Add `is_nsfw` check to channels

A new feature in Discord is warning users about NSFW channels. This can
be useful when wanting to determine if a command can be used in a
channel or not, depending on the command content.

To help with this, provide a utility function named `utils::is_nsfw`.
This function accepts a channel name, which determines if the channel
is NSFW.

This information is not provided with data from the server. It is
determined client-side based on a few rules.

The rules for a channel being NSFW are:

- must be a guild channel
- must be a text channel
- must be named `nsfw` or be prefixed with `nsfw-`

If any of these conditions are false, then the channel is not NSFW.

Additionally, provide four helper methods:

- `GuildChannel::is_nsfw`: follows rules
- `Group::is_nsfw`: always false
- `PrivateChannel::is_nsfw`: always false
- `Channel::is_nsfw`: depends on inner channel (one of 3 above)

This check is volatile, as Discord may change requirements at any time.
The check provided by the library should not be taken as being accurate
all the time.

1 files changed, 53 insertions, 0 deletions

diff --git a/src/utils/mod.rs b/src/utils/mod.rs
index adb2d45..ec19ff7 100644
--- a/src/utils/mod.rs
+++ b/src/utils/mod.rs
@@ -22,6 +22,59 @@ use ::model::{EmojiIdentifier, EmojiId};
 
 pub use self::message_builder::MessageBuilder;
 
+/// Determines if a name is NSFW.
+///
+/// This checks that the name is either `"nsfw"` or, for names longer than that,
+/// is prefixed with `"nsfw"`.
+///
+/// **Note**: Whether a channel is NSFW is done client-side, as a field for the
+/// NSFW-ness of a channel is not sent to clients. Discord's requirements for
+/// defining a channel as NSFW can change at any time.
+///
+/// # Examples
+///
+/// Check that a channel named `"nsfw"` is in fact NSFW:
+///
+/// ```rust
+/// use serenity::utils;
+///
+/// assert!(utils::is_nsfw("nsfw"));
+/// ```
+///
+/// Check that a channel named `"cats"` is _not_ NSFW:
+///
+/// ```rust
+/// use serenity::utils;
+///
+/// assert!(!utils::is_nsfw("cats"));
+/// ```
+///
+/// Check that a channel named `"nsfw-stuff"` _is_ NSFW:
+///
+/// ```rust
+/// use serenity::utils;
+///
+/// assert!(utils::is_nsfw("nsfw-stuff"));
+/// ```
+///
+/// Channels prefixed with `"nsfw"` but not the hyphen (`'-'`) are _not_
+/// considered NSFW:
+///
+/// ```rust
+/// use serenity::utils;
+///
+/// assert!(!utils::is_nsfw("nsfwstuff"));
+/// ```
+pub fn is_nsfw(name: &str) -> bool {
+    if name.len() == 4 {
+        &name[..4] == "nsfw"
+    } else if name.len() > 4 {
+        &name[..5] == "nsfw-"
+    } else {
+        false
+    }
+}
+
 /// Retrieves the "code" part of an [invite][`RichInvite`] out of a URL.
 ///
 /// # Examples