Re: [PATCH] Add support for untagged unions

[Manish Goregaokar <manish@mozilla.com>]

Commit message of this and the previous patch should probably say "in
Rust" somewhere (will amend before pushing)
-Manish


On Fri, Oct 28, 2016 at 6:02 PM, Manish Goregaokar <manish@mozilla.com> wrote:
> Rust supports untagged unions (C unions) now (using the same syntax as
> structs but with `union` instead of `struct` in the declaration).
> These are mainly used for FFI.
>
> No tests because stable Rust doesn't have these yet. Let me know if I
> should add them.
>
> From: Manish Goregaokar <manish@mozilla.com>
> Subject: [PATCH] Add support for untagged unions
>
> 2016-10-28  Manish Goregaokar  <manish@mozilla.com>
>
> gdb/ChangeLog:
>     * rust-lang.c (rust_union_is_untagged): Add function to
>     check if a union is an untagged unioni
>     * rust-lang.c (rust_val_print): Handle printing of untagged union values
>     * rust-lang.c (rust_print_type): Handle printing of untagged union types
>     * rust-lang.c (rust_evaluate_subexp): Handle evaluating field
>     access on untagged unions
> ---
>  gdb/rust-lang.c | 56 ++++++++++++++++++++++++++++++++++++++++++++++++++++----
>  1 file changed, 52 insertions(+), 4 deletions(-)
>
> diff --git a/gdb/rust-lang.c b/gdb/rust-lang.c
> index 9569584..5376efc 100644
> --- a/gdb/rust-lang.c
> +++ b/gdb/rust-lang.c
> @@ -93,6 +93,28 @@ struct disr_info
>
>  #define RUST_ENCODED_ENUM_HIDDEN 1
>
> +/* Whether or not a TYPE_CODE_UNION value is an untagged union
> +   as opposed to being a regular Rust enum.  */
> +static bool
> +rust_union_is_untagged(struct type *type) {
> +  /* Unions must have at least one field.  */
> +  if (TYPE_NFIELDS (type) == 0)
> +    return false;
> +  /* If the first field is named, but the name has the rust enum prefix,
> +      it is an enum.  */
> +  if (strncmp (TYPE_FIELD_NAME (type, 0), RUST_ENUM_PREFIX,
> +         strlen (RUST_ENUM_PREFIX)) == 0)
> +    return false;
> +  /* Unions only have named fields.  */
> +  for (int i = 0; i < TYPE_NFIELDS (type); ++i)
> +    {
> +      if (strlen (TYPE_FIELD_NAME (type, i)) == 0)
> +        return false;
> +    }
> +    return true;
> +
> +}
> +
>  /* Utility function to get discriminant info for a given value.  */
>
>  static struct disr_info
> @@ -566,6 +588,14 @@ rust_val_print (struct type *type, const gdb_byte
> *valaddr, int embedded_offset,
>      struct value_print_options opts;
>      struct cleanup *cleanup;
>
> +  /* Untagged unions are printed as if they are structs.
> +     Since the field bit positions overlap in the debuginfo,
> +     the code for printing a union is same as that for a struct,
> +     the only difference is that the input type will have overlapping
> +     fields.  */
> +  if (rust_union_is_untagged (type))
> +      goto struct_val;
> +
>      opts = *options;
>      opts.deref_ref = 0;
>
> @@ -638,6 +668,7 @@ rust_val_print (struct type *type, const gdb_byte
> *valaddr, int embedded_offset,
>        break;
>
>      case TYPE_CODE_STRUCT:
> +    struct_val:
>        {
>      int i;
>      int first_field;
> @@ -809,6 +840,7 @@ rust_print_type (struct type *type, const char *varstring,
>        break;
>
>      case TYPE_CODE_STRUCT:
> +    struct_printer:
>        {
>      int is_tuple_struct;
>
> @@ -823,7 +855,12 @@ rust_print_type (struct type *type, const char *varstring,
>      if (TYPE_N_BASECLASSES (type) > 0)
>        goto c_printer;
>
> -    fputs_filtered ("struct ", stream);
> +  /* This code path is also used by unions.  */
> +  if (TYPE_CODE (type) == TYPE_CODE_STRUCT)
> +      fputs_filtered ("struct ", stream);
> +  else
> +    fputs_filtered ("union ", stream);
> +
>      if (TYPE_TAG_NAME (type) != NULL)
>        fputs_filtered (TYPE_TAG_NAME (type), stream);
>
> @@ -899,6 +936,13 @@ rust_print_type (struct type *type, const char *varstring,
>      /* Skip the discriminant field.  */
>      int skip_to = 1;
>
> +  /* Unions and structs have the same syntax in Rust,
> +     the only difference is that structs are declared with `struct`
> +     and union with `union`. This difference is handled in the struct
> +     printer.  */
> +  if (rust_union_is_untagged (type))
> +    goto struct_printer;
> +
>      fputs_filtered ("enum ", stream);
>      if (TYPE_TAG_NAME (type) != NULL)
>        {
> @@ -1637,7 +1681,10 @@ rust_evaluate_subexp (struct type *expect_type,
> struct expression *exp,
>          lhs = evaluate_subexp (NULL_TYPE, exp, pos, noside);
>
>          type = value_type (lhs);
> -        if (TYPE_CODE (type) == TYPE_CODE_UNION)
> +        /* Untagged unions can't have anonymous field access since
> +           they can only have named fields.  */
> +        if (TYPE_CODE (type) == TYPE_CODE_UNION
> +            && !rust_union_is_untagged (type))
>        {
>          struct cleanup *cleanup;
>
> @@ -1712,8 +1759,8 @@ tuple structs, and tuple-like enum variants"));
>          lhs = evaluate_subexp (NULL_TYPE, exp, pos, noside);
>
>          type = value_type (lhs);
> -
> -        if (TYPE_CODE (type) == TYPE_CODE_UNION)
> +        if (TYPE_CODE (type) == TYPE_CODE_UNION
> +            && !rust_union_is_untagged (type))
>        {
>          int i, start;
>          struct disr_info disr;
> @@ -1762,6 +1809,7 @@ which has only anonymous fields"),
>        }
>      else
>        {
> +      /* Field access in structs and untagged unions works like C.  */
>          *pos = pc;
>          result = evaluate_subexp_standard (expect_type, exp, pos, noside);
>        }
> --
> 2.10.1