Auto merge of rust-lang#11778 - granddaifuku:fix/reduce-indexing-manual_memcpy-when-array-length-is-equal-to-range, r=blyxyas

fix: [manual_memcpy] reduce indexing suggestions when array length is equal to loop range

fixes: rust-lang#11689

This PR improves `manual_memcpy` suggestions by reducing unnecessary indexing.

For example,
```rust
let src = [0, 1, 2, 3, 4];
let mut dest = [0; 4];
for i in 0..4 {
    dest[i] = src[i];
{
```
Clippy suggests `dest[..4].copy_from_slice(&src[..4]);`.
I reduced this suggestion as `dest.copy_from_slice(&src[..4]);`. (Removed needless indexing.)

changelog: [`manual_memcpy`]: Reduce indexing suggestions when array length is equal to loop range

Author: bors

clippy_lints/src/loops/manual_memcpy.rs

@@ -178,7 +178,9 @@ fn build_manual_memcpy_suggestion<'tcx>(
     let dst_base_str = snippet(cx, dst.base.span, "???");
     let src_base_str = snippet(cx, src.base.span, "???");
 
-    let dst = if dst_offset == sugg::EMPTY && dst_limit == sugg::EMPTY {
+    let dst = if (dst_offset == sugg::EMPTY && dst_limit == sugg::EMPTY)
+        || is_array_length_equal_to_range(cx, start, end, dst.base)
+    {
         dst_base_str
     } else {
         format!("{dst_base_str}[{}..{}]", dst_offset.maybe_par(), dst_limit.maybe_par()).into()
@@ -190,11 +192,13 @@ fn build_manual_memcpy_suggestion<'tcx>(
         "clone_from_slice"
     };
 
-    format!(
-        "{dst}.{method_str}(&{src_base_str}[{}..{}]);",
-        src_offset.maybe_par(),
-        src_limit.maybe_par()
-    )
+    let src = if is_array_length_equal_to_range(cx, start, end, src.base) {
+        src_base_str
+    } else {
+        format!("{src_base_str}[{}..{}]", src_offset.maybe_par(), src_limit.maybe_par()).into()
+    };
+
+    format!("{dst}.{method_str}(&{src});")
 }
 
 /// a wrapper of `Sugg`. Besides what `Sugg` do, this removes unnecessary `0`;
@@ -452,3 +456,34 @@ fn get_loop_counters<'a, 'tcx>(
             .into()
     })
 }
+
+fn is_array_length_equal_to_range(cx: &LateContext<'_>, start: &Expr<'_>, end: &Expr<'_>, arr: &Expr<'_>) -> bool {
+    fn extract_lit_value(expr: &Expr<'_>) -> Option<u128> {
+        if let ExprKind::Lit(lit) = expr.kind
+            && let ast::LitKind::Int(value, _) = lit.node
+        {
+            Some(value)
+        } else {
+            None
+        }
+    }
+
+    let arr_ty = cx.typeck_results().expr_ty(arr).peel_refs();
+
+    if let ty::Array(_, s) = arr_ty.kind() {
+        let size: u128 = if let Some(size) = s.try_eval_target_usize(cx.tcx, cx.param_env) {
+            size.into()
+        } else {
+            return false;
+        };
+
+        let range = match (extract_lit_value(start), extract_lit_value(end)) {
+            (Some(start_value), Some(end_value)) => end_value - start_value,
+            _ => return false,
+        };
+
+        size == range
+    } else {
+        false
+    }
+}

tests/ui/manual_memcpy/without_loop_counters.rs

@@ -138,6 +138,26 @@ pub fn manual_copy(src: &[i32], dst: &mut [i32], dst2: &mut [i32]) {
     for i in 0..dst.len() {
         dst[i] = src[i];
     }
+
+    // Range is equal to array length
+    let src = [0, 1, 2, 3, 4];
+    let mut dst = [0; 4];
+    for i in 0..4 {
+        //~^ ERROR: it looks like you're manually copying between slices
+        dst[i] = src[i];
+    }
+
+    let mut dst = [0; 6];
+    for i in 0..5 {
+        //~^ ERROR: it looks like you're manually copying between slices
+        dst[i] = src[i];
+    }
+
+    let mut dst = [0; 5];
+    for i in 0..5 {
+        //~^ ERROR: it looks like you're manually copying between slices
+        dst[i] = src[i];
+    }
 }
 
 #[warn(clippy::needless_range_loop, clippy::manual_memcpy)]

tests/ui/manual_memcpy/without_loop_counters.stderr

@@ -106,7 +106,7 @@ LL | /     for i in 0..5 {
 LL | |
 LL | |         dst[i - 0] = src[i];
 LL | |     }
-   | |_____^ help: try replacing the loop by: `dst[..5].copy_from_slice(&src[..5]);`
+   | |_____^ help: try replacing the loop by: `dst[..5].copy_from_slice(&src);`
 
 error: it looks like you're manually copying between slices
   --> $DIR/without_loop_counters.rs:121:5
@@ -120,11 +120,38 @@ LL | |     }
 error: it looks like you're manually copying between slices
   --> $DIR/without_loop_counters.rs:145:5
    |
+LL | /     for i in 0..4 {
+LL | |
+LL | |         dst[i] = src[i];
+LL | |     }
+   | |_____^ help: try replacing the loop by: `dst.copy_from_slice(&src[..4]);`
+
+error: it looks like you're manually copying between slices
+  --> $DIR/without_loop_counters.rs:151:5
+   |
+LL | /     for i in 0..5 {
+LL | |
+LL | |         dst[i] = src[i];
+LL | |     }
+   | |_____^ help: try replacing the loop by: `dst[..5].copy_from_slice(&src);`
+
+error: it looks like you're manually copying between slices
+  --> $DIR/without_loop_counters.rs:157:5
+   |
+LL | /     for i in 0..5 {
+LL | |
+LL | |         dst[i] = src[i];
+LL | |     }
+   | |_____^ help: try replacing the loop by: `dst.copy_from_slice(&src);`
+
+error: it looks like you're manually copying between slices
+  --> $DIR/without_loop_counters.rs:165:5
+   |
 LL | /     for i in 0..src.len() {
 LL | |
 LL | |         dst[i] = src[i].clone();
 LL | |     }
    | |_____^ help: try replacing the loop by: `dst[..src.len()].clone_from_slice(&src[..]);`
 
-error: aborting due to 13 previous errors
+error: aborting due to 16 previous errors