From dd109c8c10ddcb3924adbcdd27baf6f525824d52 Mon Sep 17 00:00:00 2001
From: Michael Goulet <michael@errs.io>
Date: Sun, 17 Jul 2022 00:04:30 +0000
Subject: [PATCH] better error for bad depth on macro metavar expr

---
 compiler/rustc_expand/src/mbe/transcribe.rs         | 13 ++++++++++++-
 .../ui/macros/meta-variable-depth-outside-repeat.rs | 12 ++++++++++++
 .../meta-variable-depth-outside-repeat.stderr       |  8 ++++++++
 .../out-of-bounds-arguments.rs                      |  9 ++++-----
 .../out-of-bounds-arguments.stderr                  |  6 +++---
 5 files changed, 39 insertions(+), 9 deletions(-)
 create mode 100644 src/test/ui/macros/meta-variable-depth-outside-repeat.rs
 create mode 100644 src/test/ui/macros/meta-variable-depth-outside-repeat.stderr

diff --git a/compiler/rustc_expand/src/mbe/transcribe.rs b/compiler/rustc_expand/src/mbe/transcribe.rs
index fdd8dc93fc1a5..3037855ae28a6 100644
--- a/compiler/rustc_expand/src/mbe/transcribe.rs
+++ b/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>(
diff --git a/src/test/ui/macros/meta-variable-depth-outside-repeat.rs b/src/test/ui/macros/meta-variable-depth-outside-repeat.rs
new file mode 100644
index 0000000000000..b7fb947854f04
--- /dev/null
+++ b/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() {}
diff --git a/src/test/ui/macros/meta-variable-depth-outside-repeat.stderr b/src/test/ui/macros/meta-variable-depth-outside-repeat.stderr
new file mode 100644
index 0000000000000..fad150cadfca6
--- /dev/null
+++ b/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
+
diff --git a/src/test/ui/macros/rfc-3086-metavar-expr/out-of-bounds-arguments.rs b/src/test/ui/macros/rfc-3086-metavar-expr/out-of-bounds-arguments.rs
index d81c8628bab26..6a0d68bd6b16a 100644
--- a/src/test/ui/macros/rfc-3086-metavar-expr/out-of-bounds-arguments.rs
+++ b/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 });
 }
diff --git a/src/test/ui/macros/rfc-3086-metavar-expr/out-of-bounds-arguments.stderr b/src/test/ui/macros/rfc-3086-metavar-expr/out-of-bounds-arguments.stderr
index 7474c03c0f98b..236122b6465b2 100644
--- a/src/test/ui/macros/rfc-3086-metavar-expr/out-of-bounds-arguments.stderr
+++ b/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)}