Support `if let` guards in `matches!()` and `assert_matches!()`

[theemathas]

Once #141295 (stabilizing if_let_guards) is merged, we should probably support if let guards in matches!() and assert_matches!().

[Kivooeo]

In my opinion, it shouldn't. At least I couldn't come up with any useful examples. I thought about it when I was writing a blog post about 'if let' guards

[theemathas]

@Kivooeo Here's an artificial example of how it might be useful.

struct Node {
    parent_id: Option<usize>,
}

fn has_great_grandparent(curr: &Node, nodes: &[Node]) -> bool {
    matches!(curr.parent_id, Some(p)
        if let Some(gp) = nodes[p].parent_id
        && nodes[gp].parent_id.is_some())
}

[theemathas]

A more complicated example:

enum Expr {
    Assign(String, usize),
    Sum(usize, usize),
    Var(String),
    Num(i32),
}

// check if an expression is the form "x = x + 1"
fn is_increment(curr: &Expr, exprs: &[Expr]) -> bool {
    matches!(curr, Expr::Assign(lhs_name, rhs)
        if let Expr::Sum(operand1, operand2) = &exprs[rhs]
        && let Expr::Var(operand1_name) = &exprs[*operand1]
        && lhs_name == operand1_name
        && let Expr::Num(1) = exprs[*operand2]
    )
}

[theemathas]

Real world code which could benefit from let-chains in assert_matches: https://github.com/rust-lang/rust/pull/152173/changes#diff-ee51fdc73fed0b3d6074a8e236e0482fc7e4893a11dd0f2b8823415ca5ebbd09R28