clippy_lints/src/new_without_default.rs

   1 use rustc::hir::intravisit::FnKind;
   2 use rustc::hir::def_id::DefId;
   3 use rustc::hir;
   4 use rustc::lint::*;
   5 use rustc::ty;
   6 use syntax::ast;
   7 use syntax::codemap::Span;
   8 use utils::paths;
   9 use utils::{get_trait_def_id, implements_trait, in_external_macro, return_ty, same_tys, span_lint_and_then};
  10 use utils::sugg::DiagnosticBuilderExt;
  11
  12 /// **What it does:** Checks for types with a `fn new() -> Self` method and no
  13 /// implementation of
  14 /// [`Default`](https://doc.rust-lang.org/std/default/trait.Default.html).
  15 ///
  16 /// **Why is this bad?** The user might expect to be able to use
  17 /// [`Default`](https://doc.rust-lang.org/std/default/trait.Default.html) as the
  18 /// type can be constructed without arguments.
  19 ///
  20 /// **Known problems:** Hopefully none.
  21 ///
  22 /// **Example:**
  23 ///
  24 /// ```rust,ignore
  25 /// struct Foo(Bar);
  26 ///
  27 /// impl Foo {
  28 ///     fn new() -> Self {
  29 ///         Foo(Bar::new())
  30 ///     }
  31 /// }
  32 /// ```
  33 ///
  34 /// Instead, use:
  35 ///
  36 /// ```rust
  37 /// struct Foo(Bar);
  38 ///
  39 /// impl Default for Foo {
  40 ///     fn default() -> Self {
  41 ///         Foo(Bar::new())
  42 ///     }
  43 /// }
  44 /// ```
  45 ///
  46 /// You can also have `new()` call `Default::default()`.
  47 declare_lint! {
  48     pub NEW_WITHOUT_DEFAULT,
  49     Warn,
  50     "`fn new() -> Self` method without `Default` implementation"
  51 }
  52
  53 /// **What it does:** Checks for types with a `fn new() -> Self` method
  54 /// and no implementation of
  55 /// [`Default`](https://doc.rust-lang.org/std/default/trait.Default.html),
  56 /// where the `Default` can be derived by `#[derive(Default)]`.
  57 ///
  58 /// **Why is this bad?** The user might expect to be able to use
  59 /// [`Default`](https://doc.rust-lang.org/std/default/trait.Default.html) as the
  60 /// type can be constructed without arguments.
  61 ///
  62 /// **Known problems:** Hopefully none.
  63 ///
  64 /// **Example:**
  65 ///
  66 /// ```rust,ignore
  67 /// struct Foo;
  68 ///
  69 /// impl Foo {
  70 ///     fn new() -> Self {
  71 ///         Foo
  72 ///     }
  73 /// }
  74 /// ```
  75 ///
  76 /// Just prepend `#[derive(Default)]` before the `struct` definition.
  77 declare_lint! {
  78     pub NEW_WITHOUT_DEFAULT_DERIVE,
  79     Warn,
  80     "`fn new() -> Self` without `#[derive]`able `Default` implementation"
  81 }
  82
  83 #[derive(Copy,Clone)]
  84 pub struct NewWithoutDefault;
  85
  86 impl LintPass for NewWithoutDefault {
  87     fn get_lints(&self) -> LintArray {
  88         lint_array!(NEW_WITHOUT_DEFAULT, NEW_WITHOUT_DEFAULT_DERIVE)
  89     }
  90 }
  91
  92 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for NewWithoutDefault {
  93     fn check_fn(
  94         &mut self,
  95         cx: &LateContext<'a, 'tcx>,
  96         kind: FnKind<'tcx>,
  97         decl: &'tcx hir::FnDecl,
  98         _: &'tcx hir::Body,
  99         span: Span,
 100         id: ast::NodeId
 101     ) {
 102         if in_external_macro(cx, span) {
 103             return;
 104         }
 105
 106         if let FnKind::Method(name, sig, _, _) = kind {
 107             if sig.constness == hir::Constness::Const {
 108                 // can't be implemented by default
 109                 return;
 110             }
 111             let def_id = cx.tcx.hir.local_def_id(id);
 112             if decl.inputs.is_empty() && name == "new" && cx.access_levels.is_reachable(id) {
 113                 let self_ty = cx.tcx
 114                     .type_of(cx.tcx.hir.local_def_id(cx.tcx.hir.get_parent(id)));
 115                 if_let_chain!{[
 116                     self_ty.walk_shallow().next().is_none(), // implements_trait does not work with generics
 117                     same_tys(cx, self_ty, return_ty(cx, id), def_id),
 118                     let Some(default_trait_id) = get_trait_def_id(cx, &paths::DEFAULT_TRAIT),
 119                     !implements_trait(cx, self_ty, default_trait_id, &[], None)
 120                 ], {
 121                     if let Some(sp) = can_derive_default(self_ty, cx, default_trait_id) {
 122                         span_lint_and_then(cx,
 123                                            NEW_WITHOUT_DEFAULT_DERIVE, span,
 124                                            &format!("you should consider deriving a \
 125                                                      `Default` implementation for `{}`",
 126                                                     self_ty),
 127                                            |db| {
 128                             db.suggest_item_with_attr(cx, sp, "try this", "#[derive(Default)]");
 129                         });
 130                     } else {
 131                         span_lint_and_then(cx,
 132                                            NEW_WITHOUT_DEFAULT, span,
 133                                            &format!("you should consider adding a \
 134                                                     `Default` implementation for `{}`",
 135                                                     self_ty),
 136                                            |db| {
 137                         db.suggest_prepend_item(cx,
 138                                                   span,
 139                                                   "try this",
 140                                                   &format!(
 141 "impl Default for {} {{
 142     fn default() -> Self {{
 143         Self::new()
 144     }}
 145 }}",
 146                                                            self_ty));
 147                         });
 148                     }
 149                 }}
 150             }
 151         }
 152     }
 153 }
 154
 155 fn can_derive_default<'t, 'c>(ty: ty::Ty<'t>, cx: &LateContext<'c, 't>, default_trait_id: DefId) -> Option<Span> {
 156     match ty.sty {
 157         ty::TyAdt(adt_def, substs) if adt_def.is_struct() => {
 158             for field in adt_def.all_fields() {
 159                 let f_ty = field.ty(cx.tcx, substs);
 160                 if !implements_trait(cx, f_ty, default_trait_id, &[], None) {
 161                     return None;
 162                 }
 163             }
 164             cx.tcx.hir.span_if_local(adt_def.did)
 165         },
 166         _ => None,
 167     }
 168 }