rust/clippy_lints/src/ptr.rs

//! Checks for usage of  `&Vec[_]` and `&String`.

use rustc::hir::*;
use rustc::hir::map::NodeItem;
use rustc::lint::*;
use rustc::ty;
use syntax::ast::NodeId;
use utils::{match_path, match_type, paths, span_lint};

/// **What it does:** This lint checks for function arguments of type `&String` or `&Vec` unless
/// the references are mutable.
///
/// **Why is this bad?** Requiring the argument to be of the specific size makes the function less
/// useful for no benefit; slices in the form of `&[T]` or `&str` usually suffice and can be
/// obtained from other types, too.
///
/// **Known problems:** None.
///
/// **Example:**
/// ```rust
/// fn foo(&Vec<u32>) { .. }
/// ```
declare_lint! {
    pub PTR_ARG,
    Warn,
    "fn arguments of the type `&Vec<...>` or `&String`, suggesting to use `&[...]` or `&str` \
     instead, respectively"
}

/// **What it does:** This lint checks for equality comparisons with `ptr::null`
///
/// **Why is this bad?** It's easier and more readable to use the inherent `.is_null()`
/// method instead
///
/// **Known problems:** None.
///
/// **Example:**
/// ```rust
/// if x == ptr::null { .. }
/// ```
declare_lint! {
    pub CMP_NULL,
    Warn,
    "comparing a pointer to a null pointer, suggesting to use `.is_null()` instead."
}


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

impl LintPass for PointerPass {
    fn get_lints(&self) -> LintArray {
        lint_array!(PTR_ARG, CMP_NULL)
    }
}

impl LateLintPass for PointerPass {
    fn check_item(&mut self, cx: &LateContext, item: &Item) {
        if let ItemFn(ref decl, _, _, _, _, _) = item.node {
            check_fn(cx, decl, item.id);
        }
    }

    fn check_impl_item(&mut self, cx: &LateContext, item: &ImplItem) {
        if let ImplItemKind::Method(ref sig, _) = item.node {
            if let Some(NodeItem(it)) = cx.tcx.map.find(cx.tcx.map.get_parent(item.id)) {
                if let ItemImpl(_, _, _, Some(_), _, _) = it.node {
                    return; // ignore trait impls
                }
            }
            check_fn(cx, &sig.decl, item.id);
        }
    }

    fn check_trait_item(&mut self, cx: &LateContext, item: &TraitItem) {
        if let MethodTraitItem(ref sig, _) = item.node {
            check_fn(cx, &sig.decl, item.id);
        }
    }
    
    fn check_expr(&mut self, cx: &LateContext, expr: &Expr) {
        if let ExprBinary(ref op, ref l, ref r) = expr.node {
            if (op.node == BiEq || op.node == BiNe) && (is_null_path(l) || is_null_path(r)) {
                span_lint(cx,
                          CMP_NULL,
                          expr.span,
                          "Comparing with null is better expressed by the .is_null() method");
            }
        }
    }
}

fn check_fn(cx: &LateContext, decl: &FnDecl, fn_id: NodeId) {
    let fn_def_id = cx.tcx.map.local_def_id(fn_id);
    let fn_ty = cx.tcx.item_type(fn_def_id).fn_sig().skip_binder();

    for (arg, ty) in decl.inputs.iter().zip(&fn_ty.inputs) {
        if let ty::TyRef(_, ty::TypeAndMut { ty, mutbl: MutImmutable }) = ty.sty {
            if match_type(cx, ty, &paths::VEC) {
                span_lint(cx,
                          PTR_ARG,
                          arg.ty.span,
                          "writing `&Vec<_>` instead of `&[_]` involves one more reference and cannot be used \
                           with non-Vec-based slices. Consider changing the type to `&[...]`");
            } else if match_type(cx, ty, &paths::STRING) {
                span_lint(cx,
                          PTR_ARG,
                          arg.ty.span,
                          "writing `&String` instead of `&str` involves a new object where a slice will do. \
                           Consider changing the type to `&str`");
            }
        }
    }
}

fn is_null_path(expr: &Expr) -> bool {
    if let ExprCall(ref pathexp, ref args) = expr.node {
        if args.is_empty() {
            if let ExprPath(_, ref path) = pathexp.node {
                return match_path(path, &paths::PTR_NULL) || match_path(path, &paths::PTR_NULL_MUT)
            }
        }
    }
    false
}