title: Rust no_std split_at_mut function "no method named `split_mut_at` found for type" compilation error in rust stable
body: <p>I am trying to extract bits of code from an embedded rust example that does not compile. A lot of these old embedded examples don't compile because they use nightly and they quickly become broken and neglected. </p>
  
  <pre><code>let mut buffer : [u8; 2048] = [0;2048];
  // some code to fill the buffer here
  // say we want to split the buffer at position 300
  let (request_buffer, response_buffer) = buffer.split_mut_at(300);
  </code></pre>
  
  <p>This example uses <code>#![no_std]</code> so there is no standard library to link to and must have compiled at some point so the function split_mut_at must have worked at some point. I am using IntelliJ rust AND Visual Studio Code as the IDE but neither IDE's can point me to the definition of the split_mut_at function. There is a minefield of crates and use statements in the example and there is no clear way to pin-point where some function comes without huge trial and error effort. </p>
  
  <p>btw, split_at_mut can usually be found in <code>std::string::String</code></p>
  
  <p>Is there a rust command that tells you what crate a function belongs to in your project? It always takes so long to update rust-docs when doing a rust update. Surely that can help!</p>