Auto merge of #12010 - granddaifuku:fix/manual-memcpy-indexing-for-multi-dimension-arrays, r=Alexendoo

fix: `manual_memcpy` wrong indexing for multi dimensional arrays

fixes: #9334

This PR fixes an invalid suggestion for multi-dimensional arrays.

For example,
```rust
let src = vec![vec![0; 5]; 5];
let mut dst = vec![0; 5];

for i in 0..5 {
    dst[i] = src[i][i];
}
```

For the above code, Clippy suggests `dst.copy_from_slice(&src[i]);`, but it is not compilable because `i` is only used to loop the array.
I adjusted it so that Clippy `manual_memcpy` works properly for multi-dimensional arrays.

changelog: [`manual_memcpy`]: Fixes invalid indexing suggestions for multi-dimensional arrays

tests/ui/manual_memcpy/without_loop_counters.rs

@@ -1,5 +1,5 @@
-#![warn(clippy::needless_range_loop, clippy::manual_memcpy)]
-#![allow(clippy::useless_vec)]
+#![warn(clippy::manual_memcpy)]
+#![allow(clippy::useless_vec, clippy::needless_range_loop)]
 //@no-rustfix
 const LOOP_OFFSET: usize = 5000;
 
@@ -158,6 +158,59 @@ pub fn manual_copy(src: &[i32], dst: &mut [i32], dst2: &mut [i32]) {
         //~^ ERROR: it looks like you're manually copying between slices
         dst[i] = src[i];
     }
+
+    // Don't trigger lint for following multi-dimensional arrays
+    let src = [[0; 5]; 5];
+    for i in 0..4 {
+        dst[i] = src[i + 1][i];
+    }
+    for i in 0..5 {
+        dst[i] = src[i][i];
+    }
+    for i in 0..5 {
+        dst[i] = src[i][3];
+    }
+
+    let src = [0; 5];
+    let mut dst = [[0; 5]; 5];
+    for i in 0..5 {
+        dst[i][i] = src[i];
+    }
+
+    let src = [[[0; 5]; 5]; 5];
+    let mut dst = [0; 5];
+    for i in 0..5 {
+        dst[i] = src[i][i][i];
+    }
+    for i in 0..5 {
+        dst[i] = src[i][i][0];
+    }
+    for i in 0..5 {
+        dst[i] = src[i][0][i];
+    }
+    for i in 0..5 {
+        dst[i] = src[0][i][i];
+    }
+    for i in 0..5 {
+        dst[i] = src[0][i][1];
+    }
+    for i in 0..5 {
+        dst[i] = src[i][0][1];
+    }
+
+    // Trigger lint
+    let src = [[0; 5]; 5];
+    let mut dst = [0; 5];
+    for i in 0..5 {
+        //~^ ERROR: it looks like you're manually copying between slices
+        dst[i] = src[0][i];
+    }
+
+    let src = [[[0; 5]; 5]; 5];
+    for i in 0..5 {
+        //~^ ERROR: it looks like you're manually copying between slices
+        dst[i] = src[0][1][i];
+    }
 }
 
 #[warn(clippy::needless_range_loop, clippy::manual_memcpy)]

tests/ui/manual_memcpy/without_loop_counters.stderr

@@ -145,7 +145,25 @@ 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:165:5
+  --> tests/ui/manual_memcpy/without_loop_counters.rs:204:5
+   |
+LL | /     for i in 0..5 {
+LL | |
+LL | |         dst[i] = src[0][i];
+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:210:5
+   |
+LL | /     for i in 0..5 {
+LL | |
+LL | |         dst[i] = src[0][1][i];
+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:218:5
    |
 LL | /     for i in 0..src.len() {
 LL | |
@@ -153,5 +171,5 @@ LL | |         dst[i] = src[i].clone();
 LL | |     }
    | |_____^ help: try replacing the loop by: `dst[..src.len()].clone_from_slice(&src[..]);`
 
-error: aborting due to 16 previous errors
+error: aborting due to 18 previous errors