From cd22425990462d55dbd01a432dfd2aa517d5ca9c Mon Sep 17 00:00:00 2001
From: Vadim Petrochenkov <vadim.petrochenkov@gmail.com>
Date: Sat, 3 Apr 2021 18:46:25 +0300
Subject: [PATCH] expand: Do not ICE when a legacy AST-based macro attribute
 produces and empty expression

---
 compiler/rustc_expand/src/expand.rs       |  9 ++++++++-
 src/test/ui/macros/attr-empty-expr.rs     | 11 +++++++++++
 src/test/ui/macros/attr-empty-expr.stderr | 20 ++++++++++++++++++++
 3 files changed, 39 insertions(+), 1 deletion(-)
 create mode 100644 src/test/ui/macros/attr-empty-expr.rs
 create mode 100644 src/test/ui/macros/attr-empty-expr.stderr

diff --git a/compiler/rustc_expand/src/expand.rs b/compiler/rustc_expand/src/expand.rs
index 470788a972a..42332adcbab 100644
--- a/compiler/rustc_expand/src/expand.rs
+++ b/compiler/rustc_expand/src/expand.rs
@@ -735,7 +735,14 @@ impl<'a, 'b> MacroExpander<'a, 'b> {
                                     });
                                 }
                             };
-                            fragment_kind.expect_from_annotatables(items)
+                            if fragment_kind == AstFragmentKind::Expr && items.is_empty() {
+                                let msg =
+                                    "removing an expression is not supported in this position";
+                                self.cx.span_err(span, msg);
+                                fragment_kind.dummy(span)
+                            } else {
+                                fragment_kind.expect_from_annotatables(items)
+                            }
                         }
                         Err(mut err) => {
                             err.emit();
diff --git a/src/test/ui/macros/attr-empty-expr.rs b/src/test/ui/macros/attr-empty-expr.rs
new file mode 100644
index 00000000000..d4d1a3ee71e
--- /dev/null
+++ b/src/test/ui/macros/attr-empty-expr.rs
@@ -0,0 +1,11 @@
+// AST-based macro attributes expanding to an empty expression produce an error and not ICE.
+
+#![feature(custom_test_frameworks)]
+#![feature(stmt_expr_attributes)]
+#![feature(test)]
+
+fn main() {
+    let _ = #[test] 0; //~ ERROR removing an expression is not supported in this position
+    let _ = #[bench] 1; //~ ERROR removing an expression is not supported in this position
+    let _ = #[test_case] 2; //~ ERROR removing an expression is not supported in this position
+}
diff --git a/src/test/ui/macros/attr-empty-expr.stderr b/src/test/ui/macros/attr-empty-expr.stderr
new file mode 100644
index 00000000000..53721053bcc
--- /dev/null
+++ b/src/test/ui/macros/attr-empty-expr.stderr
@@ -0,0 +1,20 @@
+error: removing an expression is not supported in this position
+  --> $DIR/attr-empty-expr.rs:8:13
+   |
+LL |     let _ = #[test] 0;
+   |             ^^^^^^^
+
+error: removing an expression is not supported in this position
+  --> $DIR/attr-empty-expr.rs:9:13
+   |
+LL |     let _ = #[bench] 1;
+   |             ^^^^^^^^
+
+error: removing an expression is not supported in this position
+  --> $DIR/attr-empty-expr.rs:10:13
+   |
+LL |     let _ = #[test_case] 2;
+   |             ^^^^^^^^^^^^
+
+error: aborting due to 3 previous errors
+