Rollup merge of rust-lang#139624 - m-ou-se:unconst-format-args, r=jhpratt

Don't allow flattened format_args in const.

Fixes rust-lang#139136

Fixes rust-lang#139621

We allow `format_args!("a")` in const, but don't allow any format_args with arguments in const, such as `format_args!("{}", arg)`.

However, we accidentally allow `format_args!("hello {}", "world")` in const, as it gets flattened to `format_args!("hello world")`.

This also applies to panic in const.

This wasn't supposed to happen. I added protection against this in the format args flattening code, ~~but I accidentally marked a function as const that shouldn't have been const~~ but this was removed in rust-lang#135139.

This is a breaking change. The crater found no breakage, however.

This breaks things like:

```rust
const _: () = if false { panic!("a {}", "a") };
```

and

```rust
const F: std::fmt::Arguments<'static> = format_args!("a {}", "a");
```

Author: matthiaskrgr

core/src/fmt/rt.rs

@@ -200,8 +200,15 @@ impl Argument<'_> {
     /// let f = format_args!("{}", "a");
     /// println!("{f}");
     /// ```
+    ///
+    /// This function should _not_ be const, to make sure we don't accept
+    /// format_args!() and panic!() with arguments in const, even when not evaluated:
+    ///
+    /// ```compile_fail,E0015
+    /// const _: () = if false { panic!("a {}", "a") };
+    /// ```
     #[inline]
-    pub const fn none() -> [Self; 0] {
+    pub fn none() -> [Self; 0] {
         []
     }
 }