1.rs

#[macro_use] extern crate nickel;

use nickel::{Nickel, HttpRouter};
use std::collections::HashMap;

fn main() {
    let mut server = Nickel::new();

    // * NOTE *
    // This example is deprecated, you should use nickel_mustache from crates.io
    server.get("/", middleware! { |_, res|
        let mut data = HashMap::<&str, &str>::new();
        data.insert("name", "user");
        return res.render("examples/assets/template.tpl", &data)
    });

    server.listen("127.0.0.1:6767").unwrap();
}

// Build output
/*    Compiling synapse-password-reset v0.1.0 (file:///home/esk/dev/synapse-password-reset)
warning: unreachable expression, #[warn(unreachable_code)] on by default
  --> src/main.rs:12:21
   |
12 |     server.get("/", middleware! { |_, res|
   |                     ^
   |
   = note: this error originates in a macro outside of the current crate

    Finished debug [unoptimized + debuginfo] target(s) in 4.46 secs
*/

2.rs

#[macro_use] extern crate nickel;

use nickel::{Nickel, HttpRouter};
use std::collections::HashMap;

fn main() {
    let mut server = Nickel::new();

    // * NOTE *
    // This example is deprecated, you should use nickel_mustache from crates.io
    server.get("/", middleware! { |_, res|
        let mut data = HashMap::<&str, &str>::new();
        data.insert("name", "user");
        res.render("examples/assets/template.tpl", &data)
    });

    server.listen("127.0.0.1:6767").unwrap();
}


// build output
/*   Compiling synapse-password-reset v0.1.0 (file:///home/esk/dev/synapse-password-reset)
error[E0277]: the trait bound `nickel::Action<nickel::Response<'_, _>, nickel::Response<'_, _, hyper::net::Streaming>>: nickel::Responder<_>` is not satisfied
  --> src/main.rs:12:21
   |
12 |     server.get("/", middleware! { |_, res|
   |                     ^ the trait `nickel::Responder<_>` is not implemented for `nickel::Action<nickel::Response<'_, _>, nickel::Response<'_, _, hyper::net::Streaming>>`
   |
   = note: required because of the requirements on the impl of `nickel::Responder<_>` for `std::result::Result<nickel::Action<nickel::Response<'_, _>, nickel::Response<'_, _, hyper::net::Streaming>>, nickel::NickelError<'_, _>>`
   = note: required by `main::restrict`
   = note: this error originates in a macro outside of the current crate

error[E0277]: the trait bound `for<'e> nickel::NickelError<'e, _>: std::convert::From<(nickel::Response<'e, _>, nickel::NickelError<'_, _>)>` is not satisfied
  --> src/main.rs:12:21
   |
12 |     server.get("/", middleware! { |_, res|
   |                     ^ the trait `for<'e> std::convert::From<(nickel::Response<'e, _>, nickel::NickelError<'_, _>)>` is not implemented for `nickel::NickelError<'e, _>`
   |
   = help: the following implementations were found:
   = help:   <nickel::NickelError<'a, D> as std::convert::From<(nickel::Response<'a, D>, (nickel::status::StatusCode, T))>>
   = help:   <nickel::NickelError<'a, D> as std::convert::From<(nickel::Response<'a, D>, std::string::String)>>
   = help:   <nickel::NickelError<'a, D> as std::convert::From<(nickel::Response<'a, D>, nickel::status::StatusCode)>>
   = note: required because of the requirements on the impl of `nickel::Responder<_>` for `std::result::Result<nickel::Action<nickel::Response<'_, _>, nickel::Response<'_, _, hyper::net::Streaming>>, nickel::NickelError<'_, _>>`
   = note: required by `main::restrict`
   = note: this error originates in a macro outside of the current crate

error: aborting due to 2 previous errors

error: Could not compile `synapse-password-reset`.

To learn more, run the command again with --verbose.

  
*/

Example of how macros make things confusing.

Sourced from https://github.com/nickel-org/nickel.rs/blob/master/examples/template.rs

The implicit return is missing from the second.