[gcc r14-7664] gccrs: expand: Allow empty derive attribute

[Arthur Cohen <cohenarthur@gcc.gnu.org>]

https://gcc.gnu.org/g:da450d345c948d9be1fca29d01fa1153c2ddf3bd

commit r14-7664-gda450d345c948d9be1fca29d01fa1153c2ddf3bd
Author: Pierre-Emmanuel Patry <pierre-emmanuel.patry@embecosm.com>
Date:   Mon Jun 5 13:17:22 2023 +0200

    gccrs: expand: Allow empty derive attribute
    
    Changes derive macro expansion to accept empty derive attributes. This
    commit also add a test to prevent future regressions.
    
    gcc/rust/ChangeLog:
    
            * expand/rust-expand-visitor.cc (get_traits_to_derive): Change
            assertion to allow empty derive directives.
    
    gcc/testsuite/ChangeLog:
    
            * rust/compile/derive_empty.rs: New test.
    
    Signed-off-by: Pierre-Emmanuel Patry <pierre-emmanuel.patry@embecosm.com>

Diff:
---
 gcc/rust/expand/rust-expand-visitor.cc     | 2 +-
 gcc/testsuite/rust/compile/derive_empty.rs | 6 ++++++
 2 files changed, 7 insertions(+), 1 deletion(-)

diff --git a/gcc/rust/expand/rust-expand-visitor.cc b/gcc/rust/expand/rust-expand-visitor.cc
index 927d9225049..c7a5f573d7e 100644
--- a/gcc/rust/expand/rust-expand-visitor.cc
+++ b/gcc/rust/expand/rust-expand-visitor.cc
@@ -72,7 +72,7 @@ get_traits_to_derive (AST::Attribute &attr)
 	  = static_cast<AST::DelimTokenTree &> (input).get_token_trees ();
 
 	// erase the delimiters
-	rust_assert (tokens.size () >= 3);
+	rust_assert (tokens.size () >= 2);
 	tokens.erase (tokens.begin ());
 	tokens.pop_back ();
 
diff --git a/gcc/testsuite/rust/compile/derive_empty.rs b/gcc/testsuite/rust/compile/derive_empty.rs
new file mode 100644
index 00000000000..21c67f01d6c
--- /dev/null
+++ b/gcc/testsuite/rust/compile/derive_empty.rs
@@ -0,0 +1,6 @@
+#[derive()]
+struct UnderivedStruct;
+
+fn main() {
+    let _ = UnderivedStruct;
+}