rust-clippy/clippy_lints/src/mut_mut.rs

use rustc::hir;
use rustc::hir::intravisit;
use rustc::lint::*;
use rustc::ty;
use utils::{higher, in_external_macro, span_lint};

/// **What it does:** Checks for instances of `mut mut` references.
///
/// **Why is this bad?** Multiple `mut`s don't add anything meaningful to the
/// source. This is either a copy'n'paste error, or it shows a fundamental
/// misunderstanding of references.
///
/// **Known problems:** None.
///
/// **Example:**
/// ```rust
/// let x = &mut &mut y;
/// ```
declare_clippy_lint! {
    pub MUT_MUT,
    pedantic,
    "usage of double-mut refs, e.g. `&mut &mut ...`"
}

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

impl LintPass for MutMut {
    fn get_lints(&self) -> LintArray {
        lint_array!(MUT_MUT)
    }
}

impl<'a, 'tcx> LateLintPass<'a, 'tcx> for MutMut {
    fn check_block(&mut self, cx: &LateContext<'a, 'tcx>, block: &'tcx hir::Block) {
        intravisit::walk_block(&mut MutVisitor { cx }, block);
    }

    fn check_ty(&mut self, cx: &LateContext<'a, 'tcx>, ty: &'tcx hir::Ty) {
        use rustc::hir::intravisit::Visitor;

        MutVisitor { cx }.visit_ty(ty);
    }
}

pub struct MutVisitor<'a, 'tcx: 'a> {
    cx: &'a LateContext<'a, 'tcx>,
}

impl<'a, 'tcx> intravisit::Visitor<'tcx> for MutVisitor<'a, 'tcx> {
    fn visit_expr(&mut self, expr: &'tcx hir::Expr) {
        if in_external_macro(self.cx, expr.span) {
            return;
        }

        if let Some((_, arg, body)) = higher::for_loop(expr) {
            // A `for` loop lowers to:
            // ```rust
            // match ::std::iter::Iterator::next(&mut iter) {
            // //                                ^^^^
            // ```
            // Let's ignore the generated code.
            intravisit::walk_expr(self, arg);
            intravisit::walk_expr(self, body);
        } else if let hir::ExprAddrOf(hir::MutMutable, ref e) = expr.node {
            if let hir::ExprAddrOf(hir::MutMutable, _) = e.node {
                span_lint(
                    self.cx,
                    MUT_MUT,
                    expr.span,
                    "generally you want to avoid `&mut &mut _` if possible",
                );
            } else if let ty::TyRef(
                _,
                _,
                _,
            ) = self.cx.tables.expr_ty(e).sty
            {
                span_lint(
                    self.cx,
                    MUT_MUT,
                    expr.span,
                    "this expression mutably borrows a mutable reference. Consider reborrowing",
                );
            }
        }
    }

    fn visit_ty(&mut self, ty: &'tcx hir::Ty) {
        if let hir::TyRptr(
            _,
            hir::MutTy {
                ty: ref pty,
                mutbl: hir::MutMutable,
            },
        ) = ty.node
        {
            if let hir::TyRptr(
                _,
                hir::MutTy {
                    mutbl: hir::MutMutable,
                    ..
                },
            ) = pty.node
            {
                span_lint(
                    self.cx,
                    MUT_MUT,
                    ty.span,
                    "generally you want to avoid `&mut &mut _` if possible",
                );
            }
        }

        intravisit::walk_ty(self, ty);
    }
    fn nested_visit_map<'this>(&'this mut self) -> intravisit::NestedVisitorMap<'this, 'tcx> {
        intravisit::NestedVisitorMap::None
    }
}