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
  11 /// **What it does:** This lints about type with a `fn new() -> Self` method
  12 /// and no implementation of
  13 /// [`Default`](https://doc.rust-lang.org/std/default/trait.Default.html)
  14 ///
  15 /// **Why is this bad?** User might expect to be able to use
  16 /// [`Default`](https://doc.rust-lang.org/std/default/trait.Default.html)
  17 /// as the type can be
  18 /// 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:** This lints about type with a `fn new() -> Self` method
  54 /// and no implementation of
  55 /// [`Default`](https://doc.rust-lang.org/std/default/trait.Default.html)
  56 ///
  57 /// **Why is this bad?** User might expect to be able to use
  58 /// [`Default`](https://doc.rust-lang.org/std/default/trait.Default.html)
  59 /// as the type can be
  60 /// 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 LateLintPass for NewWithoutDefault {
  93     fn check_fn(&mut self, cx: &LateContext, kind: FnKind, decl: &hir::FnDecl, _: &hir::Block, span: Span, id: ast::NodeId) {
  94         if in_external_macro(cx, span) {
  95             return;
  96         }
  97
  98         if let FnKind::Method(name, ref sig, _, _) = kind {
  99             if sig.constness == hir::Constness::Const {
 100                 // can't be implemented by default
 101                 return;
 102             }
 103             if decl.inputs.is_empty() && name.as_str() == "new" && cx.access_levels.is_reachable(id) {
 104                 let self_ty = cx.tcx
 105                     .lookup_item_type(cx.tcx.map.local_def_id(cx.tcx.map.get_parent(id)))
 106                     .ty;
 107                 if_let_chain!{[
 108                     self_ty.walk_shallow().next().is_none(), // implements_trait does not work with generics
 109                     let Some(ret_ty) = return_ty(cx, id),
 110                     same_tys(cx, self_ty, ret_ty, id),
 111                     let Some(default_trait_id) = get_trait_def_id(cx, &paths::DEFAULT_TRAIT),
 112                     !implements_trait(cx, self_ty, default_trait_id, Vec::new())
 113                 ], {
 114                     if can_derive_default(self_ty, cx, default_trait_id) {
 115                         span_lint_and_then(cx,
 116                                            NEW_WITHOUT_DEFAULT_DERIVE, span,
 117                                            &format!("you should consider deriving a \
 118                                                      `Default` implementation for `{}`",
 119                                                     self_ty),
 120                                            |db| {
 121                                                db.span_suggestion(span, "try this", "#[derive(Default)]".into());
 122                                            });
 123                     } else {
 124                         span_lint_and_then(cx,
 125                                            NEW_WITHOUT_DEFAULT, span,
 126                                            &format!("you should consider adding a \
 127                                                     `Default` implementation for `{}`",
 128                                                     self_ty),
 129                                            |db| {
 130                                                db.span_suggestion(span,
 131                                                                   "try this",
 132                                                                   format!("impl Default for {} {{ fn default() -> \
 133                                                                           Self {{ {}::new() }} }}", self_ty, self_ty));
 134                                            });
 135                     }
 136                 }}
 137             }
 138         }
 139     }
 140 }
 141
 142 fn can_derive_default<'t, 'c>(ty: ty::Ty<'t>, cx: &LateContext<'c, 't>, default_trait_id: DefId) -> bool {
 143     match ty.sty {
 144         ty::TyStruct(ref adt_def, ref substs) => {
 145             for field in adt_def.all_fields() {
 146                 let f_ty = field.ty(cx.tcx, substs);
 147                 if !implements_trait(cx, f_ty, default_trait_id, Vec::new()) {
 148                     return false;
 149                 }
 150             }
 151             true
 152         }
 153         _ => false,
 154     }
 155 }