diff --git a/compiler/rustc_error_messages/src/lib.rs b/compiler/rustc_error_messages/src/lib.rs
index 77f87d5b007e3..a2d507328b389 100644
--- a/compiler/rustc_error_messages/src/lib.rs
+++ b/compiler/rustc_error_messages/src/lib.rs
@@ -337,19 +337,6 @@ impl DiagnosticMessage {
             }
         }
     }
-
-    /// Returns the `String` contained within the `DiagnosticMessage::Str` variant, assuming that
-    /// this diagnostic message is of the legacy, non-translatable variety. Panics if this
-    /// assumption does not hold.
-    ///
-    /// Don't use this - it exists to support some places that do comparison with diagnostic
-    /// strings.
-    pub fn expect_str(&self) -> &str {
-        match self {
-            DiagnosticMessage::Str(s) => s,
-            _ => panic!("expected non-translatable diagnostic message"),
-        }
-    }
 }
 
 /// `From` impl that enables existing diagnostic calls to functions which now take
diff --git a/compiler/rustc_expand/src/mbe/macro_rules.rs b/compiler/rustc_expand/src/mbe/macro_rules.rs
index 6d2c7aac6af49..30aa4f0fa3482 100644
--- a/compiler/rustc_expand/src/mbe/macro_rules.rs
+++ b/compiler/rustc_expand/src/mbe/macro_rules.rs
@@ -14,7 +14,7 @@ use rustc_ast::{NodeId, DUMMY_NODE_ID};
 use rustc_ast_pretty::pprust;
 use rustc_attr::{self as attr, TransparencyError};
 use rustc_data_structures::fx::{FxHashMap, FxIndexMap};
-use rustc_errors::{Applicability, Diagnostic, DiagnosticBuilder};
+use rustc_errors::{Applicability, Diagnostic, DiagnosticBuilder, DiagnosticMessage};
 use rustc_feature::Features;
 use rustc_lint_defs::builtin::{
     RUST_2021_INCOMPATIBLE_OR_PATTERNS, SEMICOLON_IN_EXPRESSIONS_FROM_MACROS,
@@ -68,19 +68,22 @@ fn emit_frag_parse_err(
     kind: AstFragmentKind,
 ) {
     // FIXME(davidtwco): avoid depending on the error message text
-    if parser.token == token::Eof && e.message[0].0.expect_str().ends_with(", found `<eof>`") {
-        if !e.span.is_dummy() {
-            // early end of macro arm (#52866)
-            e.replace_span_with(parser.sess.source_map().next_point(parser.token.span));
-        }
+    if parser.token == token::Eof
+        && let DiagnosticMessage::Str(message) = &e.message[0].0
+        && message.ends_with(", found `<eof>`")
+    {
         let msg = &e.message[0];
         e.message[0] = (
-            rustc_errors::DiagnosticMessage::Str(format!(
+            DiagnosticMessage::Str(format!(
                 "macro expansion ends with an incomplete expression: {}",
-                msg.0.expect_str().replace(", found `<eof>`", ""),
+                message.replace(", found `<eof>`", ""),
             )),
             msg.1,
         );
+        if !e.span.is_dummy() {
+            // early end of macro arm (#52866)
+            e.replace_span_with(parser.sess.source_map().next_point(parser.token.span));
+        }
     }
     if e.span.is_dummy() {
         // Get around lack of span in error (#30128)
diff --git a/src/test/ui/macros/issue-102878.rs b/src/test/ui/macros/issue-102878.rs
new file mode 100644
index 0000000000000..aac5891939e0c
--- /dev/null
+++ b/src/test/ui/macros/issue-102878.rs
@@ -0,0 +1,10 @@
+macro_rules!test{($l:expr,$_:r)=>({const:y y)}
+//~^ ERROR mismatched closing delimiter: `)`
+//~| ERROR invalid fragment specifier `r`
+//~| ERROR expected identifier, found keyword `const`
+//~| ERROR expected identifier, found keyword `const`
+//~| ERROR expected identifier, found `:`
+
+fn s(){test!(1,i)}
+
+fn main() {}
diff --git a/src/test/ui/macros/issue-102878.stderr b/src/test/ui/macros/issue-102878.stderr
new file mode 100644
index 0000000000000..e0b8855a38d05
--- /dev/null
+++ b/src/test/ui/macros/issue-102878.stderr
@@ -0,0 +1,60 @@
+error: mismatched closing delimiter: `)`
+  --> $DIR/issue-102878.rs:1:35
+   |
+LL | macro_rules!test{($l:expr,$_:r)=>({const:y y)}
+   |                                  -^         ^ mismatched closing delimiter
+   |                                  ||
+   |                                  |unclosed delimiter
+   |                                  closing delimiter possibly meant for this
+
+error: invalid fragment specifier `r`
+  --> $DIR/issue-102878.rs:1:27
+   |
+LL | macro_rules!test{($l:expr,$_:r)=>({const:y y)}
+   |                           ^^^^
+   |
+   = help: valid fragment specifiers are `ident`, `block`, `stmt`, `expr`, `pat`, `ty`, `lifetime`, `literal`, `path`, `meta`, `tt`, `item` and `vis`
+
+error: expected identifier, found keyword `const`
+  --> $DIR/issue-102878.rs:1:36
+   |
+LL | macro_rules!test{($l:expr,$_:r)=>({const:y y)}
+   |                                    ^^^^^ expected identifier, found keyword
+...
+LL | fn s(){test!(1,i)}
+   |        ---------- in this macro invocation
+   |
+   = note: this error originates in the macro `test` (in Nightly builds, run with -Z macro-backtrace for more info)
+help: escape `const` to use it as an identifier
+   |
+LL | macro_rules!test{($l:expr,$_:r)=>({r#const:y y)}
+   |                                    ++
+
+error: expected identifier, found keyword `const`
+  --> $DIR/issue-102878.rs:1:36
+   |
+LL | macro_rules!test{($l:expr,$_:r)=>({const:y y)}
+   |                                    ^^^^^ expected identifier, found keyword
+...
+LL | fn s(){test!(1,i)}
+   |        ---------- in this macro invocation
+   |
+   = note: this error originates in the macro `test` (in Nightly builds, run with -Z macro-backtrace for more info)
+help: escape `const` to use it as an identifier
+   |
+LL | macro_rules!test{($l:expr,$_:r)=>({r#const:y y)}
+   |                                    ++
+
+error: expected identifier, found `:`
+  --> $DIR/issue-102878.rs:1:41
+   |
+LL | macro_rules!test{($l:expr,$_:r)=>({const:y y)}
+   |                                         ^ expected identifier
+...
+LL | fn s(){test!(1,i)}
+   |        ---------- in this macro invocation
+   |
+   = note: this error originates in the macro `test` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: aborting due to 5 previous errors
+