Re: [RFC PATCH v3 4/4] builtin/branch: give more useful error messages when renaming

> From: Kaartic Sivaraam <kaarticsivaraam91196@gmail.com>
>
> When trying to rename an inexistent branch to with a name of a branch
> that already exists the rename failed specifying the new branch name
> exists rather than specifying that the branch trying to be renamed
> doesn't exist.
>
>     $ git branch -m tset master
>     fatal: A branch named 'master' already exists.
>
> It's conventional to report that 'tset' doesn't exist rather than
> reporting that 'master' exists, the same way the 'mv' command does.
>
>     (hypothetical)
>     $ git branch -m tset master
>     fatal: branch 'tset' doesn't exist.
>
> That has the problem that the error about an existing branch is shown
> only after the user corrects the error about inexistent branch.
>
>     $ git branch -m test master
>     fatal: A branch named 'master' already exists.
>
> This isn't useful either because the user would have corrected this error in
> a single go if he had been told this alongside the first error. So, give
> more useful error messages by giving errors about old branch name and new
> branch name at the same time. This is possible as the branch name validation
> functions now return the reason they were about to die, when requested.
>
>     $ git branch -m tset master
>     fatal: branch 'tset' doesn't exist, and branch 'master' already exists

> Signed-off-by: Kaartic Sivaraam <kaarticsivaraam91196@gmail.com>
> ---
> diff --git a/builtin/branch.c b/builtin/branch.c
> +static void get_error_msg(struct strbuf* error_msg, const char* oldname, unsigned old_branch_exists,
> +                         const char* newname, enum branch_validation_result res)
> +{
> +       const char* connector_string = _(", and ");
> +
> +       if (!old_branch_exists) {
> +               strbuf_addf(error_msg, _("branch '%s' doesn't exist"), oldname);
> +       }
> +
> +       switch (res) {
> +               case BRANCH_EXISTS_NO_FORCE:
> +                       strbuf_addf(error_msg, "%s", (!old_branch_exists) ? connector_string : "");
> +                       strbuf_addf(error_msg,_("branch '%s' already exists"), newname);
> +                       break;
> +               case CANNOT_FORCE_UPDATE_CURRENT_BRANCH:
> +                       strbuf_addf(error_msg, "%s", (!old_branch_exists) ? connector_string : "");
> +                       strbuf_addstr(error_msg, _("cannot force update the current branch"));
> +                       break;
> +               case INVALID_BRANCH_NAME:
> +                       strbuf_addf(error_msg, "%s", (!old_branch_exists) ? connector_string : "");
> +                       strbuf_addf(error_msg, _("branch name '%s' is invalid"), newname);
> +                       break;
> +               /* not necessary to handle success cases */
> +               case BRANCH_EXISTS:
> +               case BRANCH_DOESNT_EXIST:
> +                       break;
> +       }
> +}