fix!: parse block and if statements independently of expressions in statements

[asterite]

Description

Problem

Resolves #5228

Summary

The reason why this was parsed fine:

fn foo() {
  for i in 0..10 {
  }
  -1
}

but this wasn't:

fn foo() {
  {}
  -1
}

is that a for expression is parsed as a statement choice, but a block was just parsed as part of an expression. Because of this, if you had {} - 1 it was parsed as an infix expression.

Note that that infix expression is still valid if it's supposed to be an expression, for example:

let x = { 10 } - 1;

However, there's a difference if that { appears as a stand-alone statement: in that case it shouldn't be considered an expression that can form part of other expressions.

The same thing was done with if, which before this PR could never show up as a stand-alone statement.

I checked with Rust and this is the same there (I didn't check their implementation, just that our behavior now matches that of Rust).

Additional Context

This is a breaking change. If code relied on this behavior, it will now require parentheses around it. For example, this used to work (there was one case in our codebase):

if 1 { 2 } else { 3 } as Field

Now it must be written like this:

(if 1 { 2 } else { 3 }) as Field

But, again, this is the same as Rust.

Documentation

Check one:

• No documentation needed.
• Documentation included in this PR.
• [For Experimental Features] Documentation to be submitted in a separate PR.

PR Checklist*

• I have tested the changes locally.
• I have formatted the changes with Prettier and/or cargo fmt on default settings.