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:
	* gcc/testsuite/lib/prune.exp (prune_ices):
	Also prune "You broke GCC Rust. This is a feature."
	* 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

@@ -23,6 +23,7 @@
 #include "rust-ast-resolve-type.h"
 #include "rust-ast-resolve-pattern.h"
 #include "rust-ast-resolve-path.h"
+#include "diagnostic.h"
 
 namespace Rust {
 namespace Resolver {
@@ -105,6 +106,19 @@ ResolveExpr::visit (AST::AssignmentExpr &expr)
   VerifyAsignee::go (expr.get_left_expr ().get ());
 }
 
+/* The finalizer for our funny ICE.  This prints a custom message instead of
+   the default bug reporting instructions, as there is no bug to report.  */
+
+static void ATTRIBUTE_NORETURN
+funny_ice_finalizer (diagnostic_context *context, diagnostic_info *diagnostic,
+		     diagnostic_t diag_kind)
+{
+  gcc_assert (diag_kind == DK_ICE_NOBT);
+  default_diagnostic_finalizer (context, diagnostic, diag_kind);
+  fnotice (stderr, "You broke GCC Rust. This is a feature.\n");
+  exit (ICE_EXIT_CODE);
+}
+
 void
 ResolveExpr::visit (AST::IdentifierExpr &expr)
 {
@@ -120,6 +134,17 @@ ResolveExpr::visit (AST::IdentifierExpr &expr)
     {
       resolver->insert_resolved_type (expr.get_node_id (), resolved_node);
     }
+  else if (funny_error)
+    {
+      /* This was a "break rust" or "break gcc", and the identifier failed to
+	 resolve.  Emit a funny ICE.  We set the finalizer to our custom one,
+	 and use the lower-level emit_diagnostic () instead of the more common
+	 internal_error_no_backtrace () in order to pass our locus.  */
+      diagnostic_finalizer (global_dc) = funny_ice_finalizer;
+      emit_diagnostic (DK_ICE_NOBT, expr.get_locus ().gcc_location (), -1,
+		       "are you trying to break %s? how dare you?",
+		       expr.as_string ().c_str ());
+    }
   else
     {
       rust_error_at (expr.get_locus (), "failed to find name: %s",
@@ -384,7 +409,18 @@ 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/lib/prune.exp

@@ -158,6 +158,7 @@ proc prune_file_path { text } {
 proc prune_ices { text } {
   regsub -all "(^|\n)\[^\n\]*: internal compiler error:.*\nSee \[^\n\]*" $text "" text
   regsub -all "(^|\n|')*Internal compiler error:.*\nSee \[^\n\]*" $text "" text
+  regsub -all "(^|\n)\[^\n\]*: internal compiler error:.*\nYou broke GCC Rust. This is a feature." $text "" text
   return $text
 }
 

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 +1,4 @@
+fn main() {
+    break rust;
+    // { dg-ice "are you trying to break rust? how dare you?" }
+}