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