doc: mark out experimental diagnostics to avoid confusion

[hanayashiki]

rust-analyzer version: 0.3.2137-standalone (2b750da 2024-10-06) [/Users/chenyuwang/.vscode/extensions/rust-lang.rust-analyzer-0.3.2137-darwin-arm64/server/rust-analyzer]

rustc version: -

editor or extension: -

relevant settings: -
repository link (if public, optional): -
code snippet to reproduce:

fn main() {
    let n = 3;
    let b = n();
   // show `expected function, found i32rust-analyzer[E0618](https://doc.rust-lang.org/stable/error_codes/E0618.html)`
   // only when "rust-analyzer.diagnostics.experimental.enable": true
}

I'd like to test out the diagnostics https://rust-analyzer.github.io/manual.html#expected-function but it seems it is actually "experimental" and is only enabled when "rust-analyzer.diagnostics.experimental.enable": true is set to true in .vscode/settings.json.
I think the documentation is supposed to make it clear with the configuration of experimental diagnostics in the auto-generated documentation. If possible I think I can make a PR to help.

[darichey]

Whether a Diagnostic is experimental technically isn't known statically, since it's just set like

rust-analyzer/crates/ide-diagnostics/src/handlers/expected_function.rs

Lines 12 to 18 in 0fb804a

	Diagnostic::new_with_syntax_node_ptr(
	ctx,
	DiagnosticCode::RustcHardError("E0618"),
	format!("expected function, found {}", d.found.display(ctx.sema.db, ctx.edition)),
	d.call.map(|it| it.into()),
	)
	.experimental()

In practice a heuristic of "check if the text contains .experimental()" would probably work fine

rust-analyzer/xtask/src/codegen/diagnostics_docs.rs

Lines 40 to 41 in 0fb804a

	fn collect_file(acc: &mut Vec<Diagnostic>, path: PathBuf) -> io::Result<()> {
	let text = fs::read_to_string(&path)?;

Or we could just manually add a note to each experimental diagnostic, with the only risk being them becoming out of sync.

[hanayashiki]

Or just breakingly prefix them to be experimental_xxx since they are experimental anyways?