Migrate some tests away from concat-idents

`concat_idents!` is in the process of being removed, but a few things it
is used to test will still be relevant. Migrate these tests to something
other than `concat_idents`.

Author: tgross35

tests/rustdoc-js/big-result.rs

@@ -1,4 +1,3 @@
-#![feature(concat_idents)]
 #![allow(nonstandard_style)]
 /// Generate 250 items that all match the query, starting with the longest.
 /// Those long items should be dropped from the result set, and the short ones

tests/ui/issues/issue-32950.rs

@@ -1,10 +1,12 @@
-#![feature(concat_idents)]
-#![expect(deprecated)] // concat_idents is deprecated
+// Issue 32950
+// Ensure that using macros rather than a type doesn't break `derive`.
 
 #[derive(Debug)]
-struct Baz<T>(
-    concat_idents!(Foo, Bar) //~ ERROR `derive` cannot be used on items with type macros
-                             //~^ ERROR cannot find type `FooBar` in this scope
-);
+struct Nonsense<T> {
+    //~^ ERROR type parameter `T` is never used
+    should_be_vec_t: vec![T]
+    //~^ ERROR `derive` cannot be used on items with type macros
+    //~| ERROR expected type, found `expr` metavariable
+}
 
 fn main() {}

tests/ui/issues/issue-32950.stderr

@@ -1,15 +1,30 @@
 error: `derive` cannot be used on items with type macros
-  --> $DIR/issue-32950.rs:6:5
+  --> $DIR/nonsense-input-to-debug.rs:7:22
    |
-LL |     concat_idents!(Foo, Bar)
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^
+LL |     should_be_vec_t: vec![T]
+   |                      ^^^^^^^
 
-error[E0412]: cannot find type `FooBar` in this scope
-  --> $DIR/issue-32950.rs:6:5
+error: expected type, found `expr` metavariable
+  --> $DIR/nonsense-input-to-debug.rs:7:22
    |
-LL |     concat_idents!(Foo, Bar)
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^ not found in this scope
+LL |     should_be_vec_t: vec![T]
+   |                      ^^^^^^^
+   |                      |
+   |                      expected type
+   |                      in this macro invocation
+   |                      this macro call doesn't expand to a type
+   |
+   = note: this error originates in the macro `vec` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error[E0392]: type parameter `T` is never used
+  --> $DIR/nonsense-input-to-debug.rs:5:17
+   |
+LL | struct Nonsense<T> {
+   |                 ^ unused type parameter
+   |
+   = help: consider removing `T`, referring to it in a field, or using a marker such as `PhantomData`
+   = help: if you intended `T` to be a const parameter, use `const T: /* Type */` instead
 
-error: aborting due to 2 previous errors
+error: aborting due to 3 previous errors
 
-For more information about this error, try `rustc --explain E0412`.
+For more information about this error, try `rustc --explain E0392`.

tests/ui/issues/issue-50403.rs

@@ -1,6 +1,12 @@
-#![feature(concat_idents)]
-#![expect(deprecated)] // concat_idents is deprecated
+// Issue 50403
+// Ensure that `concat` can't create empty identifiers
+// FIXME(macro_metavar_expr_concat): this error message could be improved
+
+macro_rules! empty {
+    () => { ${concat()} } //~ ERROR expected identifier or string literal
+                          //~^ERROR expected expression
+}
 
 fn main() {
-    let x = concat_idents!(); //~ ERROR `concat_idents!()` takes 1 or more arguments
+    let x = empty!();
 }

tests/ui/issues/issue-50403.stderr

@@ -1,8 +1,19 @@
-error: `concat_idents!()` takes 1 or more arguments
-  --> $DIR/issue-50403.rs:5:13
+error: expected identifier or string literal
+  --> $DIR/empty-input.rs:6:14
    |
-LL |     let x = concat_idents!();
-   |             ^^^^^^^^^^^^^^^^
+LL |     () => { ${concat()} }
+   |              ^^^^^^^^^^
 
-error: aborting due to 1 previous error
+error: expected expression, found `$`
+  --> $DIR/empty-input.rs:6:13
+   |
+LL |     () => { ${concat()} }
+   |             ^ expected expression
+...
+LL |     let x = empty!();
+   |             -------- in this macro invocation
+   |
+   = note: this error originates in the macro `empty` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: aborting due to 2 previous errors