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