gccrs: better handling of incorrect file loading in include_* builtin macros

When the load_file_bytes() fails to open the file, it emits an error but
returns an empty vector (as if the file is empty).

Instead, return a tl::optional nullopt in case of error and an empty
vector in case of an empty file.

fixes #2421

gcc/rust/ChangeLog:

	* expand/rust-macro-builtins.cc
	(load_file_bytes): Return a tl::optional.
	(MacroBuiltin::include_byte_handler): Adjust and check file was
	correctly opened.
	(MacroBuiltin::include_str_handler): Likewise.

gcc/testsuite/ChangeLog:

	* rust/compile/empty_file: New file.
	* rust/compile/issue-2421_str.rs: New test.

Signed-off-by: Marc Poulhiès <dkm@kataplop.net>

2 files changed, 10 insertions, 0 deletions

diff --git a/gcc/testsuite/rust/compile/empty_file b/gcc/testsuite/rust/compile/empty_file
new file mode 100644
index 0000000..e69de29
--- /dev/null
+++ b/gcc/testsuite/rust/compile/empty_file
diff --git a/gcc/testsuite/rust/compile/issue-2421_str.rs b/gcc/testsuite/rust/compile/issue-2421_str.rs
new file mode 100644
index 0000000..2616ccb
--- /dev/null
+++ b/gcc/testsuite/rust/compile/issue-2421_str.rs
@@ -0,0 +1,10 @@
+#![feature(rustc_attrs)]
+
+#[rustc_builtin_macro]
+macro_rules! include_str {
+    () => {{}};
+}
+
+fn main() {
+    include_str!("empty_file");
+}