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