Fix tests/ui/asm/naked-invalid-attr.stderr.

`{{root}}` is supposed to be an internal-only name but it shows up in
the output.

(I'm working towards a more general fix -- a universal "joiner" function
that can be used all over the place -- but I'm not there yet, so let's
fix this one in-place for now.)

compiler/rustc_passes/src/check_attr.rs

@@ -35,7 +35,7 @@ use rustc_session::lint::builtin::{
     UNKNOWN_OR_MALFORMED_DIAGNOSTIC_ATTRIBUTES, UNUSED_ATTRIBUTES,
 };
 use rustc_session::parse::feature_err;
-use rustc_span::{BytePos, DUMMY_SP, Span, Symbol, edition, sym};
+use rustc_span::{BytePos, DUMMY_SP, Span, Symbol, edition, kw, sym};
 use rustc_trait_selection::error_reporting::InferCtxtErrorExt;
 use rustc_trait_selection::infer::{TyCtxtInferExt, ValuePairs};
 use rustc_trait_selection::traits::ObligationCtxt;
@@ -715,7 +715,10 @@ impl<'tcx> CheckAttrVisitor<'tcx> {
                         && !matches!(other_attr.path().as_slice(), [sym::rustfmt, ..])
                     {
                         let path = other_attr.path();
-                        let path: Vec<_> = path.iter().map(|s| s.as_str()).collect();
+                        let path: Vec<_> = path
+                            .iter()
+                            .map(|s| if *s == kw::PathRoot { "" } else { s.as_str() })
+                            .collect();
                         let other_attr_name = path.join("::");
 
                         self.dcx().emit_err(errors::NakedFunctionIncompatibleAttribute {

tests/ui/asm/naked-invalid-attr.stderr

@@ -37,7 +37,7 @@ error[E0736]: attribute incompatible with `#[unsafe(naked)]`
   --> $DIR/naked-invalid-attr.rs:56:1
    |
 LL | #[::a]
-   | ^^^^^^ the `{{root}}::a` attribute is incompatible with `#[unsafe(naked)]`
+   | ^^^^^^ the `::a` attribute is incompatible with `#[unsafe(naked)]`
 ...
 LL | #[unsafe(naked)]
    | ---------------- function marked with `#[unsafe(naked)]` here