rust-clippy/src/map_clone.rs

use rustc::lint::*;
use rustc_front::hir::*;
use syntax::ast::Ident;
use utils::OPTION_PATH;
use utils::{match_trait_method, match_type, snippet, span_help_and_lint};
use utils::{walk_ptrs_ty, walk_ptrs_ty_depth};

declare_lint!(pub MAP_CLONE, Warn,
              "using `.map(|x| x.clone())` to clone an iterator or option's contents (recommends \
              `.cloned()` instead)");

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

impl LateLintPass for MapClonePass {
    fn check_expr(&mut self, cx: &LateContext, expr: &Expr) {
        if_let_chain! {
            [
                // call to .map()
                let ExprMethodCall(name, _, ref args) = expr.node,
                name.node.as_str() == "map" && args.len() == 2,
                let ExprClosure(_, ref decl, ref blk) = args[1].node,
                // just one expression in the closure
                blk.stmts.is_empty(),
                let Some(ref closure_expr) = blk.expr,
                // nothing special in the argument, besides reference bindings
                // (e.g. .map(|&x| x) )
                let Some(arg_ident) = get_arg_name(&*decl.inputs[0].pat),
                // the method is being called on a known type (option or iterator)
                let Some(type_name) = get_type_name(cx, expr, &args[0])
            ], {
                // look for derefs, for .map(|x| *x)
                if only_derefs(&*closure_expr, arg_ident) &&
                    // .cloned() only removes one level of indirection, don't lint on more
                    walk_ptrs_ty_depth(cx.tcx.pat_ty(&*decl.inputs[0].pat)).1 == 1
                {
                    span_help_and_lint(cx, MAP_CLONE, expr.span, &format!(
                        "you seem to be using .map() to clone the contents of an {}, consider \
                        using `.cloned()`", type_name),
                        &format!("try\n{}.cloned()", snippet(cx, args[0].span, "..")));
                }
                // explicit clone() calls ( .map(|x| x.clone()) )
                else if let ExprMethodCall(clone_call, _, ref clone_args) = closure_expr.node {
                    if clone_call.node.as_str() == "clone" &&
                        clone_args.len() == 1 &&
                        match_trait_method(cx, closure_expr, &["core", "clone", "Clone"]) &&
                        expr_eq_ident(&clone_args[0], arg_ident)
                    {
                        span_help_and_lint(cx, MAP_CLONE, expr.span, &format!(
                            "you seem to be using .map() to clone the contents of an {}, consider \
                            using `.cloned()`", type_name),
                            &format!("try\n{}.cloned()", snippet(cx, args[0].span, "..")));
                    }
                }
            }
        }
    }
}

fn expr_eq_ident(expr: &Expr, id: Ident) -> bool {
    match expr.node {
        ExprPath(None, ref path) => {
            let arg_segment = [PathSegment { identifier: id, parameters: PathParameters::none() }];
            !path.global && path.segments == arg_segment
        },
        _ => false,
    }
}

fn get_type_name(cx: &LateContext, expr: &Expr, arg: &Expr) -> Option<&'static str> {
    if match_trait_method(cx, expr, &["core", "iter", "Iterator"]) {
        Some("iterator")
    } else if match_type(cx, walk_ptrs_ty(cx.tcx.expr_ty(arg)), &OPTION_PATH) {
        Some("Option")
    } else {
        None
    }
}

fn get_arg_name(pat: &Pat) -> Option<Ident> {
    match pat.node {
        PatIdent(_, ident, None) => Some(ident.node),
        PatRegion(ref subpat, _) => get_arg_name(subpat),
        _ => None,
    }
}

fn only_derefs(expr: &Expr, id: Ident) -> bool {
    if expr_eq_ident(expr, id) {
        true
    } else if let ExprUnary(UnDeref, ref subexpr) = expr.node {
        only_derefs(subexpr, id)
    } else {
        false
    }
}

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