Handle sizeof(type) in Rust

PR rust/21764 notes that "sizeof" does not work correctly for all types
in Rust.  The bug turns out to be an error in the conversion of the AST
to gdb expressions.  This patch fixes the bug and also avoids generating
incorrect expressions in another case.

Tested on the buildbot.  I'm checking this in.

2017-07-14  Tom Tromey  <tom@tromey.com>

	PR rust/21764:
	* rust-exp.y (convert_ast_to_expression): Add "want_type"
	parameter.
	<UNOP_SIZEOF>: Split into separate case.
	<UNOP_VAR_VALUE>: Handle want_type.  Add error case.

2017-07-14  Tom Tromey  <tom@tromey.com>

	PR rust/21764:
	* gdb.rust/simple.exp: Add tests.

1 files changed, 20 insertions, 7 deletions

diff --git a/gdb/rust-exp.y b/gdb/rust-exp.y
index 821abcd..3ac78a3 100644
--- a/gdb/rust-exp.y
+++ b/gdb/rust-exp.y
@@ -2100,7 +2100,8 @@ convert_name (struct parser_state *state, const struct rust_op *operation)
 
 static void convert_ast_to_expression (struct parser_state *state,
 				       const struct rust_op *operation,
-				       const struct rust_op *top);
+				       const struct rust_op *top,
+				       bool want_type = false);
 
 /* A helper function that converts a vec of rust_ops to a gdb
    expression.  */
@@ -2121,12 +2122,16 @@ convert_params_to_expression (struct parser_state *state,
    OPERATION is the operation to lower.  TOP is a pointer to the
    top-most operation; it is used to handle the special case where the
    top-most expression is an identifier and can be optionally lowered
-   to OP_TYPE.  */
+   to OP_TYPE.  WANT_TYPE is a flag indicating that, if the expression
+   is the name of a type, then emit an OP_TYPE for it (rather than
+   erroring).  If WANT_TYPE is set, then the similar TOP handling is
+   not done.  */
 
 static void
 convert_ast_to_expression (struct parser_state *state,
 			   const struct rust_op *operation,
-			   const struct rust_op *top)
+			   const struct rust_op *top,
+			   bool want_type)
 {
   switch (operation->opcode)
     {
@@ -2166,12 +2171,16 @@ convert_ast_to_expression (struct parser_state *state,
       }
       break;
 
+    case UNOP_SIZEOF:
+      convert_ast_to_expression (state, operation->left.op, top, true);
+      write_exp_elt_opcode (state, UNOP_SIZEOF);
+      break;
+
     case UNOP_PLUS:
     case UNOP_NEG:
     case UNOP_COMPLEMENT:
     case UNOP_IND:
     case UNOP_ADDR:
-    case UNOP_SIZEOF:
       convert_ast_to_expression (state, operation->left.op, top);
       write_exp_elt_opcode (state, operation->opcode);
       break;
@@ -2337,7 +2346,8 @@ convert_ast_to_expression (struct parser_state *state,
 	    if (type == NULL)
 	      error (_("No symbol '%s' in current context"), varname);
 
-	    if (TYPE_CODE (type) == TYPE_CODE_STRUCT
+	    if (!want_type
+		&& TYPE_CODE (type) == TYPE_CODE_STRUCT
 		&& TYPE_NFIELDS (type) == 0)
 	      {
 		/* A unit-like struct.  */
@@ -2346,13 +2356,16 @@ convert_ast_to_expression (struct parser_state *state,
 		write_exp_elt_longcst (state, 0);
 		write_exp_elt_opcode (state, OP_AGGREGATE);
 	      }
-	    else if (operation == top)
+	    else if (want_type || operation == top)
 	      {
 		write_exp_elt_opcode (state, OP_TYPE);
 		write_exp_elt_type (state, type);
 		write_exp_elt_opcode (state, OP_TYPE);
-		break;
 	      }
+	    else
+	      error (_("Found type '%s', which can't be "
+		       "evaluated in this context"),
+		     varname);
 	  }
       }
       break;