tests: adds cargo fix tests

Co-Developed-by: Eric Holk
Signed-off-by: Vincenzo Palazzo <vincenzopalazzodev@gmail.com>

tests/ui/macros/expr_2021_cargo_fix_edition.rs

@@ -0,0 +1,14 @@
+//@  compile-flags: --edition=2021
+// This test ensures that expr_2021 is not allowed on pre-2021 editions
+#![deny(edition_2024_expr_fragment_specifier)]
+
+macro_rules! m {
+    ($e:expr) => { //~ ERROR: the `expr` fragment specifier will accept more expressions in the 2024 edition.
+       //~^ WARN: this changes meaning in Rust 2024
+        $e
+    };
+}
+
+fn main() {
+    m!(());
+}

tests/ui/macros/expr_2021_cargo_fix_edition.stderr

@@ -0,0 +1,20 @@
+error: the `expr` fragment specifier will accept more expressions in the 2024 edition.
+  --> $DIR/expr_2021_cargo_fix_edition.rs:6:9
+   |
+LL |     ($e:expr) => {
+   |         ^^^^
+   |
+   = warning: this changes meaning in Rust 2024
+   = note: for more information, see issue #123742 <https://github.com/rust-lang/rust/issues/123742>
+note: the lint level is defined here
+  --> $DIR/expr_2021_cargo_fix_edition.rs:3:9
+   |
+LL | #![deny(edition_2024_expr_fragment_specifier)]
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+help: to keep the existing behavior, use the `expr_2021` fragment specifier.
+   |
+LL |     ($e:expr_2021) => {
+   |         ~~~~~~~~~
+
+error: aborting due to 1 previous error
+