Rollup merge of #81647 - m-ou-se:assert-2021-fix, r=petrochenkov

Fix bug with assert!() calling the wrong edition of panic!().

The span of `panic!` produced by the `assert` macro did not carry the right edition. This changes `assert` to call the right version.

Also adds tests for the 2021 edition of panic and assert, that would've caught this.

compiler/rustc_builtin_macros/src/assert.rs

@@ -29,11 +29,11 @@ pub fn expand_assert<'cx>(
 
     let panic_call = if let Some(tokens) = custom_message {
         let path = if span.rust_2021() {
-            // On edition 2021, we always call `$crate::panic!()`.
+            // On edition 2021, we always call `$crate::panic::panic_2021!()`.
             Path {
                 span: sp,
                 segments: cx
-                    .std_path(&[sym::panic])
+                    .std_path(&[sym::panic, sym::panic_2021])
                     .into_iter()
                     .map(|ident| PathSegment::from_ident(ident))
                     .collect(),