Fix logic for rust-analyzer.checkOnSave.features setting inheritance

[ntc2]

Problem addressed by this PR

The rust-analyzer inherits the setting of rust-analyzer.checkOnSave.features from rust-analyzer.cargo.features by default, but the elisp code here was incorrectly defaulting the former to the empty list, regardless of the value of the latter. The effect was that rust-analyzer.cargo.features disabled warnings about "inactive code", without actually turning on type checking of that code!

Besides fixing the bug, by distinguishing between "unset" (null in the rust-analyzer docs) and "set to the empty list" ([] in the rust-analyzer docs), this PR also improves the docs, by mentioning the documented rust-analyzer settings that correspond to the Elisp variables here, which should help with discoverability of these settings (I had a hard time finding them).

Example of the bug

Create a simple Rust project with a Cargo.toml like this:

[package]
name = "example"
version = "0.1.0"
edition = "2021"

[features]
foo = []

[dependencies]

And a src/main.rs like this:

fn main() {
    #[cfg(feature = "foo")]
    foo();
}

#[cfg(feature = "foo")]
fn foo() {
    // Type error:
    let x = 1 + "2";
    println!("{}", x);
}

If you open main.rs in Emacs with rust-analyzer enabled, you will see that that foo function definition and usage are inactive, because the "foo" feature is not enabled:

However, if you enable the "foo" feature in lsp by setting the lsp-rust-features variable with

M-: (setq lsp-rust-features ["foo"]) RET
M-x lsp-restart-workspace RET

then you will see that the foo function definition and usage are no longer
marked as inactive, but the type error is not highlighted:

After the fix in this PR, you instead see the type error highlighted:

The old behavior can be recovered by setting lsp-rust-analyzer-checkonsave-features to [], but that's no longer the default.

[jcs090218]

nit:

lsp-rust.el:600:2: Warning: custom-declare-variable `lsp-rust-analyzer-checkonsave-features' docstring wider than 80 characters

[ntc2]

nit:

lsp-rust.el:600:2: Warning: custom-declare-variable `lsp-rust-analyzer-checkonsave-features' docstring wider than 80 characters

Oops. I just reformatted the doc string and rewrote the commit.

Curious, how do you produce this error?

[jcs090218]

Curious, how do you produce this error?

lsp-mode uses Eask to build and test. You can do:

# Clone this repo
$ git clone https://github.com/emacs-lsp/lsp-mode

# Navigate to project dir
$ cd lsp-mode

# Install dependencies
$ eask install-deps

# Compile to see warnings/errors (if any)
$ eask compile

---

Overall, LGTM! :)