Make gethostname an optional feature

Cargo.toml

@@ -12,11 +12,12 @@ categories = ["email"]
 readme = "README.md"
 
 [features]
-default = []
+default = ["gethostname"]
+gethostname = ["dep:gethostname"]
 ludicrous_mode = []
 
 [dependencies]
-gethostname = "0.4.0"
+gethostname = { version = "0.4.0", optional = true }
 
 [dev-dependencies]
 mail-parser = "0.9"

README.md

@@ -11,6 +11,7 @@ _mail-builder_ is a flexible **e-mail builder library** written in Rust. It incl
 - Full **MIME** support (_RFC 2045 - 2049_) with automatic selection of the most optimal encoding for each message body part.
 - **Fast Base64 encoding** based on Chromium's decoder ([the fastest non-SIMD encoder](https://github.com/lemire/fastbase64)).
 - Minimal dependencies.
+  - The dependency on `gethostname` is optional.
 
 Please note that this library does not support sending or parsing e-mail messages as these functionalities are provided by the crates [`mail-send`](https://crates.io/crates/mail-send) and [`mail-parser`](https://crates.io/crates/mail-parser).
 

src/lib.rs

@@ -410,10 +410,16 @@ impl<'x> MessageBuilder<'x> {
 
         if !has_message_id {
             output.write_all(b"Message-ID: ")?;
+
+            #[cfg(feature = "gethostname")]
             generate_message_id_header(
                 &mut output,
                 gethostname::gethostname().to_str().unwrap_or("localhost"),
             )?;
+
+            #[cfg(not(feature = "gethostname"))]
+            generate_message_id_header(&mut output, "localhost")?;
+
             output.write_all(b"\r\n")?;
         }
 

src/mime.rs

@@ -108,7 +108,13 @@ thread_local!(static COUNTER: Cell<u64> = Cell::new(0));
 
 pub fn make_boundary(separator: &str) -> String {
     let mut s = DefaultHasher::new();
+
+    #[cfg(feature = "gethostname")]
     gethostname::gethostname().hash(&mut s);
+
+    #[cfg(not(feature = "gethostname"))]
+    "localhost".hash(&mut s);
+
     thread::current().id().hash(&mut s);
     let hash = s.finish();