rust-clippy/clippy_lints/src/swap.rs

use rustc::hir::*;
use rustc::lint::*;
use rustc::ty;
use utils::{differing_macro_contexts, match_type, paths, snippet, span_lint_and_then, walk_ptrs_ty, SpanlessEq};
use utils::sugg::Sugg;

/// **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.
///
/// **Known problems:** None.
///
/// **Example:**
/// ```rust,ignore
/// let t = b;
/// b = a;
/// a = t;
/// ```
declare_lint! {
    pub MANUAL_SWAP,
    Warn,
    "manual swap of two variables"
}

/// **What it does:** Checks for `foo = bar; bar = foo` sequences.
///
/// **Why is this bad?** This looks like a failed attempt to swap.
///
/// **Known problems:** None.
///
/// **Example:**
/// ```rust,ignore
/// a = b;
/// b = a;
/// ```
declare_lint! {
    pub ALMOST_SWAPPED,
    Warn,
    "`foo = bar; bar = foo` sequence"
}

#[derive(Copy, Clone)]
pub struct Swap;

impl LintPass for Swap {
    fn get_lints(&self) -> LintArray {
        lint_array![MANUAL_SWAP, ALMOST_SWAPPED]
    }
}

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

/// 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 StmtDecl(ref tmp, _) = w[0].node;
            if let DeclLocal(ref tmp) = tmp.node;
            if let Some(ref tmp_init) = tmp.init;
            if let PatKind::Binding(_, _, ref tmp_name, None) = tmp.pat.node;

            // foo() = bar();
            if let StmtSemi(ref first, _) = w[1].node;
            if let ExprAssign(ref lhs1, ref rhs1) = first.node;

            // bar() = t;
            if let StmtSemi(ref second, _) = w[2].node;
            if let ExprAssign(ref lhs2, ref rhs2) = second.node;
            if let ExprPath(QPath::Resolved(None, ref rhs2)) = rhs2.node;
            if rhs2.segments.len() == 1;

            if tmp_name.node.as_str() == rhs2.segments[0].name.as_str();
            if SpanlessEq::new(cx).ignore_fn().eq_expr(tmp_init, lhs1);
            if SpanlessEq::new(cx).ignore_fn().eq_expr(rhs1, lhs2);
            then {
                fn check_for_slice<'a>(
                    cx: &LateContext,
                    lhs1: &'a Expr,
                    lhs2: &'a Expr,
                ) -> Option<(&'a Expr, &'a Expr, &'a Expr)> {
                    if let ExprIndex(ref lhs1, ref idx1) = lhs1.node {
                        if let ExprIndex(ref lhs2, ref idx2) = lhs2.node {
                            if SpanlessEq::new(cx).ignore_fn().eq_expr(lhs1, lhs2) {
                                let ty = walk_ptrs_ty(cx.tables.expr_ty(lhs1));
    
                                if matches!(ty.sty, ty::TySlice(_)) ||
                                    matches!(ty.sty, ty::TyArray(_, _)) ||
                                    match_type(cx, ty, &paths::VEC) ||
                                    match_type(cx, ty, &paths::VEC_DEQUE) {
                                        return Some((lhs1, idx1, idx2));
                                }
                            }
                        }
                    }
    
                    None
                }
    
                let (replace, what, sugg) = if let Some((slice, idx1, idx2)) = check_for_slice(cx, lhs1, lhs2) {
                    if let Some(slice) = Sugg::hir_opt(cx, slice) {
                        (false,
                         format!(" elements of `{}`", slice),
                         format!("{}.swap({}, {})",
                                 slice.maybe_par(),
                                 snippet(cx, idx1.span, ".."),
                                 snippet(cx, idx2.span, "..")))
                    } else {
                        (false, "".to_owned(), "".to_owned())
                    }
                } else if let (Some(first), Some(second)) = (Sugg::hir_opt(cx, lhs1), Sugg::hir_opt(cx, rhs1)) {
                    (true, format!(" `{}` and `{}`", first, second),
                        format!("std::mem::swap({}, {})", first.mut_addr(), second.mut_addr()))
                } else {
                    (true, "".to_owned(), "".to_owned())
                };
    
                let span = w[0].span.to(second.span);
    
                span_lint_and_then(cx,
                                   MANUAL_SWAP,
                                   span,
                                   &format!("this looks like you are swapping{} manually", what),
                                   |db| {
                                       if !sugg.is_empty() {
                                           db.span_suggestion(span, "try", sugg);
    
                                           if replace {
                                               db.note("or maybe you should use `std::mem::replace`?");
                                           }
                                       }
                                   });
            }
        }
    }
}

/// 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 StmtSemi(ref first, _) = w[0].node;
            if let StmtSemi(ref second, _) = w[1].node;
            if !differing_macro_contexts(first.span, second.span);
            if let ExprAssign(ref lhs0, ref rhs0) = first.node;
            if let ExprAssign(ref lhs1, ref rhs1) = second.node;
            if SpanlessEq::new(cx).ignore_fn().eq_expr(lhs0, rhs1);
            if SpanlessEq::new(cx).ignore_fn().eq_expr(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 {
                    ("".to_owned(), "".to_owned(), "".to_owned())
                };
    
                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),
                                   |db| {
                                       if !what.is_empty() {
                                           db.span_suggestion(span, "try",
                                                              format!("std::mem::swap({}, {})", lhs, rhs));
                                           db.note("or maybe you should use `std::mem::replace`?");
                                       }
                                   });
            }
        }
    }
}