mbe: Extend metavariable expression tests

Add tests showing the current state to make it more clear when output
gets updated later in refactoring.

Author: tgross35

tests/ui/macros/metavar-expressions/concat-trace-errors.rs

@@ -0,0 +1,33 @@
+// Our diagnostics should be able to point to a specific input that caused an invalid
+// identifier.
+
+#![feature(macro_metavar_expr_concat)]
+
+// See what we can do without expanding anything
+macro_rules! pre_expansion {
+    ($a:ident) => {
+        ${concat("hi", " bye ")};
+        ${concat("hi", "-", "bye")};
+        ${concat($a, "-")};
+    }
+}
+
+macro_rules! post_expansion {
+    ($a:literal) => {
+        const _: () = ${concat("hi", $a, "bye")};
+        //~^ ERROR is not generating a valid identifier
+    }
+}
+
+post_expansion!("!");
+
+macro_rules! post_expansion_many {
+    ($a:ident, $b:ident, $c:ident, $d:literal, $e:ident) => {
+        const _: () = ${concat($a, $b, $c, $d, $e)};
+        //~^ ERROR is not generating a valid identifier
+    }
+}
+
+post_expansion_many!(a, b, c, ".d", e);
+
+fn main() {}

tests/ui/macros/metavar-expressions/concat-trace-errors.stderr

@@ -0,0 +1,24 @@
+error: `${concat(..)}` is not generating a valid identifier
+  --> $DIR/concat-trace-errors.rs:17:24
+   |
+LL |         const _: () = ${concat("hi", $a, "bye")};
+   |                        ^^^^^^^^^^^^^^^^^^^^^^^^^
+...
+LL | post_expansion!("!");
+   | -------------------- in this macro invocation
+   |
+   = note: this error originates in the macro `post_expansion` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: `${concat(..)}` is not generating a valid identifier
+  --> $DIR/concat-trace-errors.rs:26:24
+   |
+LL |         const _: () = ${concat($a, $b, $c, $d, $e)};
+   |                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+...
+LL | post_expansion_many!(a, b, c, ".d", e);
+   | -------------------------------------- in this macro invocation
+   |
+   = note: this error originates in the macro `post_expansion_many` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: aborting due to 2 previous errors
+

tests/ui/macros/metavar-expressions/concat-usage-errors.rs

@@ -1,6 +1,8 @@
+//@ edition: 2021
+
 #![feature(macro_metavar_expr_concat)]
 
-macro_rules! wrong_concat_declarations {
+macro_rules! syntax_errors {
     ($ex:expr) => {
         ${concat()}
         //~^ ERROR expected identifier
@@ -90,11 +92,31 @@ macro_rules! unsupported_literals {
         //~| ERROR expected pattern
         let ${concat(_a, 1)}: () = ();
         //~^ ERROR expected identifier or string literal
+        let ${concat(_a, 1.5)}: () = ();
+        //~^ ERROR expected identifier or string literal
+        let ${concat(_a, c"hi")}: () = ();
+        //~^ ERROR expected identifier or string literal
+        let ${concat(_a, b"hi")}: () = ();
+        //~^ ERROR expected identifier or string literal
+        let ${concat(_a, b'b')}: () = ();
+        //~^ ERROR expected identifier or string literal
+        let ${concat(_a, b'b')}: () = ();
+        //~^ ERROR expected identifier or string literal
 
         let ${concat($ident, 'b')}: () = ();
         //~^ ERROR expected identifier or string literal
         let ${concat($ident, 1)}: () = ();
         //~^ ERROR expected identifier or string literal
+        let ${concat($ident, 1.5)}: () = ();
+        //~^ ERROR expected identifier or string literal
+        let ${concat($ident, c"hi")}: () = ();
+        //~^ ERROR expected identifier or string literal
+        let ${concat($ident, b"hi")}: () = ();
+        //~^ ERROR expected identifier or string literal
+        let ${concat($ident, b'b')}: () = ();
+        //~^ ERROR expected identifier or string literal
+        let ${concat($ident, b'b')}: () = ();
+        //~^ ERROR expected identifier or string literal
     }};
 }