Allow also allow literals as first item of single line let chain

[est31]

Also allow literals as the first item in a single line let chain, not just idents. This will most probably not occur in most real-world code, but it makes things a little bit more consistent. While rustfmt allows any literal type, like also string literals, those will not create valid rust code. Only bool literals will.

Implements suggestion from rust-lang/rust#110568 (comment) .

[est31]

@calebcartwright @joshtriplett I found this suggestion when I read over rust-lang/rust#110568 (comment) . From what I can tell, it is consensus of the style team to allow also literals (outside of bool literals, no literal is really valid here).

[joshtriplett]

@est31 Confirmed; we don't want to put false && or true && on a line by itself.

[ytmimi]

@est31 A few minor things. I'd like the code to be a little more explicit about only allowing bool literals. Also, let-chains are still unstable, right?

[ytmimi]

Let's make the match more explicit for bool literals.

[est31]

type checking would reject any literal that's not a bool literal, but shrug, changed it.

[ytmimi]

rustfmt operates at an earlier stage of compilation, so if it parses and could be in the AST then we need to be mindful of that.

[ytmimi]

nit: I think we should be more explicit about bool literals.

Suggested change

	fn is_ident_or_lit(expr: &ast::Expr) -> bool {
	fn is_ident_or_bool_lit(expr: &ast::Expr) -> bool {

Might even be helpful to link to text from the style guide explaining the rules if they've been codified yet.

[est31]

they have not been codified yet, all there is to link is discussion threads :).

[ytmimi]

All good then. No need to link to discussions.

[est31]

let-chains are still unstable, right?

indeed they are, and this PR is one of the things I am doing to get them stabilized: to get all open questions resolved around formatting.

[ytmimi]

Thanks for making those changes.

[ytmimi]

Just ran our Diff-Check job. Looks like there will be two clippy test cases that'll change once we sync these changes with rust-lang/rust:

Diff in /tmp/rust-lang-rust-W6U1GxGa/src/tools/clippy/tests/ui/needless_late_init.rs:246:
     }
 
     let x;
-    if true
-        && let Some(n) = Some("let chains too")
-    {
+    if true && let Some(n) = Some("let chains too") {
         x = 1;
     } else {
         x = 2;

Diff in /tmp/rust-lang-rust-W6U1GxGa/src/tools/clippy/tests/ui/needless_if.rs:46:
     if let true = true
         && true
     {}
-    if true
-        && let true = true
-    {}
+    if true && let true = true {}
     // Can lint nested `if let`s
     if {
         //~^ needless_if