gccrs: Fix ICE with duplicate root item main function

Rust seems to allow duplicate HIR::Item 'main' functions but it needs
to be a root item to be the true main entry point. This means we can
use the canonical path to determine if this is a root one where
its CrateName::main or CrateName::Module::main.

Fixes Rust-GCC#3978

gcc/rust/ChangeLog:

	* backend/rust-compile-base.cc: check the canonical path

gcc/testsuite/ChangeLog:

	* rust/compile/issue-3978.rs: New test.

Signed-off-by: Philip Herron <herron.philip@googlemail.com>

1 files changed, 8 insertions, 0 deletions

diff --git a/gcc/testsuite/rust/compile/issue-3978.rs b/gcc/testsuite/rust/compile/issue-3978.rs
new file mode 100644
index 0000000..4f17d3d
--- /dev/null
+++ b/gcc/testsuite/rust/compile/issue-3978.rs
@@ -0,0 +1,8 @@
+type Dimension = usize;
+
+pub fn main() {}
+
+mod m2 {
+    fn main() {}
+    // { dg-warning "function is never used" "" { target *-*-* } .-1 }
+}