Not linting irrefutable_let_patterns on let chains

[Natural-selection1]

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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 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.

[scottmcm]

Huh, this lint actually just caught a mistake for me:

(But that'd also be handled well with a special-case "your TryFrom could be From" lint.)

[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[Natural-selection1]

I resolved the merge conflicts, can we proceed with merging this PR now?
I'm hoping to have it released together with Stabilize if let guards in version 1.95
: )

[Kobzol]

@rustbot ready

[petrochenkov]

@bors r+

[rust-bors]

📋 This PR cannot be approved because it currently has the following label: S-waiting-on-t-lang.

[petrochenkov]

@bors r+

[rust-bors]

📌 Commit 4f31ff8 has been approved by petrochenkov

It is now in the queue for this repository.