Add a test for new 2024 standard library behavior

When migrating the standard library to 2024, there will be some behavior
changes that users will be able to observe. This test should cover that
(I cannot think of any other observable differences).

Author: ehuss

tests/ui/macros/std-2024-macros.rs

@@ -0,0 +1,16 @@
+// Tests a small handful of macros in the standard library how they handle the
+// new behavior introduced in 2024.
+
+fn main() {
+    assert_eq!(0, const { 0 });
+    //~^ ERROR: no rules expected keyword `const`
+    assert_eq!(const { 0 }, const { 0 });
+    //~^ ERROR: no rules expected keyword `const`
+    assert_eq!(const { 0 }, 0);
+    //~^ ERROR: no rules expected keyword `const`
+
+    let _: Vec<Vec<String>> = vec![const { vec![] }];
+    //~^ ERROR: no rules expected keyword `const`
+    let _: Vec<Vec<String>> = vec![const { vec![] }; 10];
+    //~^ ERROR: no rules expected keyword `const`
+}

tests/ui/macros/std-2024-macros.stderr

@@ -0,0 +1,45 @@
+error: no rules expected keyword `const`
+  --> $DIR/std-2024-macros.rs:5:19
+   |
+LL |     assert_eq!(0, const { 0 });
+   |                   ^^^^^ no rules expected this token in macro call
+   |
+note: while trying to match meta-variable `$right:expr`
+  --> $SRC_DIR/core/src/macros/mod.rs:LL:COL
+
+error: no rules expected keyword `const`
+  --> $DIR/std-2024-macros.rs:7:16
+   |
+LL |     assert_eq!(const { 0 }, const { 0 });
+   |                ^^^^^ no rules expected this token in macro call
+   |
+note: while trying to match meta-variable `$left:expr`
+  --> $SRC_DIR/core/src/macros/mod.rs:LL:COL
+
+error: no rules expected keyword `const`
+  --> $DIR/std-2024-macros.rs:9:16
+   |
+LL |     assert_eq!(const { 0 }, 0);
+   |                ^^^^^ no rules expected this token in macro call
+   |
+note: while trying to match meta-variable `$left:expr`
+  --> $SRC_DIR/core/src/macros/mod.rs:LL:COL
+
+error: no rules expected keyword `const`
+  --> $DIR/std-2024-macros.rs:12:36
+   |
+LL |     let _: Vec<Vec<String>> = vec![const { vec![] }];
+   |                                    ^^^^^ no rules expected this token in macro call
+   |
+   = note: while trying to match end of macro
+
+error: no rules expected keyword `const`
+  --> $DIR/std-2024-macros.rs:14:36
+   |
+LL |     let _: Vec<Vec<String>> = vec![const { vec![] }; 10];
+   |                                    ^^^^^ no rules expected this token in macro call
+   |
+   = note: while trying to match end of macro
+
+error: aborting due to 5 previous errors
+