Migrating from Rust to Gossamer¶

Gossamer looks a lot like Rust — the lexical grammar, keyword list, and item shape are deliberate references. The differences compress into a handful of rules.

Differences that matter¶

Rust	Gossamer
Manual lifetimes (`'a`, `'static`) on references.	No explicit lifetimes. The GC owns every heap aggregate; `&T` is a plain shared reference with lifetime inferred from scope.
Ownership-by-move, `Copy` marker trait.	No move semantics. Non-trivial values are GC-heap and shared; primitives are copied the same as Rust.
Procedural and declarative macros.	No user macros at all. Six fixed `format!` / `println!`-family macros expand at parse time.
`async fn`, `Future`, `await`.	`go expr` spawns a goroutine. No futures, no awaits — blocking IO is fine.
Multiple separate compilation units, workspace member graph.	Same workspace idea (`gos new --template workspace`). Individual crates are called packages and resolve through `project.toml`.
`unsafe` blocks.	Forbidden at the language level. No `unsafe` keyword in Gossamer source. `std` is safe-Rust too.
`panic!` unwinds by default.	`panic` aborts the current goroutine; handlers observe a 500 but the process keeps running.
`Result<T, E>` + `?` + `thiserror`.	`Result<T, E>` + `?` + `std::errors::Error` (single concrete error type).

What stays the same¶

struct, enum, impl, trait syntax.
match with exhaustiveness checking, guards, or-patterns.
if let / while let.
Iterators (for n in 0..10).
Module tree (mod, use, pub).
cargo-shaped CLI: gos build, gos test, gos fmt, gos check.

Translation examples¶

Rust:

pub fn fetch(url: &str) -> Result<Vec<u8>, reqwest::Error> {
    let response = reqwest::blocking::get(url)?;
    Ok(response.bytes()?.to_vec())
}

Gossamer:

pub fn fetch(url: &str) -> Result<[u8], errors::Error> {
    let response = http::get(url)?
    Ok(response.body())
}

Rust:

struct Server { handler: Box<dyn Fn(Request) -> Response + Send + Sync> }

Gossamer:

struct Server { handler: fn(http::Request) -> http::Response }

(Trait objects stay available but rarely needed — concrete closure types are preferred and the GC keeps their captures alive.)