+use rustc_errors::Applicability;
+use rustc_hir::QPath::Resolved;
+use rustc_hir::def::Res;
+use rustc_hir::{Expr, ExprKind, Pat};
+use rustc_lint::LateContext;
+use rustc_span::source_map::Spanned;
+use rustc_span::sym;
+
+use super::MANUAL_SLICE_FILL;
+
+pub(super) fn check<'tcx>(
+    cx: &LateContext<'tcx>,
+    pat: &'tcx Pat<'_>,
+    arg: &'tcx Expr<'_>,
+    body: &'tcx Expr<'_>,
+    expr: &'tcx Expr<'_>,
+    msrv: &Msrv,
+) {
+    if !msrv.meets(msrvs::SLICE_FILL) {
+        return;
+    }
+
+    // `for _ in 0..slice.len() { slice[_] = value; }`
+    if let Some(higher::Range {
+        start: Some(start),
+        end: Some(end),
+        limits: RangeLimits::HalfOpen,
+    }) = higher::Range::hir(arg)
+        && let ExprKind::Lit(Spanned {
+            node: LitKind::Int(Pu128(0), _),
+            ..
+        }) = start.kind
+        && let ExprKind::Block(..) = body.kind
+        // Check if the body is an assignment to a slice element.
+        && let ExprKind::Assign(assignee, assignval, _) = peel_blocks_with_stmt(body).kind
+        && let ExprKind::Index(slice, _, _) = assignee.kind
+        // Check if `len()` is used for the range end.
+        && let ExprKind::MethodCall(path, recv,..) = end.kind
+        && path.ident.name == sym::len
+        // Check if the slice which is being assigned to is the same as the one being iterated over.
+        && let ExprKind::Path(Resolved(_, recv_path)) = recv.kind
+        && let ExprKind::Path(Resolved(_, slice_path)) = slice.kind
+        && recv_path.res == slice_path.res
+        && !assignval.span.from_expansion()
+        // It is generally not equivalent to use the `fill` method if `assignval` can have side effects
+        && switch_to_eager_eval(cx, assignval)
+        && span_is_local(assignval.span)
+        // The `fill` method requires that the slice's element type implements the `Clone` trait.
+        && let Some(clone_trait) = cx.tcx.lang_items().clone_trait()
+        && implements_trait(cx, cx.typeck_results().expr_ty(slice), clone_trait, &[])
+    {
+        sugg(cx, body, expr, slice.span, assignval.span);
+    }
+    // `for _ in &mut slice { *_ = value; }`
+    else if let ExprKind::AddrOf(_, _, recv) = arg.kind
+        // Check if the body is an assignment to a slice element.
+        && let ExprKind::Assign(assignee, assignval, _) = peel_blocks_with_stmt(body).kind
+        && let ExprKind::Unary(UnOp::Deref, slice_iter) = assignee.kind
+        && let ExprKind::Path(Resolved(_, recv_path)) = recv.kind
+        // Check if the slice which is being assigned to is the same as the one being iterated over.
+        && let ExprKind::Path(Resolved(_, slice_path)) = slice_iter.kind
+        && let Res::Local(local) = slice_path.res
+        && local == pat.hir_id
+        && !assignval.span.from_expansion()
+        && switch_to_eager_eval(cx, assignval)
+        && span_is_local(assignval.span)
+        // The `fill` method cannot be used if the slice's element type does not implement the `Clone` trait.
+        && let Some(clone_trait) = cx.tcx.lang_items().clone_trait()
+        && implements_trait(cx, cx.typeck_results().expr_ty(recv), clone_trait, &[])
+    {
+        sugg(cx, body, expr, recv_path.span, assignval.span);
+    }
+}
+
+fn sugg<'tcx>(
+    cx: &LateContext<'tcx>,
+    body: &'tcx Expr<'_>,
+    expr: &'tcx Expr<'_>,
+    slice_span: rustc_span::Span,
+    assignval_span: rustc_span::Span,
+) {
+    let mut app = if span_contains_comment(cx.sess().source_map(), body.span) {
+        Applicability::MaybeIncorrect // Comments may be informational.
+    } else {
+        Applicability::MachineApplicable
+    };
+
+    span_lint_and_sugg(
+        cx,
+        MANUAL_SLICE_FILL,
+        expr.span,
+        "manually filling a slice",
+        "try",
+        format!(
+            "{}.fill({});",
+            snippet_with_applicability(cx, slice_span, "..", &mut app),
+            snippet_with_applicability(cx, assignval_span, "..", &mut app),
+        ),
+        app,
+    );
+}
diff --git a/clippy_lints/src/loops/mod.rs b/clippy_lints/src/loops/mod.rs
index c5e75af2303..cdc8c18c3b7 100644
--- a/clippy_lints/src/loops/mod.rs
+++ b/clippy_lints/src/loops/mod.rs
@@ -8,6 +8,7 @@ mod iter_next_loop;
 mod manual_find;
 mod manual_flatten;
 mod manual_memcpy;
+mod manual_slice_fill;
 mod manual_while_let_some;
 mod missing_spin_loop;
 mod mut_range_bound;
@@ -714,6 +715,31 @@ declare_clippy_lint! {
     "possibly unintended infinite loop"
 }
 
+declare_clippy_lint! {
+    /// ### What it does
+    /// Checks for manually filling a slice with a value.
+    ///
+    /// ### Why is this bad?
+    /// Using the `fill` method is more idiomatic and concise.
+    ///
+    /// ### Example
+    /// ```no_run
+    /// let mut some_slice = [1, 2, 3, 4, 5];
+    /// for i in 0..some_slice.len() {
+    ///     some_slice[i] = 0;
+    /// }
+    /// ```
+    /// Use instead:
+    /// ```no_run
+    /// let mut some_slice = [1, 2, 3, 4, 5];
+    /// some_slice.fill(0);
+    /// ```
+    #[clippy::version = "1.86.0"]
+    pub MANUAL_SLICE_FILL,
+    style,
+    "manually filling a slice with a value"
+}
+
 pub struct Loops {
     msrv: Msrv,
     enforce_iter_loop_reborrow: bool,
@@ -750,6 +776,7 @@ impl_lint_pass!(Loops => [
     MANUAL_WHILE_LET_SOME,
     UNUSED_ENUMERATE_INDEX,
     INFINITE_LOOP,
+    MANUAL_SLICE_FILL,
 ]);
 
 impl<'tcx> LateLintPass<'tcx> for Loops {
@@ -823,6 +850,7 @@ impl Loops {
     ) {
         let is_manual_memcpy_triggered = manual_memcpy::check(cx, pat, arg, body, expr);
         if !is_manual_memcpy_triggered {
+            manual_slice_fill::check(cx, pat, arg, body, expr, &self.msrv);
             needless_range_loop::check(cx, pat, arg, body, expr);
             explicit_counter_loop::check(cx, pat, arg, body, expr, label);
         }
diff --git a/clippy_utils/src/msrvs.rs b/clippy_utils/src/msrvs.rs
index d73cb7e3561..6a1832d07e1 100644
--- a/clippy_utils/src/msrvs.rs
+++ b/clippy_utils/src/msrvs.rs
@@ -40,7 +40,7 @@ msrv_aliases! {
     1,53,0 { OR_PATTERNS, MANUAL_BITS, BTREE_MAP_RETAIN, BTREE_SET_RETAIN, ARRAY_INTO_ITERATOR }
     1,52,0 { STR_SPLIT_ONCE, REM_EUCLID_CONST }
     1,51,0 { BORROW_AS_PTR, SEEK_FROM_CURRENT, UNSIGNED_ABS }
-    1,50,0 { BOOL_THEN, CLAMP }
+    1,50,0 { BOOL_THEN, CLAMP, SLICE_FILL }
     1,47,0 { TAU, IS_ASCII_DIGIT_CONST, ARRAY_IMPL_ANY_LEN, SATURATING_SUB_CONST }
     1,46,0 { CONST_IF_MATCH }
     1,45,0 { STR_STRIP_PREFIX }
diff --git a/tests/ui/manual_memcpy/without_loop_counters.rs b/tests/ui/manual_memcpy/without_loop_counters.rs
index c917fa7f2d0..2f8640cd3f5 100644
--- a/tests/ui/manual_memcpy/without_loop_counters.rs
+++ b/tests/ui/manual_memcpy/without_loop_counters.rs
@@ -1,5 +1,10 @@
 #![warn(clippy::manual_memcpy)]
-#![allow(clippy::assigning_clones, clippy::useless_vec, clippy::needless_range_loop)]
+#![allow(
+    clippy::assigning_clones,
+    clippy::useless_vec,
+    clippy::needless_range_loop,
+    clippy::manual_slice_fill
+)]
 
 //@no-rustfix
 const LOOP_OFFSET: usize = 5000;
diff --git a/tests/ui/manual_memcpy/without_loop_counters.stderr b/tests/ui/manual_memcpy/without_loop_counters.stderr
index 803053b2db2..c881e3fac76 100644
--- a/tests/ui/manual_memcpy/without_loop_counters.stderr
+++ b/tests/ui/manual_memcpy/without_loop_counters.stderr
@@ -1,5 +1,5 @@
 error: it looks like you're manually copying between slices
-  --> tests/ui/manual_memcpy/without_loop_counters.rs:9:5
+  --> tests/ui/manual_memcpy/without_loop_counters.rs:14:5
    |
 LL | /     for i in 0..src.len() {
 LL | |
@@ -12,7 +12,7 @@ LL | |     }
    = help: to override `-D warnings` add `#[allow(clippy::manual_memcpy)]`
 
 error: it looks like you're manually copying between slices
-  --> tests/ui/manual_memcpy/without_loop_counters.rs:16:5
+  --> tests/ui/manual_memcpy/without_loop_counters.rs:21:5
    |
 LL | /     for i in 0..src.len() {
 LL | |
@@ -21,7 +21,7 @@ LL | |     }
    | |_____^ help: try replacing the loop by: `dst[10..(src.len() + 10)].copy_from_slice(&src[..]);`
 
 error: it looks like you're manually copying between slices
-  --> tests/ui/manual_memcpy/without_loop_counters.rs:22:5
+  --> tests/ui/manual_memcpy/without_loop_counters.rs:27:5
    |
 LL | /     for i in 0..src.len() {
 LL | |
@@ -30,7 +30,7 @@ LL | |     }
    | |_____^ help: try replacing the loop by: `dst[..src.len()].copy_from_slice(&src[10..(src.len() + 10)]);`
 
 error: it looks like you're manually copying between slices
-  --> tests/ui/manual_memcpy/without_loop_counters.rs:28:5
+  --> tests/ui/manual_memcpy/without_loop_counters.rs:33:5
    |
 LL | /     for i in 11..src.len() {
 LL | |
@@ -39,7 +39,7 @@ LL | |     }
    | |_____^ help: try replacing the loop by: `dst[11..src.len()].copy_from_slice(&src[(11 - 10)..(src.len() - 10)]);`
 
 error: it looks like you're manually copying between slices
-  --> tests/ui/manual_memcpy/without_loop_counters.rs:34:5
+  --> tests/ui/manual_memcpy/without_loop_counters.rs:39:5
    |
 LL | /     for i in 0..dst.len() {
 LL | |
@@ -48,7 +48,7 @@ LL | |     }
    | |_____^ help: try replacing the loop by: `dst.copy_from_slice(&src[..dst.len()]);`
 
 error: it looks like you're manually copying between slices
-  --> tests/ui/manual_memcpy/without_loop_counters.rs:48:5
+  --> tests/ui/manual_memcpy/without_loop_counters.rs:53:5
    |
 LL | /     for i in 10..256 {
 LL | |
@@ -64,7 +64,7 @@ LL +     dst2[(10 + 500)..(256 + 500)].copy_from_slice(&src[10..256]);
    |
 
 error: it looks like you're manually copying between slices
-  --> tests/ui/manual_memcpy/without_loop_counters.rs:61:5
+  --> tests/ui/manual_memcpy/without_loop_counters.rs:66:5
    |
 LL | /     for i in 10..LOOP_OFFSET {
 LL | |
@@ -73,7 +73,7 @@ LL | |     }
    | |_____^ help: try replacing the loop by: `dst[(10 + LOOP_OFFSET)..(LOOP_OFFSET + LOOP_OFFSET)].copy_from_slice(&src[(10 - some_var)..(LOOP_OFFSET - some_var)]);`
 
 error: it looks like you're manually copying between slices
-  --> tests/ui/manual_memcpy/without_loop_counters.rs:75:5
+  --> tests/ui/manual_memcpy/without_loop_counters.rs:80:5
    |
 LL | /     for i in 0..src_vec.len() {
 LL | |
@@ -82,7 +82,7 @@ LL | |     }
    | |_____^ help: try replacing the loop by: `dst_vec[..src_vec.len()].copy_from_slice(&src_vec[..]);`
 
 error: it looks like you're manually copying between slices
-  --> tests/ui/manual_memcpy/without_loop_counters.rs:105:5
+  --> tests/ui/manual_memcpy/without_loop_counters.rs:110:5
    |
 LL | /     for i in from..from + src.len() {
 LL | |
@@ -91,7 +91,7 @@ LL | |     }
    | |_____^ help: try replacing the loop by: `dst[from..(from + src.len())].copy_from_slice(&src[..(from + src.len() - from)]);`
 
 error: it looks like you're manually copying between slices
-  --> tests/ui/manual_memcpy/without_loop_counters.rs:110:5
+  --> tests/ui/manual_memcpy/without_loop_counters.rs:115:5
    |
 LL | /     for i in from..from + 3 {
 LL | |
@@ -100,7 +100,7 @@ LL | |     }
    | |_____^ help: try replacing the loop by: `dst[from..(from + 3)].copy_from_slice(&src[..(from + 3 - from)]);`
 
 error: it looks like you're manually copying between slices
-  --> tests/ui/manual_memcpy/without_loop_counters.rs:116:5
+  --> tests/ui/manual_memcpy/without_loop_counters.rs:121:5
    |
 LL | /     for i in 0..5 {
 LL | |
@@ -109,7 +109,7 @@ LL | |     }
    | |_____^ help: try replacing the loop by: `dst[..5].copy_from_slice(&src);`
 
 error: it looks like you're manually copying between slices
-  --> tests/ui/manual_memcpy/without_loop_counters.rs:122:5
+  --> tests/ui/manual_memcpy/without_loop_counters.rs:127:5
    |
 LL | /     for i in 0..0 {
 LL | |
@@ -118,7 +118,7 @@ LL | |     }
    | |_____^ help: try replacing the loop by: `dst[..0].copy_from_slice(&src[..0]);`
 
 error: it looks like you're manually copying between slices
-  --> tests/ui/manual_memcpy/without_loop_counters.rs:146:5
+  --> tests/ui/manual_memcpy/without_loop_counters.rs:151:5
    |
 LL | /     for i in 0..4 {
 LL | |
@@ -127,7 +127,7 @@ LL | |     }
    | |_____^ help: try replacing the loop by: `dst.copy_from_slice(&src[..4]);`
 
 error: it looks like you're manually copying between slices
-  --> tests/ui/manual_memcpy/without_loop_counters.rs:152:5
+  --> tests/ui/manual_memcpy/without_loop_counters.rs:157:5
    |
 LL | /     for i in 0..5 {
 LL | |
@@ -136,7 +136,7 @@ LL | |     }
    | |_____^ help: try replacing the loop by: `dst[..5].copy_from_slice(&src);`
 
 error: it looks like you're manually copying between slices
-  --> tests/ui/manual_memcpy/without_loop_counters.rs:158:5
+  --> tests/ui/manual_memcpy/without_loop_counters.rs:163:5
    |
 LL | /     for i in 0..5 {
 LL | |
@@ -145,7 +145,7 @@ LL | |     }
    | |_____^ help: try replacing the loop by: `dst.copy_from_slice(&src);`
 
 error: it looks like you're manually copying between slices
-  --> tests/ui/manual_memcpy/without_loop_counters.rs:205:5
+  --> tests/ui/manual_memcpy/without_loop_counters.rs:210:5
    |
 LL | /     for i in 0..5 {
 LL | |
@@ -154,7 +154,7 @@ LL | |     }
    | |_____^ help: try replacing the loop by: `dst.copy_from_slice(&src[0]);`
 
 error: it looks like you're manually copying between slices
-  --> tests/ui/manual_memcpy/without_loop_counters.rs:211:5
+  --> tests/ui/manual_memcpy/without_loop_counters.rs:216:5
    |
 LL | /     for i in 0..5 {
 LL | |
@@ -163,7 +163,7 @@ LL | |     }
    | |_____^ help: try replacing the loop by: `dst.copy_from_slice(&src[0][1]);`
 
 error: it looks like you're manually copying between slices
-  --> tests/ui/manual_memcpy/without_loop_counters.rs:219:5
+  --> tests/ui/manual_memcpy/without_loop_counters.rs:224:5
    |
 LL | /     for i in 0..src.len() {
 LL | |
diff --git a/tests/ui/manual_slice_fill.fixed b/tests/ui/manual_slice_fill.fixed
new file mode 100644
index 00000000000..397a156a2dc
--- /dev/null
+++ b/tests/ui/manual_slice_fill.fixed
@@ -0,0 +1,101 @@
+#![warn(clippy::manual_slice_fill)]
+#![allow(clippy::needless_range_loop)]
+
+macro_rules! assign_element {
+    ($slice:ident, $index:expr) => {
+        $slice[$index] = 0;
+    };
+}
+
+macro_rules! assign_element_2 {
+    ($i:expr) => {
+        $i = 0;
+    };
+}
+
+struct NoClone;
+
+fn num() -> usize {
+    5
+}
+
+fn should_lint() {
+    let mut some_slice = [1, 2, 3, 4, 5];
+
+    some_slice.fill(0);
+
+    let x = 5;
+    some_slice.fill(x);
+
+    some_slice.fill(0);
+
+    // This should trigger `manual_slice_fill`, but the applicability is `MaybeIncorrect` since comments
+    // within the loop might be purely informational.
+    some_slice.fill(0);
+}
+
+fn should_not_lint() {
+    let mut some_slice = [1, 2, 3, 4, 5];
+
+    // Should not lint because we can't determine if the scope of the loop is intended to access all the
+    // elements of the slice.
+    for i in 0..5 {
+        some_slice[i] = 0;
+    }
+
+    // Should not lint, as using a function to assign values to elements might be
+    // intentional. For example, the contents of `num()` could be temporary and subject to change
+    // later.
+    for i in 0..some_slice.len() {
+        some_slice[i] = num();
+    }
+
+    // Should not lint because this loop isn't equivalent to `fill`.
+    for i in 0..some_slice.len() {
+        some_slice[i] = 0;
+        println!("foo");
+    }
+
+    // Should not lint because it may be intentional to use a macro to perform an operation equivalent
+    // to `fill`.
+    for i in 0..some_slice.len() {
+        assign_element!(some_slice, i);
+    }
+
+    let another_slice = [1, 2, 3];
+    // Should not lint because the range is not for `some_slice`.
+    for i in 0..another_slice.len() {
+        some_slice[i] = 0;
+    }
+
+    let mut vec: Vec<Option<NoClone>> = Vec::with_capacity(5);
+    // Should not lint because `NoClone` does not have `Clone` trait.
+    for i in 0..vec.len() {
+        vec[i] = None;
+    }
+
+    // Should not lint, as using a function to assign values to elements might be
+    // intentional. For example, the contents of `num()` could be temporary and subject to change
+    // later.
+    for i in &mut some_slice {
+        *i = num();
+    }
+
+    // Should not lint because this loop isn't equivalent to `fill`.
+    for i in &mut some_slice {
+        *i = 0;
+        println!("foo");
+    }
+
+    // Should not lint because it may be intentional to use a macro to perform an operation equivalent
+    // to `fill`.
+    for i in &mut some_slice {
+        assign_element_2!(*i);
+    }
+
+    let mut vec: Vec<Option<NoClone>> = Vec::with_capacity(5);
+    // Should not lint because `NoClone` does not have `Clone` trait.
+    for i in &mut vec {
+        *i = None;
+    }
+}
diff --git a/tests/ui/manual_slice_fill.rs b/tests/ui/manual_slice_fill.rs
new file mode 100644
index 00000000000..c25127ca613
--- /dev/null
+++ b/tests/ui/manual_slice_fill.rs
@@ -0,0 +1,110 @@
+#![warn(clippy::manual_slice_fill)]
+#![allow(clippy::needless_range_loop)]
+
+macro_rules! assign_element {
+    ($slice:ident, $index:expr) => {
+        $slice[$index] = 0;
+    };
+}
+
+macro_rules! assign_element_2 {
+    ($i:expr) => {
+        $i = 0;
+    };
+}
+
+struct NoClone;
+
+fn num() -> usize {
+    5
+}
+
+fn should_lint() {
+    let mut some_slice = [1, 2, 3, 4, 5];
+
+    for i in 0..some_slice.len() {
+        some_slice[i] = 0;
+    }
+
+    let x = 5;
+    for i in 0..some_slice.len() {
+        some_slice[i] = x;
+    }
+
+    for i in &mut some_slice {
+        *i = 0;
+    }
+
+    // This should trigger `manual_slice_fill`, but the applicability is `MaybeIncorrect` since comments
+    // within the loop might be purely informational.
+    for i in 0..some_slice.len() {
+        some_slice[i] = 0;
+        // foo
+    }
+}
+
+fn should_not_lint() {
+    let mut some_slice = [1, 2, 3, 4, 5];
+
+    // Should not lint because we can't determine if the scope of the loop is intended to access all the
+    // elements of the slice.
+    for i in 0..5 {
+        some_slice[i] = 0;
+    }
+
+    // Should not lint, as using a function to assign values to elements might be
+    // intentional. For example, the contents of `num()` could be temporary and subject to change
+    // later.
+    for i in 0..some_slice.len() {
+        some_slice[i] = num();
+    }
+
+    // Should not lint because this loop isn't equivalent to `fill`.
+    for i in 0..some_slice.len() {
+        some_slice[i] = 0;
+        println!("foo");
+    }
+
+    // Should not lint because it may be intentional to use a macro to perform an operation equivalent
+    // to `fill`.
+    for i in 0..some_slice.len() {
+        assign_element!(some_slice, i);
+    }
+
+    let another_slice = [1, 2, 3];
+    // Should not lint because the range is not for `some_slice`.
+    for i in 0..another_slice.len() {
+        some_slice[i] = 0;
+    }
+
+    let mut vec: Vec<Option<NoClone>> = Vec::with_capacity(5);
+    // Should not lint because `NoClone` does not have `Clone` trait.
+    for i in 0..vec.len() {
+        vec[i] = None;
+    }
+
+    // Should not lint, as using a function to assign values to elements might be
+    // intentional. For example, the contents of `num()` could be temporary and subject to change
+    // later.
+    for i in &mut some_slice {
+        *i = num();
+    }
+
+    // Should not lint because this loop isn't equivalent to `fill`.
+    for i in &mut some_slice {
+        *i = 0;
+        println!("foo");
+    }
+
+    // Should not lint because it may be intentional to use a macro to perform an operation equivalent
+    // to `fill`.
+    for i in &mut some_slice {
+        assign_element_2!(*i);
+    }
+
+    let mut vec: Vec<Option<NoClone>> = Vec::with_capacity(5);
+    // Should not lint because `NoClone` does not have `Clone` trait.
+    for i in &mut vec {
+        *i = None;
+    }
+}
diff --git a/tests/ui/manual_slice_fill.stderr b/tests/ui/manual_slice_fill.stderr
new file mode 100644
index 00000000000..3aa980f6919
--- /dev/null
+++ b/tests/ui/manual_slice_fill.stderr
@@ -0,0 +1,38 @@
+error: manually filling a slice
+  --> tests/ui/manual_slice_fill.rs:25:5
+   |
+LL | /     for i in 0..some_slice.len() {
+LL | |         some_slice[i] = 0;
+LL | |     }
+   | |_____^ help: try: `some_slice.fill(0);`
+   |
+   = note: `-D clippy::manual-slice-fill` implied by `-D warnings`
+   = help: to override `-D warnings` add `#[allow(clippy::manual_slice_fill)]`
+
+error: manually filling a slice
+  --> tests/ui/manual_slice_fill.rs:30:5
+   |
+LL | /     for i in 0..some_slice.len() {
+LL | |         some_slice[i] = x;
+LL | |     }
+   | |_____^ help: try: `some_slice.fill(x);`
+
+error: manually filling a slice
+  --> tests/ui/manual_slice_fill.rs:34:5
+   |
+LL | /     for i in &mut some_slice {
+LL | |         *i = 0;
+LL | |     }
+   | |_____^ help: try: `some_slice.fill(0);`
+
+error: manually filling a slice
+  --> tests/ui/manual_slice_fill.rs:40:5
+   |
+LL | /     for i in 0..some_slice.len() {
+LL | |         some_slice[i] = 0;
+LL | |         // foo
+LL | |     }
+   | |_____^ help: try: `some_slice.fill(0);`
+
+error: aborting due to 4 previous errors
+
diff --git a/tests/ui/needless_range_loop.rs b/tests/ui/needless_range_loop.rs
index 3f242195330..75f1896eded 100644
--- a/tests/ui/needless_range_loop.rs
+++ b/tests/ui/needless_range_loop.rs
@@ -2,7 +2,8 @@
 #![allow(
     clippy::uninlined_format_args,
     clippy::unnecessary_literal_unwrap,
-    clippy::useless_vec
+    clippy::useless_vec,
+    clippy::manual_slice_fill
 )]
 //@no-rustfix
 static STATIC: [usize; 4] = [0, 1, 8, 16];
diff --git a/tests/ui/needless_range_loop.stderr b/tests/ui/needless_range_loop.stderr
index dc2cf437e02..503d796e5e8 100644
--- a/tests/ui/needless_range_loop.stderr
+++ b/tests/ui/needless_range_loop.stderr
@@ -1,5 +1,5 @@
 error: the loop variable `i` is only used to index `vec`
-  --> tests/ui/needless_range_loop.rs:15:14
+  --> tests/ui/needless_range_loop.rs:16:14
    |
 LL |     for i in 0..vec.len() {
    |              ^^^^^^^^^^^^
@@ -12,7 +12,7 @@ LL |     for <item> in &vec {
    |         ~~~~~~    ~~~~
 
 error: the loop variable `i` is only used to index `vec`
-  --> tests/ui/needless_range_loop.rs:26:14
+  --> tests/ui/needless_range_loop.rs:27:14
    |
 LL |     for i in 0..vec.len() {
    |              ^^^^^^^^^^^^
@@ -23,7 +23,7 @@ LL |     for <item> in &vec {
    |         ~~~~~~    ~~~~
 
 error: the loop variable `j` is only used to index `STATIC`
-  --> tests/ui/needless_range_loop.rs:32:14
+  --> tests/ui/needless_range_loop.rs:33:14
    |
 LL |     for j in 0..4 {
    |              ^^^^
@@ -34,7 +34,7 @@ LL |     for <item> in &STATIC {
    |         ~~~~~~    ~~~~~~~
 
 error: the loop variable `j` is only used to index `CONST`
-  --> tests/ui/needless_range_loop.rs:37:14
+  --> tests/ui/needless_range_loop.rs:38:14
    |
 LL |     for j in 0..4 {
    |              ^^^^
@@ -45,7 +45,7 @@ LL |     for <item> in &CONST {
    |         ~~~~~~    ~~~~~~
 
 error: the loop variable `i` is used to index `vec`
-  --> tests/ui/needless_range_loop.rs:42:14
+  --> tests/ui/needless_range_loop.rs:43:14
    |
 LL |     for i in 0..vec.len() {
    |              ^^^^^^^^^^^^
@@ -56,7 +56,7 @@ LL |     for (i, <item>) in vec.iter().enumerate() {
    |         ~~~~~~~~~~~    ~~~~~~~~~~~~~~~~~~~~~~
 
 error: the loop variable `i` is only used to index `vec2`
-  --> tests/ui/needless_range_loop.rs:51:14
+  --> tests/ui/needless_range_loop.rs:52:14
    |
 LL |     for i in 0..vec.len() {
    |              ^^^^^^^^^^^^
@@ -67,7 +67,7 @@ LL |     for <item> in vec2.iter().take(vec.len()) {
    |         ~~~~~~    ~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 error: the loop variable `i` is only used to index `vec`
-  --> tests/ui/needless_range_loop.rs:56:14
+  --> tests/ui/needless_range_loop.rs:57:14
    |
 LL |     for i in 5..vec.len() {
    |              ^^^^^^^^^^^^
@@ -78,7 +78,7 @@ LL |     for <item> in vec.iter().skip(5) {
    |         ~~~~~~    ~~~~~~~~~~~~~~~~~~
 
 error: the loop variable `i` is only used to index `vec`
-  --> tests/ui/needless_range_loop.rs:61:14
+  --> tests/ui/needless_range_loop.rs:62:14
    |
 LL |     for i in 0..MAX_LEN {
    |              ^^^^^^^^^^
@@ -89,7 +89,7 @@ LL |     for <item> in vec.iter().take(MAX_LEN) {
    |         ~~~~~~    ~~~~~~~~~~~~~~~~~~~~~~~~
 
 error: the loop variable `i` is only used to index `vec`
-  --> tests/ui/needless_range_loop.rs:66:14
+  --> tests/ui/needless_range_loop.rs:67:14
    |
 LL |     for i in 0..=MAX_LEN {
    |              ^^^^^^^^^^^
@@ -100,7 +100,7 @@ LL |     for <item> in vec.iter().take(MAX_LEN + 1) {
    |         ~~~~~~    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 error: the loop variable `i` is only used to index `vec`
-  --> tests/ui/needless_range_loop.rs:71:14
+  --> tests/ui/needless_range_loop.rs:72:14
    |
 LL |     for i in 5..10 {
    |              ^^^^^
@@ -111,7 +111,7 @@ LL |     for <item> in vec.iter().take(10).skip(5) {
    |         ~~~~~~    ~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 error: the loop variable `i` is only used to index `vec`
-  --> tests/ui/needless_range_loop.rs:76:14
+  --> tests/ui/needless_range_loop.rs:77:14
    |
 LL |     for i in 5..=10 {
    |              ^^^^^^
@@ -122,7 +122,7 @@ LL |     for <item> in vec.iter().take(10 + 1).skip(5) {
    |         ~~~~~~    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 error: the loop variable `i` is used to index `vec`
-  --> tests/ui/needless_range_loop.rs:81:14
+  --> tests/ui/needless_range_loop.rs:82:14
    |
 LL |     for i in 5..vec.len() {
    |              ^^^^^^^^^^^^
@@ -133,7 +133,7 @@ LL |     for (i, <item>) in vec.iter().enumerate().skip(5) {
    |         ~~~~~~~~~~~    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 error: the loop variable `i` is used to index `vec`
-  --> tests/ui/needless_range_loop.rs:86:14
+  --> tests/ui/needless_range_loop.rs:87:14
    |
 LL |     for i in 5..10 {
    |              ^^^^^
@@ -144,7 +144,7 @@ LL |     for (i, <item>) in vec.iter().enumerate().take(10).skip(5) {
    |         ~~~~~~~~~~~    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 error: the loop variable `i` is used to index `vec`
-  --> tests/ui/needless_range_loop.rs:92:14
+  --> tests/ui/needless_range_loop.rs:93:14
    |
 LL |     for i in 0..vec.len() {
    |              ^^^^^^^^^^^^