resolve: Add "break rust" Easter egg

When we encounter a "break rust" statement, emit a funny error message
and intentionally cause an ICE. This matches the corresponding Easter
egg in rustc. As a GNU extension, "break gcc" is also supported.

The conditions for this to happen are:
* The break expression must be literally "rust" or "gcc". For instance,
  "break (rust)" will not trigger the Easter egg.
* The name ("rust" or "gcc") must not be in scope; if it is, no error
  is emitted, and the compilation proceeds as usual. In other words,
  this only affects how GCC diagnoses programs that would fail to
  compile anyway.

Closes #1996

gcc/rust/ChangeLog:
	* resolve/rust-ast-resolve-expr.cc: Add "break rust" Easter egg

gcc/testsuite/ChangeLog:
	* rust/compile/break-rust1.rs: New test
	* rust/compile/break-rust2.rs: New test
	* rust/compile/break-rust3.rs: New test

Signed-off-by: Sergey Bugaev <[email protected]>

Author: bugaevc

gcc/rust/resolve/rust-ast-resolve-expr.cc

@@ -120,6 +120,11 @@ ResolveExpr::visit (AST::IdentifierExpr &expr)
     {
       resolver->insert_resolved_type (expr.get_node_id (), resolved_node);
     }
+  else if (funny_error)
+    {
+      rust_error_at (expr.get_locus (), "oopsie woopsie! you broke GCC Rust");
+      internal_error_no_backtrace ("you broke me");
+    }
   else
     {
       rust_error_at (expr.get_locus (), "failed to find name: %s",
@@ -384,7 +389,18 @@ ResolveExpr::visit (AST::BreakExpr &expr)
     }
 
   if (expr.has_break_expr ())
-    ResolveExpr::go (expr.get_break_expr ().get (), prefix, canonical_prefix);
+    {
+      bool funny_error = false;
+      AST::Expr &break_expr = *expr.get_break_expr ().get ();
+      if (break_expr.get_ast_kind () == AST::Kind::IDENTIFIER)
+	{
+	  std::string ident
+	    = static_cast<AST::IdentifierExpr &> (break_expr).as_string ();
+	  if (ident == "rust" || ident == "gcc")
+	    funny_error = true;
+	}
+      ResolveExpr::go (&break_expr, prefix, canonical_prefix, funny_error);
+    }
 }
 
 void

gcc/testsuite/rust/compile/break-rust1.rs

@@ -0,0 +1,7 @@
+fn main() {
+    let rust = "crab";
+    let res = loop {
+        // { dg-warning "unused name" "" { target *-*-* } .-1 }
+        break rust;
+    };
+}

gcc/testsuite/rust/compile/break-rust2.rs

@@ -0,0 +1,4 @@
+fn main() {
+    break (rust);
+    // { dg-error "failed to find name: rust" "" { target *-*-* } .-1 }
+}

gcc/testsuite/rust/compile/break-rust3.rs

@@ -0,0 +1,5 @@
+fn main() {
+    break rust;
+    // { dg-error "oopsie woopsie! you broke GCC Rust"  "" { target *-*-* } .-1 }
+    // { dg-ice "you broke me" }
+}