Add ast validation check on union variant number

Unions with zero fields are forbidden. Add regression test for empty
unions.

gcc/rust/ChangeLog:

	* checks/errors/rust-ast-validation.cc (ASTValidation::visit): Add
	zero field check during ast validation pass.
	* checks/errors/rust-ast-validation.h: Add union visit function
	prototype.

gcc/testsuite/ChangeLog:

	* rust/compile/const_generics_8.rs: Fill the union with dummy values.
	* rust/compile/empty_union.rs: New test.

Signed-off-by: Pierre-Emmanuel Patry <[email protected]>

gcc/rust/checks/errors/rust-ast-validation.cc

@@ -18,6 +18,7 @@
 
 #include "rust-ast-validation.h"
 #include "rust-diagnostics.h"
+#include "rust-item.h"
 #include "rust-keyword-values.h"
 
 namespace Rust {
@@ -81,6 +82,15 @@ ASTValidation::visit (AST::ExternalFunctionItem &item)
   AST::ContextualASTVisitor::visit (item);
 }
 
+void
+ASTValidation::visit (AST::Union &item)
+{
+  if (item.get_variants ().empty ())
+    rust_error_at (item.get_locus (), "unions cannot have zero fields");
+
+  AST::ContextualASTVisitor::visit (item);
+}
+
 void
 ASTValidation::visit (AST::Function &function)
 {

gcc/rust/checks/errors/rust-ast-validation.h

@@ -21,6 +21,7 @@
 
 #include "rust-ast-visitor.h"
 #include "rust-ast-full.h"
+#include "rust-item.h"
 
 namespace Rust {
 
@@ -37,6 +38,7 @@ class ASTValidation : public AST::ContextualASTVisitor
   virtual void visit (AST::Lifetime &lifetime);
   virtual void visit (AST::LoopLabel &label);
   virtual void visit (AST::ExternalFunctionItem &item);
+  virtual void visit (AST::Union &item);
   virtual void visit (AST::Function &function);
   virtual void visit (AST::Trait &trait);
 };

gcc/testsuite/rust/compile/const_generics_8.rs

@@ -8,7 +8,12 @@ enum Bidoule<const N: i32 = 15> {}
 type Bipboupe<const N: i32 = 15> = Bidule;
 trait Fooable<const N: i32 = 15> {}
 
-union Bidoulepe<const N: i32 = 15> {} // { dg-error "default values for const generic parameters are not allowed in .union. items" }
+union Bidoulepe<const N: i32 = 15> {
+    // { dg-error "default values for const generic parameters are not allowed in .union. items"  "" {target *-*-* } .-1 }
+    int: i32,
+    float: f32,
+}
+
 fn const_default<const N: i32 = 15>() {} // { dg-error "default values for const generic parameters are not allowed in .function. items" }
 
 // Note - missing generic parameter - needs name resolution on const generics

gcc/testsuite/rust/compile/empty_union.rs

@@ -0,0 +1,2 @@
+#[repr(C)]
+union MyUnion {} // { dg-error "unions cannot have zero fields" }