Not linting irrefutable_let_patterns on let chains

[Natural-selection1]

Description

this PR makes the lint irrefutable_let_patterns not check for let chains,
only check for single if let, while let, and if let guard.

Motivation

Since let chains were stabilized, the following code has become common:

fn max() -> usize { 42 }

fn main() {
    if let mx = max() && mx < usize::MAX { /* */ }
}

This code naturally expresses "please call that function and then do something if the return value satisfies a condition".
Putting the let binding outside the if would be bad as then it remains in scope after the if, which is not the intent.

Current Output:

warning: leading irrefutable pattern in let chain
 --> src/main.rs:7:8
  |
7 |     if let mx = max() && mx < usize::MAX {
  |        ^^^^^^^^^^^^^^
  |
  = note: this pattern will always match
  = help: consider moving it outside of the construct
  = note: `#[warn(irrefutable_let_patterns)]` on by default

Another common case is progressively destructuring a struct with enum fields, or an enum with struct variants:

struct NameOfOuterStruct {
    middle: NameOfMiddleEnum,
    other: (),
}
enum NameOfMiddleEnum {
    Inner(NameOfInnerStruct),
    Other(()),
}
struct NameOfInnerStruct {
    id: u32,
}

fn test(outer: NameOfOuterStruct) {
    if let NameOfOuterStruct { middle, .. } = outer
        && let NameOfMiddleEnum::Inner(inner) = middle
        && let NameOfInnerStruct { id } = inner
    {
        /* */
    }
}

Current Output:

warning: leading irrefutable pattern in let chain
  --> src\main.rs:17:8
   |
17 |     if let NameOfOuterStruct { middle, .. } = outer
   |        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: this pattern will always match
   = help: consider moving it outside of the construct
   = note: `#[warn(irrefutable_let_patterns)]` on by default


warning: trailing irrefutable pattern in let chain
  --> src\main.rs:19:12
   |
19 |         && let NameOfInnerStruct { id } = inner
   |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: this pattern will always match
   = help: consider moving it into the body

To avoid the warning, the readability would be much worse:

fn test(outer: NameOfOuterStruct) {
    if let NameOfOuterStruct {
        middle: NameOfMiddleEnum::Inner(NameOfInnerStruct { id }),
        ..
    } = outer
    {
        /* */
    }
}

related issue

• Spurious irrefutable_let_patterns warning with let-chain #139369

possible questions

1. Moving the irrefutable pattern at the head of the chain out of it would cause a variable that was intended to be temporary to remain in scope, so we remove it.
   However, should we keep the check for moving the irrefutable pattern at the tail into the body?

2. Should we still lint entire chain is made up of irrefutable let?

---

This is my first time contributing non-documentation code to Rust. If there are any irregularities, please feel free to point them out.
: )

[rustbot]

Some changes occurred in match checking

cc @Nadrieril

The Miri subtree was changed

cc @rust-lang/miri

[Natural-selection1]

It seems that only modifying tests/ui/rfcs/rfc-2497-if-let-chains/irrefutable-lets.rs is needed to cover this change. Do I still need to add an additional new test?

[traviscross]

It seems the check you're adding should cause a reworking of the logic that follows.

[traviscross]

It seems that only modifying tests/ui/rfcs/rfc-2497-if-let-chains/irrefutable-lets.rs is needed to cover this change. Do I still need to add an additional new test?

All the lints seem to be removed in that test, but you'll want to ensure there are tests that cover the edges of what is still linted here.

@rustbot author

[rustbot]

Reminder, once the PR becomes ready for a review, use @rustbot ready.

[Natural-selection1]

but you'll want to ensure there are tests that cover the edges of what is still linted here.

match chain_refutabilities.len() {
    0   => "`assert!(self.let_source != LetSource::None)` ensure this case doesn't exist"
    2.. => "`if let [Some((_, refutability))] = chain_refutabilities[..]` ensure not checked"  // The purpose of this PR: don't check let chains
    1 if chain_refutabilities[0].is_none() 
        => "A boolean expression at the start of `while let` | `if let` | `else if let`, this case cannot pass syntax analysis"
    1 if chain_refutabilities[0].is_some() 
        => "There is exactly one let binding in `while let` | `if let` | `else if let` statement, proceed to check"
}

[rustbot]

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

[Natural-selection1]

I tried adding a new test file, but I don't know why ./x test tests/ui/binding/irrefutable-in-let-chains.rs --bless is not generating the .stderr file. I'm not sure what I did incorrectly.
: (

edit: adding //@ check-pass can fix it : )

[Natural-selection1]

@rustbot ready

[traviscross]

Please add a comment at the top of this test describing the purpose of this test overall and comments throughout describing the nature of the cases that we're testing.

[traviscross]

This isn't a good comment in this position. What follows is something that will lint in some cases, and that's what should be described.

[traviscross]

@rustbot author

[Natural-selection1]

@rustbot ready

[traviscross]

This will need an FCP by the lang team. For that, this needs a description of what we'll be stabilizing exactly and the reasons for it. Have a look at other PRs that are labeled T-lang and finished-final-comment-period to see examples of this.

@rustbot author

[Natural-selection1]

this needs a description of what we'll be stabilizing exactly and the reasons for it. Have a look at other PRs that are labeled T-lang and finished-final-comment-period to see examples of this.

Is there anything I can do to move this forward?

[RalfJung]

Yes, you can write such a description. :)

[Natural-selection1]

I have updated the top comment of this PR. Does it now have the proper format for a FCP? If so, what are the next steps to get it started?