clippy_lints/src/ptr.rs

   1 //! Checks for usage of  `&Vec[_]` and `&String`.
   2
   3 use std::borrow::Cow;
   4 use crate::rustc::hir::*;
   5 use crate::rustc::hir::QPath;
   6 use crate::rustc::lint::{LateContext, LateLintPass, LintArray, LintPass};
   7 use crate::rustc::{declare_tool_lint, lint_array};
   8 use if_chain::if_chain;
   9 use crate::rustc::ty;
  10 use crate::syntax::ast::NodeId;
  11 use crate::syntax::source_map::Span;
  12 use crate::syntax_pos::MultiSpan;
  13 use crate::utils::{match_qpath, match_type, paths, snippet_opt, span_lint, span_lint_and_then, walk_ptrs_hir_ty};
  14 use crate::utils::ptr::get_spans;
  15 use crate::rustc_errors::Applicability;
  16
  17 /// **What it does:** This lint checks for function arguments of type `&String`
  18 /// or `&Vec` unless the references are mutable. It will also suggest you
  19 /// replace `.clone()` calls with the appropriate `.to_owned()`/`to_string()`
  20 /// calls.
  21 ///
  22 /// **Why is this bad?** Requiring the argument to be of the specific size
  23 /// makes the function less useful for no benefit; slices in the form of `&[T]`
  24 /// or `&str` usually suffice and can be obtained from other types, too.
  25 ///
  26 /// **Known problems:** The lint does not follow data. So if you have an
  27 /// argument `x` and write `let y = x; y.clone()` the lint will not suggest
  28 /// changing that `.clone()` to `.to_owned()`.
  29 ///
  30 /// Other functions called from this function taking a `&String` or `&Vec`
  31 /// argument may also fail to compile if you change the argument. Applying
  32 /// this lint on them will fix the problem, but they may be in other crates.
  33 ///
  34 /// Also there may be `fn(&Vec)`-typed references pointing to your function.
  35 /// If you have them, you will get a compiler error after applying this lint's
  36 /// suggestions. You then have the choice to undo your changes or change the
  37 /// type of the reference.
  38 ///
  39 /// Note that if the function is part of your public interface, there may be
  40 /// other crates referencing it you may not be aware. Carefully deprecate the
  41 /// function before applying the lint suggestions in this case.
  42 ///
  43 /// **Example:**
  44 /// ```rust
  45 /// fn foo(&Vec<u32>) { .. }
  46 /// ```
  47 declare_clippy_lint! {
  48     pub PTR_ARG,
  49     style,
  50     "fn arguments of the type `&Vec<...>` or `&String`, suggesting to use `&[...]` or `&str` \
  51      instead, respectively"
  52 }
  53
  54 /// **What it does:** This lint checks for equality comparisons with `ptr::null`
  55 ///
  56 /// **Why is this bad?** It's easier and more readable to use the inherent
  57 /// `.is_null()`
  58 /// method instead
  59 ///
  60 /// **Known problems:** None.
  61 ///
  62 /// **Example:**
  63 /// ```rust
  64 /// if x == ptr::null { .. }
  65 /// ```
  66 declare_clippy_lint! {
  67     pub CMP_NULL,
  68     style,
  69     "comparing a pointer to a null pointer, suggesting to use `.is_null()` instead."
  70 }
  71
  72 /// **What it does:** This lint checks for functions that take immutable
  73 /// references and return
  74 /// mutable ones.
  75 ///
  76 /// **Why is this bad?** This is trivially unsound, as one can create two
  77 /// mutable references
  78 /// from the same (immutable!) source. This
  79 /// [error](https://github.com/rust-lang/rust/issues/39465)
  80 /// actually lead to an interim Rust release 1.15.1.
  81 ///
  82 /// **Known problems:** To be on the conservative side, if there's at least one
  83 /// mutable reference
  84 /// with the output lifetime, this lint will not trigger. In practice, this
  85 /// case is unlikely anyway.
  86 ///
  87 /// **Example:**
  88 /// ```rust
  89 /// fn foo(&Foo) -> &mut Bar { .. }
  90 /// ```
  91 declare_clippy_lint! {
  92     pub MUT_FROM_REF,
  93     correctness,
  94     "fns that create mutable refs from immutable ref args"
  95 }
  96
  97 #[derive(Copy, Clone)]
  98 pub struct PointerPass;
  99
 100 impl LintPass for PointerPass {
 101     fn get_lints(&self) -> LintArray {
 102         lint_array!(PTR_ARG, CMP_NULL, MUT_FROM_REF)
 103     }
 104 }
 105
 106 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for PointerPass {
 107     fn check_item(&mut self, cx: &LateContext<'a, 'tcx>, item: &'tcx Item) {
 108         if let ItemKind::Fn(ref decl, _, _, body_id) = item.node {
 109             check_fn(cx, decl, item.id, Some(body_id));
 110         }
 111     }
 112
 113     fn check_impl_item(&mut self, cx: &LateContext<'a, 'tcx>, item: &'tcx ImplItem) {
 114         if let ImplItemKind::Method(ref sig, body_id) = item.node {
 115             if let Some(Node::Item(it)) = cx.tcx.hir.find(cx.tcx.hir.get_parent(item.id)) {
 116                 if let ItemKind::Impl(_, _, _, _, Some(_), _, _) = it.node {
 117                     return; // ignore trait impls
 118                 }
 119             }
 120             check_fn(cx, &sig.decl, item.id, Some(body_id));
 121         }
 122     }
 123
 124     fn check_trait_item(&mut self, cx: &LateContext<'a, 'tcx>, item: &'tcx TraitItem) {
 125         if let TraitItemKind::Method(ref sig, ref trait_method) = item.node {
 126             let body_id = if let TraitMethod::Provided(b) = *trait_method {
 127                 Some(b)
 128             } else {
 129                 None
 130             };
 131             check_fn(cx, &sig.decl, item.id, body_id);
 132         }
 133     }
 134
 135     fn check_expr(&mut self, cx: &LateContext<'a, 'tcx>, expr: &'tcx Expr) {
 136         if let ExprKind::Binary(ref op, ref l, ref r) = expr.node {
 137             if (op.node == BinOpKind::Eq || op.node == BinOpKind::Ne) && (is_null_path(l) || is_null_path(r)) {
 138                 span_lint(
 139                     cx,
 140                     CMP_NULL,
 141                     expr.span,
 142                     "Comparing with null is better expressed by the .is_null() method",
 143                 );
 144             }
 145         }
 146     }
 147 }
 148
 149 fn check_fn(cx: &LateContext<'_, '_>, decl: &FnDecl, fn_id: NodeId, opt_body_id: Option<BodyId>) {
 150     let fn_def_id = cx.tcx.hir.local_def_id(fn_id);
 151     let sig = cx.tcx.fn_sig(fn_def_id);
 152     let fn_ty = sig.skip_binder();
 153
 154     for (idx, (arg, ty)) in decl.inputs.iter().zip(fn_ty.inputs()).enumerate() {
 155         if let ty::Ref(
 156             _,
 157             ty,
 158             MutImmutable
 159         ) = ty.sty
 160         {
 161             if match_type(cx, ty, &paths::VEC) {
 162                 let mut ty_snippet = None;
 163                 if_chain! {
 164                     if let TyKind::Path(QPath::Resolved(_, ref path)) = walk_ptrs_hir_ty(arg).node;
 165                     if let Some(&PathSegment{args: Some(ref parameters), ..}) = path.segments.last();
 166                     then {
 167                         let types: Vec<_> = parameters.args.iter().filter_map(|arg| match arg {
 168                             GenericArg::Type(ty) => Some(ty),
 169                             _ => None,
 170                         }).collect();
 171                         if types.len() == 1 {
 172                             ty_snippet = snippet_opt(cx, types[0].span);
 173                         }
 174                     }
 175                 };
 176                 if let Some(spans) = get_spans(cx, opt_body_id, idx, &[("clone", ".to_owned()")]) {
 177                     span_lint_and_then(
 178                         cx,
 179                         PTR_ARG,
 180                         arg.span,
 181                         "writing `&Vec<_>` instead of `&[_]` involves one more reference and cannot be used \
 182                          with non-Vec-based slices.",
 183                         |db| {
 184                             if let Some(ref snippet) = ty_snippet {
 185                                 db.span_suggestion_with_applicability(
 186                                             arg.span,
 187                                             "change this to",
 188                                             format!("&[{}]", snippet),
 189                                             Applicability::Unspecified,
 190                                             );
 191                             }
 192                             for (clonespan, suggestion) in spans {
 193                                 db.span_suggestion_with_applicability(
 194                                     clonespan,
 195                                     &snippet_opt(cx, clonespan).map_or(
 196                                         "change the call to".into(),
 197                                         |x| Cow::Owned(format!("change `{}` to", x)),
 198                                     ),
 199                                     suggestion.into(),
 200                                     Applicability::Unspecified,
 201                                 );
 202                             }
 203                         },
 204                     );
 205                 }
 206             } else if match_type(cx, ty, &paths::STRING) {
 207                 if let Some(spans) = get_spans(cx, opt_body_id, idx, &[("clone", ".to_string()"), ("as_str", "")]) {
 208                     span_lint_and_then(
 209                         cx,
 210                         PTR_ARG,
 211                         arg.span,
 212                         "writing `&String` instead of `&str` involves a new object where a slice will do.",
 213                         |db| {
 214                             db.span_suggestion_with_applicability(
 215                                 arg.span,
 216                                 "change this to",
 217                                 "&str".into(),
 218                                 Applicability::Unspecified,
 219                             );
 220                             for (clonespan, suggestion) in spans {
 221                                 db.span_suggestion_short_with_applicability(
 222                                     clonespan,
 223                                     &snippet_opt(cx, clonespan).map_or(
 224                                         "change the call to".into(),
 225                                         |x| Cow::Owned(format!("change `{}` to", x)),
 226                                     ),
 227                                     suggestion.into(),
 228                                     Applicability::Unspecified,
 229                                 );
 230                             }
 231                         },
 232                     );
 233                 }
 234             } else if match_type(cx, ty, &paths::COW) {
 235                 if_chain! {
 236                     if let TyKind::Rptr(_, MutTy { ref ty, ..} ) = arg.node;
 237                     if let TyKind::Path(ref path) = ty.node;
 238                     if let QPath::Resolved(None, ref pp) = *path;
 239                     if let [ref bx] = *pp.segments;
 240                     if let Some(ref params) = bx.args;
 241                     if !params.parenthesized;
 242                     if let Some(inner) = params.args.iter().find_map(|arg| match arg {
 243                         GenericArg::Type(ty) => Some(ty),
 244                         GenericArg::Lifetime(_) => None,
 245                     });
 246                     then {
 247                         let replacement = snippet_opt(cx, inner.span);
 248                         if let Some(r) = replacement {
 249                             span_lint_and_then(
 250                                 cx,
 251                                 PTR_ARG,
 252                                 arg.span,
 253                                 "using a reference to `Cow` is not recommended.",
 254                                 |db| {
 255                                     db.span_suggestion_with_applicability(
 256                                         arg.span,
 257                                         "change this to",
 258                                         "&".to_owned() + &r,
 259                                         Applicability::Unspecified,
 260                                     );
 261                                 },
 262                             );
 263                         }
 264                     }
 265                 }
 266             }
 267         }
 268     }
 269
 270     if let FunctionRetTy::Return(ref ty) = decl.output {
 271         if let Some((out, MutMutable, _)) = get_rptr_lm(ty) {
 272             let mut immutables = vec![];
 273             for (_, ref mutbl, ref argspan) in decl.inputs
 274                 .iter()
 275                 .filter_map(|ty| get_rptr_lm(ty))
 276                 .filter(|&(lt, _, _)| lt.name == out.name)
 277             {
 278                 if *mutbl == MutMutable {
 279                     return;
 280                 }
 281                 immutables.push(*argspan);
 282             }
 283             if immutables.is_empty() {
 284                 return;
 285             }
 286             span_lint_and_then(cx, MUT_FROM_REF, ty.span, "mutable borrow from immutable input(s)", |db| {
 287                 let ms = MultiSpan::from_spans(immutables);
 288                 db.span_note(ms, "immutable borrow here");
 289             });
 290         }
 291     }
 292 }
 293
 294 fn get_rptr_lm(ty: &Ty) -> Option<(&Lifetime, Mutability, Span)> {
 295     if let TyKind::Rptr(ref lt, ref m) = ty.node {
 296         Some((lt, m.mutbl, ty.span))
 297     } else {
 298         None
 299     }
 300 }
 301
 302 fn is_null_path(expr: &Expr) -> bool {
 303     if let ExprKind::Call(ref pathexp, ref args) = expr.node {
 304         if args.is_empty() {
 305             if let ExprKind::Path(ref path) = pathexp.node {
 306                 return match_qpath(path, &paths::PTR_NULL) || match_qpath(path, &paths::PTR_NULL_MUT);
 307             }
 308         }
 309     }
 310     false
 311 }