src/tools/clippy/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_allowed, is_type_diagnostic_item, match_qpath, match_type, paths, snippet_opt, span_lint, span_lint_and_sugg,
   6     span_lint_and_then, walk_ptrs_hir_ty,
   7 };
   8 use if_chain::if_chain;
   9 use rustc_errors::Applicability;
  10 use rustc_hir::{
  11     BinOpKind, BodyId, Expr, ExprKind, FnDecl, FnRetTy, GenericArg, HirId, Impl, ImplItem, ImplItemKind, Item,
  12     ItemKind, Lifetime, MutTy, Mutability, Node, PathSegment, QPath, TraitFn, TraitItem, TraitItemKind, Ty, TyKind,
  13 };
  14 use rustc_lint::{LateContext, LateLintPass};
  15 use rustc_middle::ty;
  16 use rustc_session::{declare_lint_pass, declare_tool_lint};
  17 use rustc_span::source_map::Span;
  18 use rustc_span::{sym, MultiSpan};
  19 use std::borrow::Cow;
  20
  21 declare_clippy_lint! {
  22     /// **What it does:** This lint checks for function arguments of type `&String`
  23     /// or `&Vec` unless the references are mutable. It will also suggest you
  24     /// replace `.clone()` calls with the appropriate `.to_owned()`/`to_string()`
  25     /// calls.
  26     ///
  27     /// **Why is this bad?** Requiring the argument to be of the specific size
  28     /// makes the function less useful for no benefit; slices in the form of `&[T]`
  29     /// or `&str` usually suffice and can be obtained from other types, too.
  30     ///
  31     /// **Known problems:** The lint does not follow data. So if you have an
  32     /// argument `x` and write `let y = x; y.clone()` the lint will not suggest
  33     /// changing that `.clone()` to `.to_owned()`.
  34     ///
  35     /// Other functions called from this function taking a `&String` or `&Vec`
  36     /// argument may also fail to compile if you change the argument. Applying
  37     /// this lint on them will fix the problem, but they may be in other crates.
  38     ///
  39     /// One notable example of a function that may cause issues, and which cannot
  40     /// easily be changed due to being in the standard library is `Vec::contains`.
  41     /// when called on a `Vec<Vec<T>>`. If a `&Vec` is passed to that method then
  42     /// it will compile, but if a `&[T]` is passed then it will not compile.
  43     ///
  44     /// ```ignore
  45     /// fn cannot_take_a_slice(v: &Vec<u8>) -> bool {
  46     ///     let vec_of_vecs: Vec<Vec<u8>> = some_other_fn();
  47     ///
  48     ///     vec_of_vecs.contains(v)
  49     /// }
  50     /// ```
  51     ///
  52     /// Also there may be `fn(&Vec)`-typed references pointing to your function.
  53     /// If you have them, you will get a compiler error after applying this lint's
  54     /// suggestions. You then have the choice to undo your changes or change the
  55     /// type of the reference.
  56     ///
  57     /// Note that if the function is part of your public interface, there may be
  58     /// other crates referencing it, of which you may not be aware. Carefully
  59     /// deprecate the function before applying the lint suggestions in this case.
  60     ///
  61     /// **Example:**
  62     /// ```ignore
  63     /// // Bad
  64     /// fn foo(&Vec<u32>) { .. }
  65     ///
  66     /// // Good
  67     /// fn foo(&[u32]) { .. }
  68     /// ```
  69     pub PTR_ARG,
  70     style,
  71     "fn arguments of the type `&Vec<...>` or `&String`, suggesting to use `&[...]` or `&str` instead, respectively"
  72 }
  73
  74 declare_clippy_lint! {
  75     /// **What it does:** This lint checks for equality comparisons with `ptr::null`
  76     ///
  77     /// **Why is this bad?** It's easier and more readable to use the inherent
  78     /// `.is_null()`
  79     /// method instead
  80     ///
  81     /// **Known problems:** None.
  82     ///
  83     /// **Example:**
  84     /// ```ignore
  85     /// // Bad
  86     /// if x == ptr::null {
  87     ///     ..
  88     /// }
  89     ///
  90     /// // Good
  91     /// if x.is_null() {
  92     ///     ..
  93     /// }
  94     /// ```
  95     pub CMP_NULL,
  96     style,
  97     "comparing a pointer to a null pointer, suggesting to use `.is_null()` instead."
  98 }
  99
 100 declare_clippy_lint! {
 101     /// **What it does:** This lint checks for functions that take immutable
 102     /// references and return mutable ones.
 103     ///
 104     /// **Why is this bad?** This is trivially unsound, as one can create two
 105     /// mutable references from the same (immutable!) source.
 106     /// This [error](https://github.com/rust-lang/rust/issues/39465)
 107     /// actually lead to an interim Rust release 1.15.1.
 108     ///
 109     /// **Known problems:** To be on the conservative side, if there's at least one
 110     /// mutable reference with the output lifetime, this lint will not trigger.
 111     /// In practice, this case is unlikely anyway.
 112     ///
 113     /// **Example:**
 114     /// ```ignore
 115     /// fn foo(&Foo) -> &mut Bar { .. }
 116     /// ```
 117     pub MUT_FROM_REF,
 118     correctness,
 119     "fns that create mutable refs from immutable ref args"
 120 }
 121
 122 declare_lint_pass!(Ptr => [PTR_ARG, CMP_NULL, MUT_FROM_REF]);
 123
 124 impl<'tcx> LateLintPass<'tcx> for Ptr {
 125     fn check_item(&mut self, cx: &LateContext<'tcx>, item: &'tcx Item<'_>) {
 126         if let ItemKind::Fn(ref sig, _, body_id) = item.kind {
 127             check_fn(cx, &sig.decl, item.hir_id(), Some(body_id));
 128         }
 129     }
 130
 131     fn check_impl_item(&mut self, cx: &LateContext<'tcx>, item: &'tcx ImplItem<'_>) {
 132         if let ImplItemKind::Fn(ref sig, body_id) = item.kind {
 133             let parent_item = cx.tcx.hir().get_parent_item(item.hir_id());
 134             if let Some(Node::Item(it)) = cx.tcx.hir().find(parent_item) {
 135                 if let ItemKind::Impl(Impl { of_trait: Some(_), .. }) = it.kind {
 136                     return; // ignore trait impls
 137                 }
 138             }
 139             check_fn(cx, &sig.decl, item.hir_id(), Some(body_id));
 140         }
 141     }
 142
 143     fn check_trait_item(&mut self, cx: &LateContext<'tcx>, item: &'tcx TraitItem<'_>) {
 144         if let TraitItemKind::Fn(ref sig, ref trait_method) = item.kind {
 145             let body_id = if let TraitFn::Provided(b) = *trait_method {
 146                 Some(b)
 147             } else {
 148                 None
 149             };
 150             check_fn(cx, &sig.decl, item.hir_id(), body_id);
 151         }
 152     }
 153
 154     fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
 155         if let ExprKind::Binary(ref op, ref l, ref r) = expr.kind {
 156             if (op.node == BinOpKind::Eq || op.node == BinOpKind::Ne) && (is_null_path(l) || is_null_path(r)) {
 157                 span_lint(
 158                     cx,
 159                     CMP_NULL,
 160                     expr.span,
 161                     "comparing with null is better expressed by the `.is_null()` method",
 162                 );
 163             }
 164         }
 165     }
 166 }
 167
 168 #[allow(clippy::too_many_lines)]
 169 fn check_fn(cx: &LateContext<'_>, decl: &FnDecl<'_>, fn_id: HirId, opt_body_id: Option<BodyId>) {
 170     let fn_def_id = cx.tcx.hir().local_def_id(fn_id);
 171     let sig = cx.tcx.fn_sig(fn_def_id);
 172     let fn_ty = sig.skip_binder();
 173     let body = opt_body_id.map(|id| cx.tcx.hir().body(id));
 174
 175     for (idx, (arg, ty)) in decl.inputs.iter().zip(fn_ty.inputs()).enumerate() {
 176         // Honor the allow attribute on parameters. See issue 5644.
 177         if let Some(body) = &body {
 178             if is_allowed(cx, PTR_ARG, body.params[idx].hir_id) {
 179                 continue;
 180             }
 181         }
 182
 183         if let ty::Ref(_, ty, Mutability::Not) = ty.kind() {
 184             if is_type_diagnostic_item(cx, ty, sym::vec_type) {
 185                 if let Some(spans) = get_spans(cx, opt_body_id, idx, &[("clone", ".to_owned()")]) {
 186                     span_lint_and_then(
 187                         cx,
 188                         PTR_ARG,
 189                         arg.span,
 190                         "writing `&Vec<_>` instead of `&[_]` involves one more reference and cannot be used \
 191                          with non-Vec-based slices",
 192                         |diag| {
 193                             if let Some(ref snippet) = get_only_generic_arg_snippet(cx, arg) {
 194                                 diag.span_suggestion(
 195                                     arg.span,
 196                                     "change this to",
 197                                     format!("&[{}]", snippet),
 198                                     Applicability::Unspecified,
 199                                 );
 200                             }
 201                             for (clonespan, suggestion) in spans {
 202                                 diag.span_suggestion(
 203                                     clonespan,
 204                                     &snippet_opt(cx, clonespan).map_or("change the call to".into(), |x| {
 205                                         Cow::Owned(format!("change `{}` to", x))
 206                                     }),
 207                                     suggestion.into(),
 208                                     Applicability::Unspecified,
 209                                 );
 210                             }
 211                         },
 212                     );
 213                 }
 214             } else if is_type_diagnostic_item(cx, ty, sym::string_type) {
 215                 if let Some(spans) = get_spans(cx, opt_body_id, idx, &[("clone", ".to_string()"), ("as_str", "")]) {
 216                     span_lint_and_then(
 217                         cx,
 218                         PTR_ARG,
 219                         arg.span,
 220                         "writing `&String` instead of `&str` involves a new object where a slice will do",
 221                         |diag| {
 222                             diag.span_suggestion(arg.span, "change this to", "&str".into(), Applicability::Unspecified);
 223                             for (clonespan, suggestion) in spans {
 224                                 diag.span_suggestion_short(
 225                                     clonespan,
 226                                     &snippet_opt(cx, clonespan).map_or("change the call to".into(), |x| {
 227                                         Cow::Owned(format!("change `{}` to", x))
 228                                     }),
 229                                     suggestion.into(),
 230                                     Applicability::Unspecified,
 231                                 );
 232                             }
 233                         },
 234                     );
 235                 }
 236             } else if is_type_diagnostic_item(cx, ty, sym::PathBuf) {
 237                 if let Some(spans) = get_spans(cx, opt_body_id, idx, &[("clone", ".to_path_buf()"), ("as_path", "")]) {
 238                     span_lint_and_then(
 239                         cx,
 240                         PTR_ARG,
 241                         arg.span,
 242                         "writing `&PathBuf` instead of `&Path` involves a new object where a slice will do",
 243                         |diag| {
 244                             diag.span_suggestion(
 245                                 arg.span,
 246                                 "change this to",
 247                                 "&Path".into(),
 248                                 Applicability::Unspecified,
 249                             );
 250                             for (clonespan, suggestion) in spans {
 251                                 diag.span_suggestion_short(
 252                                     clonespan,
 253                                     &snippet_opt(cx, clonespan).map_or("change the call to".into(), |x| {
 254                                         Cow::Owned(format!("change `{}` to", x))
 255                                     }),
 256                                     suggestion.into(),
 257                                     Applicability::Unspecified,
 258                                 );
 259                             }
 260                         },
 261                     );
 262                 }
 263             } else if match_type(cx, ty, &paths::COW) {
 264                 if_chain! {
 265                     if let TyKind::Rptr(_, MutTy { ref ty, ..} ) = arg.kind;
 266                     if let TyKind::Path(QPath::Resolved(None, ref pp)) = ty.kind;
 267                     if let [ref bx] = *pp.segments;
 268                     if let Some(ref params) = bx.args;
 269                     if !params.parenthesized;
 270                     if let Some(inner) = params.args.iter().find_map(|arg| match arg {
 271                         GenericArg::Type(ty) => Some(ty),
 272                         _ => None,
 273                     });
 274                     then {
 275                         let replacement = snippet_opt(cx, inner.span);
 276                         if let Some(r) = replacement {
 277                             span_lint_and_sugg(
 278                                 cx,
 279                                 PTR_ARG,
 280                                 arg.span,
 281                                 "using a reference to `Cow` is not recommended",
 282                                 "change this to",
 283                                 "&".to_owned() + &r,
 284                                 Applicability::Unspecified,
 285                             );
 286                         }
 287                     }
 288                 }
 289             }
 290         }
 291     }
 292
 293     if let FnRetTy::Return(ref ty) = decl.output {
 294         if let Some((out, Mutability::Mut, _)) = get_rptr_lm(ty) {
 295             let mut immutables = vec![];
 296             for (_, ref mutbl, ref argspan) in decl
 297                 .inputs
 298                 .iter()
 299                 .filter_map(|ty| get_rptr_lm(ty))
 300                 .filter(|&(lt, _, _)| lt.name == out.name)
 301             {
 302                 if *mutbl == Mutability::Mut {
 303                     return;
 304                 }
 305                 immutables.push(*argspan);
 306             }
 307             if immutables.is_empty() {
 308                 return;
 309             }
 310             span_lint_and_then(
 311                 cx,
 312                 MUT_FROM_REF,
 313                 ty.span,
 314                 "mutable borrow from immutable input(s)",
 315                 |diag| {
 316                     let ms = MultiSpan::from_spans(immutables);
 317                     diag.span_note(ms, "immutable borrow here");
 318                 },
 319             );
 320         }
 321     }
 322 }
 323
 324 fn get_only_generic_arg_snippet(cx: &LateContext<'_>, arg: &Ty<'_>) -> Option<String> {
 325     if_chain! {
 326         if let TyKind::Path(QPath::Resolved(_, ref path)) = walk_ptrs_hir_ty(arg).kind;
 327         if let Some(&PathSegment{args: Some(ref parameters), ..}) = path.segments.last();
 328         let types: Vec<_> = parameters.args.iter().filter_map(|arg| match arg {
 329             GenericArg::Type(ty) => Some(ty),
 330             _ => None,
 331         }).collect();
 332         if types.len() == 1;
 333         then {
 334             snippet_opt(cx, types[0].span)
 335         } else {
 336             None
 337         }
 338     }
 339 }
 340
 341 fn get_rptr_lm<'tcx>(ty: &'tcx Ty<'tcx>) -> Option<(&'tcx Lifetime, Mutability, Span)> {
 342     if let TyKind::Rptr(ref lt, ref m) = ty.kind {
 343         Some((lt, m.mutbl, ty.span))
 344     } else {
 345         None
 346     }
 347 }
 348
 349 fn is_null_path(expr: &Expr<'_>) -> bool {
 350     if let ExprKind::Call(ref pathexp, ref args) = expr.kind {
 351         if args.is_empty() {
 352             if let ExprKind::Path(ref path) = pathexp.kind {
 353                 return match_qpath(path, &paths::PTR_NULL) || match_qpath(path, &paths::PTR_NULL_MUT);
 354             }
 355         }
 356     }
 357     false
 358 }