Allow for reparsing failure when reparsing a pasted metavar.

compiler/rustc_parse/src/parser/mod.rs

@@ -782,9 +782,16 @@ impl<'a> Parser<'a> {
             // Recovery is disabled when parsing macro arguments, so it must
             // also be disabled when reparsing pasted macro arguments,
             // otherwise we get inconsistent results (e.g. #137874).
-            let res = self.with_recovery(Recovery::Forbidden, |this| {
-                f(this).expect("failed to reparse {mv_kind:?}")
-            });
+            let res = self.with_recovery(Recovery::Forbidden, |this| f(this));
+
+            let res = match res {
+                Ok(res) => res,
+                Err(err) => {
+                    // This can occur in unusual error cases, e.g. #139445.
+                    err.delay_as_bug();
+                    return None;
+                }
+            };
 
             if let token::CloseDelim(delim) = self.token.kind
                 && let Delimiter::Invisible(InvisibleOrigin::MetaVar(mv_kind)) = delim
@@ -793,7 +800,12 @@ impl<'a> Parser<'a> {
                 self.bump();
                 Some(res)
             } else {
-                panic!("no close delim when reparsing {mv_kind:?}");
+                // This can occur when invalid syntax is passed to a decl macro. E.g. see #139248,
+                // where the reparse attempt of an invalid expr consumed the trailing invisible
+                // delimiter.
+                self.dcx()
+                    .span_delayed_bug(self.token.span, "no close delim with reparsing {mv_kind:?}");
+                None
             }
         } else {
             None

tests/ui/macros/failed-to-reparse-issue-139445.rs

@@ -0,0 +1,6 @@
+fn main() {
+    assert_eq!(3, 'a,)
+    //~^ ERROR expected `while`, `for`, `loop` or `{` after a label
+    //~| ERROR expected `while`, `for`, `loop` or `{` after a label
+    //~| ERROR expected expression, found ``
+}

tests/ui/macros/failed-to-reparse-issue-139445.stderr

@@ -0,0 +1,24 @@
+error: expected `while`, `for`, `loop` or `{` after a label
+  --> $DIR/failed-to-reparse-issue-139445.rs:2:21
+   |
+LL |     assert_eq!(3, 'a,)
+   |                     ^ expected `while`, `for`, `loop` or `{` after a label
+
+error: expected `while`, `for`, `loop` or `{` after a label
+  --> $DIR/failed-to-reparse-issue-139445.rs:2:5
+   |
+LL |     assert_eq!(3, 'a,)
+   |     ^^^^^^^^^^^^^^^^^^ expected `while`, `for`, `loop` or `{` after a label
+   |
+   = note: this error originates in the macro `assert_eq` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: expected expression, found ``
+  --> $DIR/failed-to-reparse-issue-139445.rs:2:5
+   |
+LL |     assert_eq!(3, 'a,)
+   |     ^^^^^^^^^^^^^^^^^^ expected expression
+   |
+   = note: this error originates in the macro `assert_eq` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: aborting due to 3 previous errors
+

tests/ui/macros/no-close-delim-issue-139248.rs

@@ -0,0 +1,14 @@
+// This code caused a "no close delim when reparsing Expr" ICE in #139248.
+
+macro_rules! m {
+    (static a : () = $e:expr) => {
+        static a : () = $e;
+        //~^ ERROR macro expansion ends with an incomplete expression: expected expression
+    }
+}
+
+m! { static a : () = (if b) }
+//~^ ERROR expected `{`, found `)`
+//~| ERROR expected `{`, found `)`
+
+fn main() {}

tests/ui/macros/no-close-delim-issue-139248.stderr

@@ -0,0 +1,33 @@
+error: expected `{`, found `)`
+  --> $DIR/no-close-delim-issue-139248.rs:10:27
+   |
+LL | m! { static a : () = (if b) }
+   |                           ^ expected `{`
+   |
+note: the `if` expression is missing a block after this condition
+  --> $DIR/no-close-delim-issue-139248.rs:10:26
+   |
+LL | m! { static a : () = (if b) }
+   |                          ^
+
+error: expected `{`, found `)`
+  --> $DIR/no-close-delim-issue-139248.rs:10:27
+   |
+LL | m! { static a : () = (if b) }
+   |                           ^ expected `{`
+   |
+note: the `if` expression is missing a block after this condition
+  --> $DIR/no-close-delim-issue-139248.rs:10:26
+   |
+LL | m! { static a : () = (if b) }
+   |                          ^
+   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
+
+error: macro expansion ends with an incomplete expression: expected expression
+  --> $DIR/no-close-delim-issue-139248.rs:5:28
+   |
+LL |         static a : () = $e;
+   |                            ^ expected expression
+
+error: aborting due to 3 previous errors
+