Auto merge of #92066 - Smittyvb:concat_bytes-repeat, r=nagisa

Support [x; n] expressions in concat_bytes!

Currently trying to use `concat_bytes!` with a repeating array value like `[42; 5]` results in an error:
```
error: expected a byte literal
 --> src/main.rs:3:27
  |
3 |     let x = concat_bytes!([3; 4]);
  |                           ^^^^^^
  |
  = note: only byte literals (like `b"foo"`, `b's'`, and `[3, 4, 5]`) can be passed to `concat_bytes!()`
```

This makes it so repeating array syntax can be used the same way normal arrays can be. The RFC doesn't explicitly mention repeat expressions, but it seems reasonable to allow them as well, since normal arrays are allowed.

It is possible to make the compiler get stuck compiling forever with `concat_bytes!([3; 999999999])`, but I don't think that's much of an issue since you can do that already with `const X: [u8; 999999999] = [3; 999999999];`.

Contributes to #87555.

3 files changed, 70 insertions, 2 deletions

diff --git a/src/test/ui/macros/concat-bytes-error.rs b/src/test/ui/macros/concat-bytes-error.rs
index 9b4a9c2cf81..db5d3cab0bd 100644
--- a/src/test/ui/macros/concat-bytes-error.rs
+++ b/src/test/ui/macros/concat-bytes-error.rs
@@ -39,4 +39,12 @@ fn main() {
     ]);
     concat_bytes!(5u16); //~ ERROR cannot concatenate numeric literals
     concat_bytes!([5u16]); //~ ERROR numeric literal is not a `u8`
+    concat_bytes!([3; ()]); //~ ERROR repeat count is not a positive number
+    concat_bytes!([3; -2]); //~ ERROR repeat count is not a positive number
+    concat_bytes!([pie; -2]); //~ ERROR repeat count is not a positive number
+    concat_bytes!([pie; 2]); //~ ERROR expected a byte literal
+    concat_bytes!([2.2; 0]); //~ ERROR cannot concatenate float literals
+    concat_bytes!([5.5; ()]); //~ ERROR repeat count is not a positive number
+    concat_bytes!([[1, 2, 3]; 3]); //~ ERROR cannot concatenate doubly nested array
+    concat_bytes!([[42; 2]; 3]); //~ ERROR cannot concatenate doubly nested array
 }
diff --git a/src/test/ui/macros/concat-bytes-error.stderr b/src/test/ui/macros/concat-bytes-error.stderr
index 1fc2d5c4843..d6cd1a3d178 100644
--- a/src/test/ui/macros/concat-bytes-error.stderr
+++ b/src/test/ui/macros/concat-bytes-error.stderr
@@ -127,5 +127,55 @@ error: numeric literal is not a `u8`
 LL |     concat_bytes!([5u16]);
    |                    ^^^^
 
-error: aborting due to 20 previous errors
+error: repeat count is not a positive number
+  --> $DIR/concat-bytes-error.rs:42:23
+   |
+LL |     concat_bytes!([3; ()]);
+   |                       ^^
+
+error: repeat count is not a positive number
+  --> $DIR/concat-bytes-error.rs:43:23
+   |
+LL |     concat_bytes!([3; -2]);
+   |                       ^^
+
+error: repeat count is not a positive number
+  --> $DIR/concat-bytes-error.rs:44:25
+   |
+LL |     concat_bytes!([pie; -2]);
+   |                         ^^
+
+error: expected a byte literal
+  --> $DIR/concat-bytes-error.rs:45:20
+   |
+LL |     concat_bytes!([pie; 2]);
+   |                    ^^^
+   |
+   = note: only byte literals (like `b"foo"`, `b's'`, and `[3, 4, 5]`) can be passed to `concat_bytes!()`
+
+error: cannot concatenate float literals
+  --> $DIR/concat-bytes-error.rs:46:20
+   |
+LL |     concat_bytes!([2.2; 0]);
+   |                    ^^^
+
+error: repeat count is not a positive number
+  --> $DIR/concat-bytes-error.rs:47:25
+   |
+LL |     concat_bytes!([5.5; ()]);
+   |                         ^^
+
+error: cannot concatenate doubly nested array
+  --> $DIR/concat-bytes-error.rs:48:20
+   |
+LL |     concat_bytes!([[1, 2, 3]; 3]);
+   |                    ^^^^^^^^^
+
+error: cannot concatenate doubly nested array
+  --> $DIR/concat-bytes-error.rs:49:20
+   |
+LL |     concat_bytes!([[42; 2]; 3]);
+   |                    ^^^^^^^
+
+error: aborting due to 28 previous errors
 
diff --git a/src/test/ui/macros/concat-bytes.rs b/src/test/ui/macros/concat-bytes.rs
index 5415cf3fe22..fd8f99417ec 100644
--- a/src/test/ui/macros/concat-bytes.rs
+++ b/src/test/ui/macros/concat-bytes.rs
@@ -3,5 +3,15 @@
 
 fn main() {
     assert_eq!(concat_bytes!(), &[]);
-    assert_eq!(concat_bytes!(b'A', b"BC", [68, b'E', 70]), b"ABCDEF");
+    assert_eq!(
+        concat_bytes!(b'A', b"BC", [68, b'E', 70], [b'G'; 1], [72; 2], [73u8; 3], [65; 0]),
+        b"ABCDEFGHHIII",
+    );
+    assert_eq!(
+        concat_bytes!(
+            concat_bytes!(b"AB", b"CD"),
+            concat_bytes!(b"EF", b"GH"),
+        ),
+        b"ABCDEFGH",
+    );
 }