clippy_lints/src/new_without_default.rs

   1 // Copyright 2014-2018 The Rust Project Developers. See the COPYRIGHT
   2 // file at the top-level directory of this distribution.
   3 //
   4 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
   5 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
   6 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
   7 // option. This file may not be copied, modified, or distributed
   8 // except according to those terms.
   9
  10 use crate::utils::paths;
  11 use crate::utils::sugg::DiagnosticBuilderExt;
  12 use crate::utils::{get_trait_def_id, implements_trait, return_ty, same_tys, span_lint_node_and_then};
  13 use if_chain::if_chain;
  14 use rustc::hir;
  15 use rustc::hir::def_id::DefId;
  16 use rustc::lint::{in_external_macro, LateContext, LateLintPass, LintArray, LintContext, LintPass};
  17 use rustc::ty::{self, Ty};
  18 use rustc::util::nodemap::NodeSet;
  19 use rustc::{declare_tool_lint, lint_array};
  20 use rustc_errors::Applicability;
  21 use syntax::source_map::Span;
  22
  23 /// **What it does:** Checks for types with a `fn new() -> Self` method and no
  24 /// implementation of
  25 /// [`Default`](https://doc.rust-lang.org/std/default/trait.Default.html).
  26 ///
  27 /// It detects both the case when a manual
  28 /// [`Default`](https://doc.rust-lang.org/std/default/trait.Default.html)
  29 /// implementation is required and also when it can be created with
  30 /// `#[derive(Default)]
  31 ///
  32 /// **Why is this bad?** The user might expect to be able to use
  33 /// [`Default`](https://doc.rust-lang.org/std/default/trait.Default.html) as the
  34 /// type can be constructed without arguments.
  35 ///
  36 /// **Known problems:** Hopefully none.
  37 ///
  38 /// **Example:**
  39 ///
  40 /// ```rust
  41 /// struct Foo(Bar);
  42 ///
  43 /// impl Foo {
  44 ///     fn new() -> Self {
  45 ///         Foo(Bar::new())
  46 ///     }
  47 /// }
  48 /// ```
  49 ///
  50 /// Instead, use:
  51 ///
  52 /// ```rust
  53 /// struct Foo(Bar);
  54 ///
  55 /// impl Default for Foo {
  56 ///     fn default() -> Self {
  57 ///         Foo(Bar::new())
  58 ///     }
  59 /// }
  60 /// ```
  61 ///
  62 /// Or, if
  63 /// [`Default`](https://doc.rust-lang.org/std/default/trait.Default.html)
  64 /// can be derived by `#[derive(Default)]`:
  65 ///
  66 /// ```rust
  67 /// struct Foo;
  68 ///
  69 /// impl Foo {
  70 ///     fn new() -> Self {
  71 ///         Foo
  72 ///     }
  73 /// }
  74 /// ```
  75 ///
  76 /// Instead, use:
  77 ///
  78 /// ```rust
  79 /// #[derive(Default)]
  80 /// struct Foo;
  81 ///
  82 /// impl Foo {
  83 ///     fn new() -> Self {
  84 ///         Foo
  85 ///     }
  86 /// }
  87 /// ```
  88 ///
  89 /// You can also have `new()` call `Default::default()`.
  90 declare_clippy_lint! {
  91     pub NEW_WITHOUT_DEFAULT,
  92     style,
  93     "`fn new() -> Self` method without `Default` implementation"
  94 }
  95
  96 #[derive(Clone, Default)]
  97 pub struct NewWithoutDefault {
  98     impling_types: Option<NodeSet>,
  99 }
 100
 101 impl LintPass for NewWithoutDefault {
 102     fn get_lints(&self) -> LintArray {
 103         lint_array!(NEW_WITHOUT_DEFAULT)
 104     }
 105 }
 106
 107 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for NewWithoutDefault {
 108     fn check_item(&mut self, cx: &LateContext<'a, 'tcx>, item: &'tcx hir::Item) {
 109         if let hir::ItemKind::Impl(_, _, _, _, None, _, ref items) = item.node {
 110             for assoc_item in items {
 111                 if let hir::AssociatedItemKind::Method { has_self: false } = assoc_item.kind {
 112                     let impl_item = cx.tcx.hir().impl_item(assoc_item.id);
 113                     if in_external_macro(cx.sess(), impl_item.span) {
 114                         return;
 115                     }
 116                     if let hir::ImplItemKind::Method(ref sig, _) = impl_item.node {
 117                         let name = impl_item.ident.name;
 118                         let id = impl_item.id;
 119                         if sig.header.constness == hir::Constness::Const {
 120                             // can't be implemented by default
 121                             return;
 122                         }
 123                         if sig.header.unsafety == hir::Unsafety::Unsafe {
 124                             // can't be implemented for unsafe new
 125                             return;
 126                         }
 127                         if impl_item.generics.params.iter().any(|gen| match gen.kind {
 128                             hir::GenericParamKind::Type { .. } => true,
 129                             _ => false,
 130                         }) {
 131                             // when the result of `new()` depends on a type parameter we should not require
 132                             // an
 133                             // impl of `Default`
 134                             return;
 135                         }
 136                         if sig.decl.inputs.is_empty() && name == "new" && cx.access_levels.is_reachable(id) {
 137                             let self_did = cx.tcx.hir().local_def_id(cx.tcx.hir().get_parent(id));
 138                             let self_ty = cx.tcx.type_of(self_did);
 139                             if_chain! {
 140                                 if same_tys(cx, self_ty, return_ty(cx, id));
 141                                 if let Some(default_trait_id) = get_trait_def_id(cx, &paths::DEFAULT_TRAIT);
 142                                 then {
 143                                     if self.impling_types.is_none() {
 144                                         let mut impls = NodeSet::default();
 145                                         cx.tcx.for_each_impl(default_trait_id, |d| {
 146                                             if let Some(ty_def) = cx.tcx.type_of(d).ty_adt_def() {
 147                                                 if let Some(node_id) = cx.tcx.hir().as_local_node_id(ty_def.did) {
 148                                                     impls.insert(node_id);
 149                                                 }
 150                                             }
 151                                         });
 152                                         self.impling_types = Some(impls);
 153                                     }
 154
 155                                     // Check if a Default implementation exists for the Self type, regardless of
 156                                     // generics
 157                                     if_chain! {
 158                                         if let Some(ref impling_types) = self.impling_types;
 159                                         if let Some(self_def) = cx.tcx.type_of(self_did).ty_adt_def();
 160                                         if self_def.did.is_local();
 161                                         then {
 162                                             let self_id = cx.tcx.hir().local_def_id_to_node_id(self_def.did.to_local());
 163                                             if impling_types.contains(&self_id) {
 164                                                 return;
 165                                             }
 166                                         }
 167                                     }
 168
 169                                     if let Some(sp) = can_derive_default(self_ty, cx, default_trait_id) {
 170                                         span_lint_node_and_then(
 171                                             cx,
 172                                             NEW_WITHOUT_DEFAULT,
 173                                             id,
 174                                             impl_item.span,
 175                                             &format!(
 176                                                 "you should consider deriving a `Default` implementation for `{}`",
 177                                                 self_ty
 178                                             ),
 179                                             |db| {
 180                                                 db.suggest_item_with_attr(
 181                                                     cx,
 182                                                     sp,
 183                                                     "try this",
 184                                                     "#[derive(Default)]",
 185                                                     Applicability::MaybeIncorrect,
 186                                                 );
 187                                             });
 188                                     } else {
 189                                         span_lint_node_and_then(
 190                                             cx,
 191                                             NEW_WITHOUT_DEFAULT,
 192                                             id,
 193                                             impl_item.span,
 194                                             &format!(
 195                                                 "you should consider adding a `Default` implementation for `{}`",
 196                                                 self_ty
 197                                             ),
 198                                             |db| {
 199                                                 db.suggest_prepend_item(
 200                                                     cx,
 201                                                     item.span,
 202                                                     "try this",
 203                                                     &create_new_without_default_suggest_msg(self_ty),
 204                                                     Applicability::MaybeIncorrect,
 205                                                 );
 206                                             },
 207                                         );
 208                                     }
 209                                 }
 210                             }
 211                         }
 212                     }
 213                 }
 214             }
 215         }
 216     }
 217 }
 218
 219 fn create_new_without_default_suggest_msg(ty: Ty<'_>) -> String {
 220     #[rustfmt::skip]
 221     format!(
 222 "impl Default for {} {{
 223     fn default() -> Self {{
 224         Self::new()
 225     }}
 226 }}", ty)
 227 }
 228
 229 fn can_derive_default<'t, 'c>(ty: Ty<'t>, cx: &LateContext<'c, 't>, default_trait_id: DefId) -> Option<Span> {
 230     match ty.sty {
 231         ty::Adt(adt_def, substs) if adt_def.is_struct() => {
 232             for field in adt_def.all_fields() {
 233                 let f_ty = field.ty(cx.tcx, substs);
 234                 if !implements_trait(cx, f_ty, default_trait_id, &[]) {
 235                     return None;
 236                 }
 237             }
 238             Some(cx.tcx.def_span(adt_def.did))
 239         },
 240         _ => None,
 241     }
 242 }