if clauses following an if let are compiled to match guards

[nox]

https://is.gd/2ZnMND

is this code:

struct Foo;

impl Foo {
    fn bar(&mut self) -> bool { true }
}

fn error(foo: &mut Foo) {
    if let Some(_) = Some(true) {
    } else if foo.bar() {}
}

fn ok(foo: &mut Foo) {
    if let Some(_) = Some(true) {
    } else {
        if foo.bar() {}
    }
}

fn main() {}

yielding:

rustc 1.16.0 (30cf806ef 2017-03-10)
error[E0301]: cannot mutably borrow in a pattern guard
 --> <anon>:9:15
  |
9 |     } else if foo.bar() {}
  |               ^^^ borrowed mutably in pattern guard

[eddyb]

Where this happens. I don't know why we do this, the "else" part of "if" or "if-let" is an arbitrary expression:

if let pat = expr1 {...} else expr2
// can be lowered to
match expr1 {
    pat => {...}
    _ => expr2
}

This seems simpler, less error prone and it won't hit arbitrary limitations specific to match guards.

[nikomatsakis]

The RFC states:

Optionally, one or more else if (not else if let) blocks can be placed in the same match using pattern guards on _. This could be done to simplify the code when pretty-printing the expansion result. Otherwise, this is an unnecessary transformation.

This doesn't feel like a compelling reason, particularly since the desugaring is not otherwise user visible. I would call this a bug.

[nikomatsakis]

triage: P-medium

[nikomatsakis]

This is a great mentoring bug! I'm going to write up some very quick pointers for now. Please reach out for more details. In short, the code that needs to be changed is in the "HIR lowering", which converts from the AST (what the user wrote) to the compiler's internal HIR (also an abstract syntax tree, but somewhat desugared). The code in question is in lowering.rs, specifically right around here. There is also some more logic. You'll want to change how it is desugaring along the lines that @eddyb suggested in this comment.

[cuviper]

I'm interested in taking this bug.

[ghost]

@nikomatsakis @eddyb @nox Hello I'm new to open source and Rust project. I wanted to contribute to the project. May I work on this bug?

[eddyb]

@suchithjn225 Sure thing! Let us know how far you get with #41272 (comment). We're also on IRC.

[cuviper]

@suchithjn225 I'll yield to you -- good luck!

[ghost]

struct Foo;

impl Foo {
    fn bar(&mut self) -> bool { true }
}

fn error(foo: &mut Foo) {
    if let Some(_) = Some(true) {
    } else if foo.bar() {}
}

fn desugar_test(foo: &mut Foo) {
	let x = Some(true);
	match x {
		Some(_) => {},
		_ => {
			if foo.bar() { }
		}
	}
}

fn desugar_error(foo: &mut Foo) {
	let x = Some(true);
	match x {
		Some(_) => {},
		_ if foo.bar() => {},
		_ => {}
	}
}

fn ok(foo: &mut Foo) {
    if let Some(_) = Some(true) {
    } else {
        if foo.bar() {}
    }
}

fn main() {}

If IfLet expression is desugared to the one shown in desugar_test() then it works as suggested by @eddyb. I have also reproduced the bug after desugaring the expression.