rust/clippy_lints/src/swap.rs

use clippy_utils::diagnostics::{span_lint_and_sugg, span_lint_and_then};
use clippy_utils::source::snippet_with_applicability;
use clippy_utils::sugg::Sugg;
use clippy_utils::ty::is_type_diagnostic_item;
use clippy_utils::{can_mut_borrow_both, differing_macro_contexts, eq_expr_value};
use if_chain::if_chain;
use rustc_errors::Applicability;
use rustc_hir::{BinOpKind, Block, Expr, ExprKind, PatKind, QPath, Stmt, StmtKind};
use rustc_lint::{LateContext, LateLintPass};
use rustc_middle::ty;
use rustc_session::{declare_lint_pass, declare_tool_lint};
use rustc_span::source_map::Spanned;
use rustc_span::{sym, Span};

declare_clippy_lint! {
    /// ### What it does
    /// Checks for manual swapping.
    ///
    /// ### Why is this bad?
    /// The `std::mem::swap` function exposes the intent better
    /// without deinitializing or copying either variable.
    ///
    /// ### Example
    /// ```rust
    /// let mut a = 42;
    /// let mut b = 1337;
    ///
    /// let t = b;
    /// b = a;
    /// a = t;
    /// ```
    /// Use std::mem::swap():
    /// ```rust
    /// let mut a = 1;
    /// let mut b = 2;
    /// std::mem::swap(&mut a, &mut b);
    /// ```
    pub MANUAL_SWAP,
    complexity,
    "manual swap of two variables"
}

declare_clippy_lint! {
    /// ### What it does
    /// Checks for `foo = bar; bar = foo` sequences.
    ///
    /// ### Why is this bad?
    /// This looks like a failed attempt to swap.
    ///
    /// ### Example
    /// ```rust
    /// # let mut a = 1;
    /// # let mut b = 2;
    /// a = b;
    /// b = a;
    /// ```
    /// If swapping is intended, use `swap()` instead:
    /// ```rust
    /// # let mut a = 1;
    /// # let mut b = 2;
    /// std::mem::swap(&mut a, &mut b);
    /// ```
    pub ALMOST_SWAPPED,
    correctness,
    "`foo = bar; bar = foo` sequence"
}

declare_lint_pass!(Swap => [MANUAL_SWAP, ALMOST_SWAPPED]);

impl<'tcx> LateLintPass<'tcx> for Swap {
    fn check_block(&mut self, cx: &LateContext<'tcx>, block: &'tcx Block<'_>) {
        check_manual_swap(cx, block);
        check_suspicious_swap(cx, block);
        check_xor_swap(cx, block);
    }
}

fn generate_swap_warning(cx: &LateContext<'_>, e1: &Expr<'_>, e2: &Expr<'_>, span: Span, is_xor_based: bool) {
    let mut applicability = Applicability::MachineApplicable;

    if !can_mut_borrow_both(cx, e1, e2) {
        if let ExprKind::Index(lhs1, idx1) = e1.kind {
            if let ExprKind::Index(lhs2, idx2) = e2.kind {
                if eq_expr_value(cx, lhs1, lhs2) {
                    let ty = cx.typeck_results().expr_ty(lhs1).peel_refs();

                    if matches!(ty.kind(), ty::Slice(_))
                        || matches!(ty.kind(), ty::Array(_, _))
                        || is_type_diagnostic_item(cx, ty, sym::Vec)
                        || is_type_diagnostic_item(cx, ty, sym::VecDeque)
                    {
                        let slice = Sugg::hir_with_applicability(cx, lhs1, "<slice>", &mut applicability);
                        span_lint_and_sugg(
                            cx,
                            MANUAL_SWAP,
                            span,
                            &format!("this looks like you are swapping elements of `{}` manually", slice),
                            "try",
                            format!(
                                "{}.swap({}, {})",
                                slice.maybe_par(),
                                snippet_with_applicability(cx, idx1.span, "..", &mut applicability),
                                snippet_with_applicability(cx, idx2.span, "..", &mut applicability),
                            ),
                            applicability,
                        );
                    }
                }
            }
        }
        return;
    }

    let first = Sugg::hir_with_applicability(cx, e1, "..", &mut applicability);
    let second = Sugg::hir_with_applicability(cx, e2, "..", &mut applicability);
    span_lint_and_then(
        cx,
        MANUAL_SWAP,
        span,
        &format!("this looks like you are swapping `{}` and `{}` manually", first, second),
        |diag| {
            diag.span_suggestion(
                span,
                "try",
                format!("std::mem::swap({}, {})", first.mut_addr(), second.mut_addr()),
                applicability,
            );
            if !is_xor_based {
                diag.note("or maybe you should use `std::mem::replace`?");
            }
        },
    );
}

/// Implementation of the `MANUAL_SWAP` lint.
fn check_manual_swap(cx: &LateContext<'_>, block: &Block<'_>) {
    for w in block.stmts.windows(3) {
        if_chain! {
            // let t = foo();
            if let StmtKind::Local(tmp) = w[0].kind;
            if let Some(tmp_init) = tmp.init;
            if let PatKind::Binding(.., ident, None) = tmp.pat.kind;

            // foo() = bar();
            if let StmtKind::Semi(first) = w[1].kind;
            if let ExprKind::Assign(lhs1, rhs1, _) = first.kind;

            // bar() = t;
            if let StmtKind::Semi(second) = w[2].kind;
            if let ExprKind::Assign(lhs2, rhs2, _) = second.kind;
            if let ExprKind::Path(QPath::Resolved(None, rhs2)) = rhs2.kind;
            if rhs2.segments.len() == 1;

            if ident.name == rhs2.segments[0].ident.name;
            if eq_expr_value(cx, tmp_init, lhs1);
            if eq_expr_value(cx, rhs1, lhs2);
            then {
                let span = w[0].span.to(second.span);
                generate_swap_warning(cx, lhs1, lhs2, span, false);
            }
        }
    }
}

/// Implementation of the `ALMOST_SWAPPED` lint.
fn check_suspicious_swap(cx: &LateContext<'_>, block: &Block<'_>) {
    for w in block.stmts.windows(2) {
        if_chain! {
            if let StmtKind::Semi(first) = w[0].kind;
            if let StmtKind::Semi(second) = w[1].kind;
            if !differing_macro_contexts(first.span, second.span);
            if let ExprKind::Assign(lhs0, rhs0, _) = first.kind;
            if let ExprKind::Assign(lhs1, rhs1, _) = second.kind;
            if eq_expr_value(cx, lhs0, rhs1);
            if eq_expr_value(cx, lhs1, rhs0);
            then {
                let lhs0 = Sugg::hir_opt(cx, lhs0);
                let rhs0 = Sugg::hir_opt(cx, rhs0);
                let (what, lhs, rhs) = if let (Some(first), Some(second)) = (lhs0, rhs0) {
                    (
                        format!(" `{}` and `{}`", first, second),
                        first.mut_addr().to_string(),
                        second.mut_addr().to_string(),
                    )
                } else {
                    (String::new(), String::new(), String::new())
                };

                let span = first.span.to(second.span);

                span_lint_and_then(cx,
                                   ALMOST_SWAPPED,
                                   span,
                                   &format!("this looks like you are trying to swap{}", what),
                                   |diag| {
                                       if !what.is_empty() {
                                           diag.span_suggestion(
                                               span,
                                               "try",
                                               format!(
                                                   "std::mem::swap({}, {})",
                                                   lhs,
                                                   rhs,
                                               ),
                                               Applicability::MaybeIncorrect,
                                           );
                                           diag.note("or maybe you should use `std::mem::replace`?");
                                       }
                                   });
            }
        }
    }
}

/// Implementation of the xor case for `MANUAL_SWAP` lint.
fn check_xor_swap(cx: &LateContext<'_>, block: &Block<'_>) {
    for window in block.stmts.windows(3) {
        if_chain! {
            if let Some((lhs0, rhs0)) = extract_sides_of_xor_assign(&window[0]);
            if let Some((lhs1, rhs1)) = extract_sides_of_xor_assign(&window[1]);
            if let Some((lhs2, rhs2)) = extract_sides_of_xor_assign(&window[2]);
            if eq_expr_value(cx, lhs0, rhs1);
            if eq_expr_value(cx, lhs2, rhs1);
            if eq_expr_value(cx, lhs1, rhs0);
            if eq_expr_value(cx, lhs1, rhs2);
            then {
                let span = window[0].span.to(window[2].span);
                generate_swap_warning(cx, lhs0, rhs0, span, true);
            }
        };
    }
}

/// Returns the lhs and rhs of an xor assignment statement.
fn extract_sides_of_xor_assign<'a, 'hir>(stmt: &'a Stmt<'hir>) -> Option<(&'a Expr<'hir>, &'a Expr<'hir>)> {
    if let StmtKind::Semi(expr) = stmt.kind {
        if let ExprKind::AssignOp(
            Spanned {
                node: BinOpKind::BitXor,
                ..
            },
            lhs,
            rhs,
        ) = expr.kind
        {
            return Some((lhs, rhs));
        }
    }
    None
}