Trait std::convert::From<String> not implemented for hyper::body::Body - rust

Question

lazy_static::lazy_static! {
static ref file_data: String = fs::read_to_string("static/login.html").expect("unable to read from static/login.html");
}
#[tokio::main]
async fn main() {
// code omitted
let login = warp::path("login").map(move || warp::reply::html(file_data));
// code omitted
}
Compile error:
error[E0277]: the trait bound `hyper::body::body::Body: std::convert::From<file_data>` is not satisfied
--> src/main.rs:45:49
|
45 | let login = warp::path("login").map(move || warp::reply::html(file_data));
| ^^^^^^^^^^^^^^^^^ the trait `std::convert::From<file_data>` is not implemented for `hyper::body::body::Body`
|
::: /home/ichi/.cargo/registry/src/github.com-1ecc6299db9ec823/warp-0.2.3/src/reply.rs:170:11
|
170 | Body: From<T>,
| ------- required by this bound in `warp::reply::html`
|
= help: the following implementations were found:
<hyper::body::body::Body as std::convert::From<&'static [u8]>>
<hyper::body::body::Body as std::convert::From<&'static str>>
<hyper::body::body::Body as std::convert::From<bytes::bytes::Bytes>>
<hyper::body::body::Body as std::convert::From<std::borrow::Cow<'static, [u8]>>>
and 4 others
I am trying to pass a String to a closure. According to the documentation, From<String> is implemented for hyper::body::Body and file_data is of type String. So why am I getting this error?

Answer 1

From lazy_static
For a given static ref NAME: TYPE = EXPR;, the macro generates a unique type that implements Deref<TYPE> and stores it in a static with name NAME. (Attributes end up attaching to this type.)
That is, the type of the variable is not TYPE!
This is why you see the error
the trait `std::convert::From<file_data>` is not implemented for `hyper::body::body::Body`
^^^^^^^^^
You'd probably have better luck explicitly de-referencing or cloning the global variable: warp::reply::html(&*file_data) or warp::reply::html(file_data.clone()).

Answer 2

The issue here is that lazy_static creates a wrapper type that references your data, and hyper doesn't know how to handle it. You could use file_data.clone() to clone the referenced data and construct a body from that, but in this case there's actually a simpler method. Since your string has a fixed value and Body implements From<&'static str>, you don't actually need a heap-allocated String or lazy_static at all: You can use the following code which just uses a constant string slice that can be used directly.
const FILE_DATA: &str = "test";
#[tokio::main]
async fn main() {
// code omitted
let login = warp::path("login").map(move || warp::reply::html(FILE_DATA));
// code omitted
}