From 550e3087d1b521f2b3b669ebaaf6d16e4c593fca Mon Sep 17 00:00:00 2001
From: Mu42 <mu001999@outlook.com>
Date: Fri, 17 Mar 2023 14:36:22 +0800
Subject: [PATCH] Suggest surrounding the macro with `{}` to interpret as a
 statement

---
 compiler/rustc_expand/src/mbe/diagnostics.rs | 24 +++++++++++++++-----
 tests/ui/macros/issue-109237.rs              |  7 ++++++
 tests/ui/macros/issue-109237.stderr          | 18 +++++++++++++++
 3 files changed, 43 insertions(+), 6 deletions(-)
 create mode 100644 tests/ui/macros/issue-109237.rs
 create mode 100644 tests/ui/macros/issue-109237.stderr

diff --git a/compiler/rustc_expand/src/mbe/diagnostics.rs b/compiler/rustc_expand/src/mbe/diagnostics.rs
index b1d9cea277397..6bc393c6534f3 100644
--- a/compiler/rustc_expand/src/mbe/diagnostics.rs
+++ b/compiler/rustc_expand/src/mbe/diagnostics.rs
@@ -245,12 +245,24 @@ pub(super) fn emit_frag_parse_err(
                 e.note(
                     "the macro call doesn't expand to an expression, but it can expand to a statement",
                 );
-                e.span_suggestion_verbose(
-                    site_span.shrink_to_hi(),
-                    "add `;` to interpret the expansion as a statement",
-                    ";",
-                    Applicability::MaybeIncorrect,
-                );
+
+                if parser.token == token::Semi {
+                    if let Ok(snippet) = parser.sess.source_map().span_to_snippet(site_span) {
+                        e.span_suggestion_verbose(
+                            site_span,
+                            "surround the macro invocation with `{}` to interpret the expansion as a statement",
+                            format!("{{ {}; }}", snippet),
+                            Applicability::MaybeIncorrect,
+                        );
+                    }
+                } else {
+                    e.span_suggestion_verbose(
+                        site_span.shrink_to_hi(),
+                        "add `;` to interpret the expansion as a statement",
+                        ";",
+                        Applicability::MaybeIncorrect,
+                    );
+                }
             }
         },
         _ => annotate_err_with_kind(&mut e, kind, site_span),
diff --git a/tests/ui/macros/issue-109237.rs b/tests/ui/macros/issue-109237.rs
new file mode 100644
index 0000000000000..86a193c9e44ac
--- /dev/null
+++ b/tests/ui/macros/issue-109237.rs
@@ -0,0 +1,7 @@
+macro_rules! statement {
+    () => {;}; //~ ERROR expected expression
+}
+
+fn main() {
+    let _ = statement!();
+}
diff --git a/tests/ui/macros/issue-109237.stderr b/tests/ui/macros/issue-109237.stderr
new file mode 100644
index 0000000000000..d125cff63eab2
--- /dev/null
+++ b/tests/ui/macros/issue-109237.stderr
@@ -0,0 +1,18 @@
+error: expected expression, found `;`
+  --> $DIR/issue-109237.rs:2:12
+   |
+LL |     () => {;};
+   |            ^ expected expression
+...
+LL |     let _ = statement!();
+   |             ------------ in this macro invocation
+   |
+   = note: the macro call doesn't expand to an expression, but it can expand to a statement
+   = note: this error originates in the macro `statement` (in Nightly builds, run with -Z macro-backtrace for more info)
+help: surround the macro invocation with `{}` to interpret the expansion as a statement
+   |
+LL |     let _ = { statement!(); };
+   |             ~~~~~~~~~~~~~~~~~
+
+error: aborting due to previous error
+