rust-clippy/clippy_lints/src/needless_borrowed_ref.rs

use clippy_utils::diagnostics::span_lint_and_then;
use rustc_errors::Applicability;
use rustc_hir::{BindingMode, Mutability, Node, Pat, PatKind};
use rustc_lint::{LateContext, LateLintPass};
use rustc_session::declare_lint_pass;

declare_clippy_lint! {
    /// ### What it does
    /// Checks for bindings that needlessly destructure a reference and borrow the inner
    /// value with `&ref`.
    ///
    /// ### Why is this bad?
    /// This pattern has no effect in almost all cases.
    ///
    /// ### Example
    /// ```no_run
    /// let mut v = Vec::<String>::new();
    /// v.iter_mut().filter(|&ref a| a.is_empty());
    ///
    /// if let &[ref first, ref second] = v.as_slice() {}
    /// ```
    ///
    /// Use instead:
    /// ```no_run
    /// let mut v = Vec::<String>::new();
    /// v.iter_mut().filter(|a| a.is_empty());
    ///
    /// if let [first, second] = v.as_slice() {}
    /// ```
    #[clippy::version = "pre 1.29.0"]
    pub NEEDLESS_BORROWED_REFERENCE,
    complexity,
    "destructuring a reference and borrowing the inner value"
}

declare_lint_pass!(NeedlessBorrowedRef => [NEEDLESS_BORROWED_REFERENCE]);

impl<'tcx> LateLintPass<'tcx> for NeedlessBorrowedRef {
    fn check_pat(&mut self, cx: &LateContext<'tcx>, ref_pat: &'tcx Pat<'_>) {
        if ref_pat.span.from_expansion() {
            // OK, simple enough, lints doesn't check in macro.
            return;
        }

        // Do not lint patterns that are part of an OR `|` pattern, the binding mode must match in all arms
        for (_, node) in cx.tcx.hir().parent_iter(ref_pat.hir_id) {
            let Node::Pat(pat) = node else { break };

            if matches!(pat.kind, PatKind::Or(_)) {
                return;
            }
        }

        // Only lint immutable refs, because `&mut ref T` may be useful.
        let PatKind::Ref(pat, Mutability::Not) = ref_pat.kind else {
            return;
        };

        match pat.kind {
            // Check sub_pat got a `ref` keyword (excluding `ref mut`).
            PatKind::Binding(BindingMode::REF, _, ident, None) => {
                span_lint_and_then(
                    cx,
                    NEEDLESS_BORROWED_REFERENCE,
                    ref_pat.span,
                    "this pattern takes a reference on something that is being dereferenced",
                    |diag| {
                        // `&ref ident`
                        //  ^^^^^
                        let span = ref_pat.span.until(ident.span);
                        diag.span_suggestion_verbose(
                            span,
                            "try removing the `&ref` part",
                            String::new(),
                            Applicability::MachineApplicable,
                        );
                    },
                );
            },
            // Slices where each element is `ref`: `&[ref a, ref b, ..., ref z]`
            PatKind::Slice(
                before,
                None
                | Some(Pat {
                    kind: PatKind::Wild, ..
                }),
                after,
            ) => {
                check_subpatterns(
                    cx,
                    "dereferencing a slice pattern where every element takes a reference",
                    ref_pat,
                    pat,
                    itertools::chain(before, after),
                );
            },
            PatKind::Tuple(subpatterns, _) | PatKind::TupleStruct(_, subpatterns, _) => {
                check_subpatterns(
                    cx,
                    "dereferencing a tuple pattern where every element takes a reference",
                    ref_pat,
                    pat,
                    subpatterns,
                );
            },
            PatKind::Struct(_, fields, _) => {
                check_subpatterns(
                    cx,
                    "dereferencing a struct pattern where every field's pattern takes a reference",
                    ref_pat,
                    pat,
                    fields.iter().map(|field| field.pat),
                );
            },
            _ => {},
        }
    }
}

fn check_subpatterns<'tcx>(
    cx: &LateContext<'tcx>,
    message: &'static str,
    ref_pat: &Pat<'_>,
    pat: &Pat<'_>,
    subpatterns: impl IntoIterator<Item = &'tcx Pat<'tcx>>,
) {
    let mut suggestions = Vec::new();

    for subpattern in subpatterns {
        match subpattern.kind {
            PatKind::Binding(BindingMode::REF, _, ident, None) => {
                // `ref ident`
                //  ^^^^
                let span = subpattern.span.until(ident.span);
                suggestions.push((span, String::new()));
            },
            PatKind::Wild => {},
            _ => return,
        }
    }

    if !suggestions.is_empty() {
        span_lint_and_then(cx, NEEDLESS_BORROWED_REFERENCE, ref_pat.span, message, |diag| {
            // `&pat`
            //  ^
            let span = ref_pat.span.until(pat.span);
            suggestions.push((span, String::new()));

            diag.multipart_suggestion(
                "try removing the `&` and `ref` parts",
                suggestions,
                Applicability::MachineApplicable,
            );
        });
    }
}