gccrs: Add fatal_error when experimental flag is not present

This commits ports over the flag/env variable checking from #1540 which only
allows compilation using gccrs if a certain flag is specified

	gcc/rust/
	* lang.opt (-frust-incomplete-and-experimental-compiler-do-not-use):
	New.
	* rust-session-manager.cc (Session::compile_crate): Check it.
	* Make-lang.in (RUST_SELFTEST_FLAGS): Add it.
	gcc/testsuite/
	* lib/rust.exp (rust_init): Add
	'-frust-incomplete-and-experimental-compiler-do-not-use'.

gcc/rust/Make-lang.in

@@ -267,7 +267,7 @@ rust.uninstall:
 # Enable selftests for the rust frontend
 selftest-rust: s-selftest-rust
 
-RUST_SELFTEST_FLAGS = -xrust $(SELFTEST_FLAGS)
+RUST_SELFTEST_FLAGS = -xrust -frust-incomplete-and-experimental-compiler-do-not-use $(SELFTEST_FLAGS)
 RUST_SELFTEST_DEPS = rust1$(exeext) $(SELFTEST_DEPS)
 
 # Run the rust selftests

gcc/rust/lang.opt

@@ -66,6 +66,10 @@ frust-dump-
 Rust Joined RejectNegative
 -frust-dump-<type>	Dump Rust frontend internal information.
 
+frust-incomplete-and-experimental-compiler-do-not-use
+Rust Var(flag_rust_experimental)
+Enable experimental compilation of Rust files at your own risk
+
 frust-max-recursion-depth=
 Rust RejectNegative Type(int) Var(rust_max_recursion_depth) Init(64)
 -frust-max-recursion-depth=integer

gcc/rust/rust-session-manager.cc

@@ -417,6 +417,28 @@ Session::handle_crate_name (const AST::Crate &parsed_crate)
 void
 Session::compile_crate (const char *filename)
 {
+  if (!flag_rust_experimental
+      && !std::getenv ("GCCRS_INCOMPLETE_AND_EXPERIMENTAL_COMPILER_DO_NOT_USE"))
+    rust_fatal_error (
+      Location (), "%s",
+      "gccrs is not yet able to compile Rust code "
+      "properly. Most of the errors produced will be gccrs' fault and not the "
+      "crate you are trying to compile. Because of this, please reports issues "
+      "to us directly instead of opening issues on said crate's "
+      "repository.\n\nOur github repository: "
+      "https://github.com/rust-gcc/gccrs\nOur bugzilla tracker: "
+      "https://gcc.gnu.org/bugzilla/"
+      "buglist.cgi?bug_status=__open__&component=rust&product=gcc\n\n"
+      "If you understand this, and understand that the binaries produced might "
+      "not behave accordingly, you may attempt to use gccrs in an experimental "
+      "manner by passing the following flag:\n\n"
+      "`-frust-incomplete-and-experimental-compiler-do-not-use`\n\nor by "
+      "defining the following environment variable (any value will "
+      "do)\n\nGCCRS_INCOMPLETE_AND_EXPERIMENTAL_COMPILER_DO_NOT_USE\n\nFor"
+      "cargo-gccrs, this means passing\n\n"
+      "GCCRS_EXTRA_FLAGS=\"-frust-incomplete-and-experimental-compiler-do-not-"
+      "use\"\n\nas an environment variable.");
+
   RAIIFile file_wrap (filename);
   if (!file_wrap.ok ())
     {

gcc/testsuite/lib/rust.exp

@@ -149,6 +149,8 @@ proc rust_init { args } {
 	lappend ALWAYS_RUSTFLAGS "additional_flags=$TOOL_OPTIONS"
     }
 
+	lappend ALWAYS_RUSTFLAGS "additional_flags=-frust-incomplete-and-experimental-compiler-do-not-use"	
+	
     verbose -log "ALWAYS_RUSTFLAGS set to $ALWAYS_RUSTFLAGS"
 
     set gcc_warning_prefix "warning:"