Zed - enable inlay type hints for Rust projects

Inlay hints or inline type hints allow you to see the derived types of a variable.

For example when you assign the return value of a function to a variable but you don't explicitely write the type of the variable, Rust will derive (deduct) the type based on the function signature.

If you declare a variable let x = 2;, Rust will derived that it is an i32 and the inlay type hints will show let x: i32 = 2;.

In another case if add together two variables holding numbers (that default to i32) and declare the resulting variable to be i8 then Rust will derived that the original two variables also must be i8.

So

    let x = 2;
    let y = 3;

is understood as

    let x: i32 = 2;
    let y: i32 = 3;

but

    let x = 2;
    let y = 3;
    let z: i8 = x +y;

is understood as

    let x: i8 = 2;
    let y: i8 = 3;
    let z: i8 = x +y;

The inlay type hints will help you understand this.

Inline (inlay) type hints:

I configured the following to make it work. I am not sure everything is really needed.

• Installed rust-analyzers

rustup component add rust-analyzer

• settings.json

In the settings.json file I added the following:

  "inlay_hints": {
    "enabled": true,
  },
  "enable_language_server": true,
  "lsp": {
    "rust-analyzer": {
      "binary": {
        "path": "/home/gabor/.cargo/bin/rust-analyzer"
      },
      "initialization_options": {
        "inlayHints": {
          "maxLength": null,
          "lifetimeElisionHints": {
            "enable": "always",
            "useParameterNames": true,
          },
          "closureReturnTypeHints": {
            "enable": "always",
          },
        },
      },
    },
  },

• Trusted code

Opened the crate I was working on as a "project" and then in the top-left corner of the IDE I clicked on "Trusted project".

Then I could also use Crtl-: to toggle the inline hint.