Add clippy lint: use_destructuring

[emilk]

New lint: [use_destructuring]

Adds a pedantic lint that suggests using destructuring when a function accesses all fields of a struct via the self parameter (e.g. self.x, self.y, self.z), and the struct has 3 or more fields. This helps keep code in sync with struct definitions — the compiler will error on the destructuring if a field is added.

Works for both named structs and tuple structs.

Configuration

Two options in clippy.toml:

Option	Type	Default	Description
use-destructuring-min-fields	u64	3	Minimum number of struct fields for the lint to trigger.
use-destructuring-scope	string	"self"	Which variables to lint. See below.

use-destructuring-scope values:

Value	Description
"self"	Only lint the self parameter (default, most conservative).
"Self"	Lint any variable whose type matches Self in the enclosing impl block.
"crate"	Lint any variable whose type is defined in the current crate.
"*"	Lint all struct types, including external ones.

Suppression

The lint does NOT fire when:

• The struct has fewer fields than the configured minimum (default 3).
• Not all fields are accessed.
• The variable is also used as a whole (passed to a function, returned, etc.).
• Any field is mutated (self.x = 5).
• The struct is #[non_exhaustive].
• The struct is a union or enum.
• The field access is from macro expansion.
• Destructuring would cause a name collision with an existing local variable.

Example

struct Point { x: f32, y: f32, z: f32 }

impl std::fmt::Display for Point {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        // Before:
        write!(f, "({}, {}, {})", self.x, self.y, self.z)

        // After:
        // let Self { x, y, z } = self;
        // write!(f, "({x}, {y}, {z})")
    }
}

---

• Followed lint naming conventions
• Added passing UI tests (including committed .stderr file)
• cargo test passes locally
• Executed cargo dev update_lints
• Added lint documentation
• Run cargo dev fmt

changelog: [use_destructuring]: new pedantic lint suggesting destructuring when all fields of a struct are accessed individually

---

Implemented with a lot of help from Claude Code

[github-actions]

Lintcheck changes for 801a92f

Lint	Added	Removed	Changed
clippy::use_destructuring	522	0	0

This comment will be updated if you push new changes

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[dswij]

r? clippy

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

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[CommanderStorm]

👋🏻 Triage here.

We have recently adopted an new policy regarding LLM use.
Please read https://forge.rust-lang.org/policies/llm-usage.html

Concretely regarding your PR

• Please disclose clearly which parts of the PR were generated or assisted by Claude and which were written manually.
  A phrase like "lots of help" is too vague for a reviewer to properly evaluate the contribution and if they want to review your PR.
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Please help us change this PR to also abide by this policy and make this a suceess 🤗.

@rustbot author
@rustbot label +llm-assisted
r? @ghost

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