clippy_lints/src/ptr.rs

   1 //! Checks for usage of  `&Vec[_]` and `&String`.
   2
   3 use crate::utils::ptr::get_spans;
   4 use crate::utils::{
   5     is_type_diagnostic_item, match_qpath, match_type, paths, snippet_opt, span_lint, span_lint_and_then,
   6     walk_ptrs_hir_ty,
   7 };
   8 use if_chain::if_chain;
   9 use rustc::declare_lint_pass;
  10 use rustc::hir::QPath;
  11 use rustc::hir::*;
  12 use rustc::lint::{LateContext, LateLintPass, LintArray, LintPass};
  13 use rustc::ty;
  14 use rustc_errors::Applicability;
  15 use rustc_session::declare_tool_lint;
  16 use std::borrow::Cow;
  17 use syntax::source_map::Span;
  18 use syntax_pos::{MultiSpan, Symbol};
  19
  20 declare_clippy_lint! {
  21     /// **What it does:** This lint checks for function arguments of type `&String`
  22     /// or `&Vec` unless the references are mutable. It will also suggest you
  23     /// replace `.clone()` calls with the appropriate `.to_owned()`/`to_string()`
  24     /// calls.
  25     ///
  26     /// **Why is this bad?** Requiring the argument to be of the specific size
  27     /// makes the function less useful for no benefit; slices in the form of `&[T]`
  28     /// or `&str` usually suffice and can be obtained from other types, too.
  29     ///
  30     /// **Known problems:** The lint does not follow data. So if you have an
  31     /// argument `x` and write `let y = x; y.clone()` the lint will not suggest
  32     /// changing that `.clone()` to `.to_owned()`.
  33     ///
  34     /// Other functions called from this function taking a `&String` or `&Vec`
  35     /// argument may also fail to compile if you change the argument. Applying
  36     /// this lint on them will fix the problem, but they may be in other crates.
  37     ///
  38     /// Also there may be `fn(&Vec)`-typed references pointing to your function.
  39     /// If you have them, you will get a compiler error after applying this lint's
  40     /// suggestions. You then have the choice to undo your changes or change the
  41     /// type of the reference.
  42     ///
  43     /// Note that if the function is part of your public interface, there may be
  44     /// other crates referencing it you may not be aware. Carefully deprecate the
  45     /// function before applying the lint suggestions in this case.
  46     ///
  47     /// **Example:**
  48     /// ```ignore
  49     /// fn foo(&Vec<u32>) { .. }
  50     /// ```
  51     pub PTR_ARG,
  52     style,
  53     "fn arguments of the type `&Vec<...>` or `&String`, suggesting to use `&[...]` or `&str` instead, respectively"
  54 }
  55
  56 declare_clippy_lint! {
  57     /// **What it does:** This lint checks for equality comparisons with `ptr::null`
  58     ///
  59     /// **Why is this bad?** It's easier and more readable to use the inherent
  60     /// `.is_null()`
  61     /// method instead
  62     ///
  63     /// **Known problems:** None.
  64     ///
  65     /// **Example:**
  66     /// ```ignore
  67     /// if x == ptr::null {
  68     ///     ..
  69     /// }
  70     /// ```
  71     pub CMP_NULL,
  72     style,
  73     "comparing a pointer to a null pointer, suggesting to use `.is_null()` instead."
  74 }
  75
  76 declare_clippy_lint! {
  77     /// **What it does:** This lint checks for functions that take immutable
  78     /// references and return
  79     /// mutable ones.
  80     ///
  81     /// **Why is this bad?** This is trivially unsound, as one can create two
  82     /// mutable references
  83     /// from the same (immutable!) source. This
  84     /// [error](https://github.com/rust-lang/rust/issues/39465)
  85     /// actually lead to an interim Rust release 1.15.1.
  86     ///
  87     /// **Known problems:** To be on the conservative side, if there's at least one
  88     /// mutable reference
  89     /// with the output lifetime, this lint will not trigger. In practice, this
  90     /// case is unlikely anyway.
  91     ///
  92     /// **Example:**
  93     /// ```ignore
  94     /// fn foo(&Foo) -> &mut Bar { .. }
  95     /// ```
  96     pub MUT_FROM_REF,
  97     correctness,
  98     "fns that create mutable refs from immutable ref args"
  99 }
 100
 101 declare_lint_pass!(Ptr => [PTR_ARG, CMP_NULL, MUT_FROM_REF]);
 102
 103 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for Ptr {
 104     fn check_item(&mut self, cx: &LateContext<'a, 'tcx>, item: &'tcx Item) {
 105         if let ItemKind::Fn(ref sig, _, body_id) = item.kind {
 106             check_fn(cx, &sig.decl, item.hir_id, Some(body_id));
 107         }
 108     }
 109
 110     fn check_impl_item(&mut self, cx: &LateContext<'a, 'tcx>, item: &'tcx ImplItem) {
 111         if let ImplItemKind::Method(ref sig, body_id) = item.kind {
 112             let parent_item = cx.tcx.hir().get_parent_item(item.hir_id);
 113             if let Some(Node::Item(it)) = cx.tcx.hir().find(parent_item) {
 114                 if let ItemKind::Impl(_, _, _, _, Some(_), _, _) = it.kind {
 115                     return; // ignore trait impls
 116                 }
 117             }
 118             check_fn(cx, &sig.decl, item.hir_id, Some(body_id));
 119         }
 120     }
 121
 122     fn check_trait_item(&mut self, cx: &LateContext<'a, 'tcx>, item: &'tcx TraitItem) {
 123         if let TraitItemKind::Method(ref sig, ref trait_method) = item.kind {
 124             let body_id = if let TraitMethod::Provided(b) = *trait_method {
 125                 Some(b)
 126             } else {
 127                 None
 128             };
 129             check_fn(cx, &sig.decl, item.hir_id, body_id);
 130         }
 131     }
 132
 133     fn check_expr(&mut self, cx: &LateContext<'a, 'tcx>, expr: &'tcx Expr) {
 134         if let ExprKind::Binary(ref op, ref l, ref r) = expr.kind {
 135             if (op.node == BinOpKind::Eq || op.node == BinOpKind::Ne) && (is_null_path(l) || is_null_path(r)) {
 136                 span_lint(
 137                     cx,
 138                     CMP_NULL,
 139                     expr.span,
 140                     "Comparing with null is better expressed by the .is_null() method",
 141                 );
 142             }
 143         }
 144     }
 145 }
 146
 147 #[allow(clippy::too_many_lines)]
 148 fn check_fn(cx: &LateContext<'_, '_>, decl: &FnDecl, fn_id: HirId, opt_body_id: Option<BodyId>) {
 149     let fn_def_id = cx.tcx.hir().local_def_id(fn_id);
 150     let sig = cx.tcx.fn_sig(fn_def_id);
 151     let fn_ty = sig.skip_binder();
 152
 153     for (idx, (arg, ty)) in decl.inputs.iter().zip(fn_ty.inputs()).enumerate() {
 154         if let ty::Ref(_, ty, Mutability::Not) = ty.kind {
 155             if is_type_diagnostic_item(cx, ty, Symbol::intern("vec_type")) {
 156                 let mut ty_snippet = None;
 157                 if_chain! {
 158                     if let TyKind::Path(QPath::Resolved(_, ref path)) = walk_ptrs_hir_ty(arg).kind;
 159                     if let Some(&PathSegment{args: Some(ref parameters), ..}) = path.segments.last();
 160                     then {
 161                         let types: Vec<_> = parameters.args.iter().filter_map(|arg| match arg {
 162                             GenericArg::Type(ty) => Some(ty),
 163                             _ => None,
 164                         }).collect();
 165                         if types.len() == 1 {
 166                             ty_snippet = snippet_opt(cx, types[0].span);
 167                         }
 168                     }
 169                 };
 170                 if let Some(spans) = get_spans(cx, opt_body_id, idx, &[("clone", ".to_owned()")]) {
 171                     span_lint_and_then(
 172                         cx,
 173                         PTR_ARG,
 174                         arg.span,
 175                         "writing `&Vec<_>` instead of `&[_]` involves one more reference and cannot be used \
 176                          with non-Vec-based slices.",
 177                         |db| {
 178                             if let Some(ref snippet) = ty_snippet {
 179                                 db.span_suggestion(
 180                                     arg.span,
 181                                     "change this to",
 182                                     format!("&[{}]", snippet),
 183                                     Applicability::Unspecified,
 184                                 );
 185                             }
 186                             for (clonespan, suggestion) in spans {
 187                                 db.span_suggestion(
 188                                     clonespan,
 189                                     &snippet_opt(cx, clonespan).map_or("change the call to".into(), |x| {
 190                                         Cow::Owned(format!("change `{}` to", x))
 191                                     }),
 192                                     suggestion.into(),
 193                                     Applicability::Unspecified,
 194                                 );
 195                             }
 196                         },
 197                     );
 198                 }
 199             } else if match_type(cx, ty, &paths::STRING) {
 200                 if let Some(spans) = get_spans(cx, opt_body_id, idx, &[("clone", ".to_string()"), ("as_str", "")]) {
 201                     span_lint_and_then(
 202                         cx,
 203                         PTR_ARG,
 204                         arg.span,
 205                         "writing `&String` instead of `&str` involves a new object where a slice will do.",
 206                         |db| {
 207                             db.span_suggestion(arg.span, "change this to", "&str".into(), Applicability::Unspecified);
 208                             for (clonespan, suggestion) in spans {
 209                                 db.span_suggestion_short(
 210                                     clonespan,
 211                                     &snippet_opt(cx, clonespan).map_or("change the call to".into(), |x| {
 212                                         Cow::Owned(format!("change `{}` to", x))
 213                                     }),
 214                                     suggestion.into(),
 215                                     Applicability::Unspecified,
 216                                 );
 217                             }
 218                         },
 219                     );
 220                 }
 221             } else if match_type(cx, ty, &paths::COW) {
 222                 if_chain! {
 223                     if let TyKind::Rptr(_, MutTy { ref ty, ..} ) = arg.kind;
 224                     if let TyKind::Path(ref path) = ty.kind;
 225                     if let QPath::Resolved(None, ref pp) = *path;
 226                     if let [ref bx] = *pp.segments;
 227                     if let Some(ref params) = bx.args;
 228                     if !params.parenthesized;
 229                     if let Some(inner) = params.args.iter().find_map(|arg| match arg {
 230                         GenericArg::Type(ty) => Some(ty),
 231                         _ => None,
 232                     });
 233                     then {
 234                         let replacement = snippet_opt(cx, inner.span);
 235                         if let Some(r) = replacement {
 236                             span_lint_and_then(
 237                                 cx,
 238                                 PTR_ARG,
 239                                 arg.span,
 240                                 "using a reference to `Cow` is not recommended.",
 241                                 |db| {
 242                                     db.span_suggestion(
 243                                         arg.span,
 244                                         "change this to",
 245                                         "&".to_owned() + &r,
 246                                         Applicability::Unspecified,
 247                                     );
 248                                 },
 249                             );
 250                         }
 251                     }
 252                 }
 253             }
 254         }
 255     }
 256
 257     if let FunctionRetTy::Return(ref ty) = decl.output {
 258         if let Some((out, Mutability::Mut, _)) = get_rptr_lm(ty) {
 259             let mut immutables = vec![];
 260             for (_, ref mutbl, ref argspan) in decl
 261                 .inputs
 262                 .iter()
 263                 .filter_map(|ty| get_rptr_lm(ty))
 264                 .filter(|&(lt, _, _)| lt.name == out.name)
 265             {
 266                 if *mutbl == Mutability::Mut {
 267                     return;
 268                 }
 269                 immutables.push(*argspan);
 270             }
 271             if immutables.is_empty() {
 272                 return;
 273             }
 274             span_lint_and_then(
 275                 cx,
 276                 MUT_FROM_REF,
 277                 ty.span,
 278                 "mutable borrow from immutable input(s)",
 279                 |db| {
 280                     let ms = MultiSpan::from_spans(immutables);
 281                     db.span_note(ms, "immutable borrow here");
 282                 },
 283             );
 284         }
 285     }
 286 }
 287
 288 fn get_rptr_lm(ty: &Ty) -> Option<(&Lifetime, Mutability, Span)> {
 289     if let TyKind::Rptr(ref lt, ref m) = ty.kind {
 290         Some((lt, m.mutbl, ty.span))
 291     } else {
 292         None
 293     }
 294 }
 295
 296 fn is_null_path(expr: &Expr) -> bool {
 297     if let ExprKind::Call(ref pathexp, ref args) = expr.kind {
 298         if args.is_empty() {
 299             if let ExprKind::Path(ref path) = pathexp.kind {
 300                 return match_qpath(path, &paths::PTR_NULL) || match_qpath(path, &paths::PTR_NULL_MUT);
 301             }
 302         }
 303     }
 304     false
 305 }