better error for bad depth parameter on macro metavar expr

compiler/rustc_expand/src/mbe/transcribe.rs

@@ -512,7 +512,18 @@ fn out_of_bounds_err<'a>(
     span: Span,
     ty: &str,
 ) -> DiagnosticBuilder<'a, ErrorGuaranteed> {
-    cx.struct_span_err(span, &format!("{ty} depth must be less than {max}"))
+    let msg = if max == 0 {
+        format!(
+            "meta-variable expression `{ty}` with depth parameter \
+             must be called inside of a macro repetition"
+        )
+    } else {
+        format!(
+            "depth parameter on meta-variable expression `{ty}` \
+             must be less than {max}"
+        )
+    };
+    cx.struct_span_err(span, &msg)
 }
 
 fn transcribe_metavar_expr<'a>(

src/test/ui/macros/meta-variable-depth-outside-repeat.rs

@@ -0,0 +1,12 @@
+#![feature(macro_metavar_expr)]
+
+macro_rules! metavar {
+    ( $i:expr ) => {
+        ${length(0)}
+        //~^ ERROR meta-variable expression `length` with depth parameter must be called inside of a macro repetition
+    };
+}
+
+const _: i32 = metavar!(0);
+
+fn main() {}

src/test/ui/macros/meta-variable-depth-outside-repeat.stderr

@@ -0,0 +1,8 @@
+error: meta-variable expression `length` with depth parameter must be called inside of a macro repetition
+  --> $DIR/meta-variable-depth-outside-repeat.rs:5:10
+   |
+LL |         ${length(0)}
+   |          ^^^^^^^^^^^
+
+error: aborting due to previous error
+

src/test/ui/macros/rfc-3086-metavar-expr/out-of-bounds-arguments.rs

@@ -5,7 +5,7 @@ macro_rules! a {
         (
             ${count(foo, 0)},
             ${count(foo, 10)},
-            //~^ ERROR count depth must be less than 4
+            //~^ ERROR depth parameter on meta-variable expression `count` must be less than 4
         )
     };
 }
@@ -17,7 +17,7 @@ macro_rules! b {
                 ${ignore(foo)}
                 ${index(0)},
                 ${index(10)},
-                //~^ ERROR index depth must be less than 3
+                //~^ ERROR depth parameter on meta-variable expression `index` must be less than 3
             )* )* )*
         )
     };
@@ -30,15 +30,14 @@ macro_rules! c {
                 ${ignore(foo)}
                 ${length(0)}
                 ${length(10)}
-                //~^ ERROR length depth must be less than 2
+                //~^ ERROR depth parameter on meta-variable expression `length` must be less than 2
             )* )*
         )
     };
 }
 
-
 fn main() {
     a!( { [ (a) ] [ (b c) ] } );
     b!( { [ a b ] } );
-    c!( { a } );
+    c!({ a });
 }

src/test/ui/macros/rfc-3086-metavar-expr/out-of-bounds-arguments.stderr

@@ -1,16 +1,16 @@
-error: count depth must be less than 4
+error: depth parameter on meta-variable expression `count` must be less than 4
   --> $DIR/out-of-bounds-arguments.rs:7:14
    |
 LL |             ${count(foo, 10)},
    |              ^^^^^^^^^^^^^^^^
 
-error: index depth must be less than 3
+error: depth parameter on meta-variable expression `index` must be less than 3
   --> $DIR/out-of-bounds-arguments.rs:19:18
    |
 LL |                 ${index(10)},
    |                  ^^^^^^^^^^^
 
-error: length depth must be less than 2
+error: depth parameter on meta-variable expression `length` must be less than 2
   --> $DIR/out-of-bounds-arguments.rs:32:18
    |
 LL |                 ${length(10)}