src/tools/clippy/clippy_lints/src/inherent_to_string.rs

   1 use clippy_utils::diagnostics::span_lint_and_help;
   2 use clippy_utils::ty::{implements_trait, is_type_lang_item};
   3 use clippy_utils::{return_ty, trait_ref_of_method};
   4 use if_chain::if_chain;
   5 use rustc_hir::{GenericParamKind, ImplItem, ImplItemKind, LangItem};
   6 use rustc_lint::{LateContext, LateLintPass};
   7 use rustc_session::{declare_lint_pass, declare_tool_lint};
   8 use rustc_span::sym;
   9
  10 declare_clippy_lint! {
  11     /// ### What it does
  12     /// Checks for the definition of inherent methods with a signature of `to_string(&self) -> String`.
  13     ///
  14     /// ### Why is this bad?
  15     /// This method is also implicitly defined if a type implements the `Display` trait. As the functionality of `Display` is much more versatile, it should be preferred.
  16     ///
  17     /// ### Example
  18     /// ```rust
  19     /// pub struct A;
  20     ///
  21     /// impl A {
  22     ///     pub fn to_string(&self) -> String {
  23     ///         "I am A".to_string()
  24     ///     }
  25     /// }
  26     /// ```
  27     ///
  28     /// Use instead:
  29     /// ```rust
  30     /// use std::fmt;
  31     ///
  32     /// pub struct A;
  33     ///
  34     /// impl fmt::Display for A {
  35     ///     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
  36     ///         write!(f, "I am A")
  37     ///     }
  38     /// }
  39     /// ```
  40     #[clippy::version = "1.38.0"]
  41     pub INHERENT_TO_STRING,
  42     style,
  43     "type implements inherent method `to_string()`, but should instead implement the `Display` trait"
  44 }
  45
  46 declare_clippy_lint! {
  47     /// ### What it does
  48     /// Checks for the definition of inherent methods with a signature of `to_string(&self) -> String` and if the type implementing this method also implements the `Display` trait.
  49     ///
  50     /// ### Why is this bad?
  51     /// This method is also implicitly defined if a type implements the `Display` trait. The less versatile inherent method will then shadow the implementation introduced by `Display`.
  52     ///
  53     /// ### Example
  54     /// ```rust
  55     /// use std::fmt;
  56     ///
  57     /// pub struct A;
  58     ///
  59     /// impl A {
  60     ///     pub fn to_string(&self) -> String {
  61     ///         "I am A".to_string()
  62     ///     }
  63     /// }
  64     ///
  65     /// impl fmt::Display for A {
  66     ///     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
  67     ///         write!(f, "I am A, too")
  68     ///     }
  69     /// }
  70     /// ```
  71     ///
  72     /// Use instead:
  73     /// ```rust
  74     /// use std::fmt;
  75     ///
  76     /// pub struct A;
  77     ///
  78     /// impl fmt::Display for A {
  79     ///     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
  80     ///         write!(f, "I am A")
  81     ///     }
  82     /// }
  83     /// ```
  84     #[clippy::version = "1.38.0"]
  85     pub INHERENT_TO_STRING_SHADOW_DISPLAY,
  86     correctness,
  87     "type implements inherent method `to_string()`, which gets shadowed by the implementation of the `Display` trait"
  88 }
  89
  90 declare_lint_pass!(InherentToString => [INHERENT_TO_STRING, INHERENT_TO_STRING_SHADOW_DISPLAY]);
  91
  92 impl<'tcx> LateLintPass<'tcx> for InherentToString {
  93     fn check_impl_item(&mut self, cx: &LateContext<'tcx>, impl_item: &'tcx ImplItem<'_>) {
  94         if impl_item.span.from_expansion() {
  95             return;
  96         }
  97
  98         if_chain! {
  99             // Check if item is a method, called to_string and has a parameter 'self'
 100             if let ImplItemKind::Fn(ref signature, _) = impl_item.kind;
 101             if impl_item.ident.name == sym::to_string;
 102             let decl = &signature.decl;
 103             if decl.implicit_self.has_implicit_self();
 104             if decl.inputs.len() == 1;
 105             if impl_item.generics.params.iter().all(|p| matches!(p.kind, GenericParamKind::Lifetime { .. }));
 106
 107             // Check if return type is String
 108             if is_type_lang_item(cx, return_ty(cx, impl_item.hir_id()), LangItem::String);
 109
 110             // Filters instances of to_string which are required by a trait
 111             if trait_ref_of_method(cx, impl_item.owner_id.def_id).is_none();
 112
 113             then {
 114                 show_lint(cx, impl_item);
 115             }
 116         }
 117     }
 118 }
 119
 120 fn show_lint(cx: &LateContext<'_>, item: &ImplItem<'_>) {
 121     let display_trait_id = cx
 122         .tcx
 123         .get_diagnostic_item(sym::Display)
 124         .expect("Failed to get trait ID of `Display`!");
 125
 126     // Get the real type of 'self'
 127     let self_type = cx.tcx.fn_sig(item.owner_id).input(0);
 128     let self_type = self_type.skip_binder().peel_refs();
 129
 130     // Emit either a warning or an error
 131     if implements_trait(cx, self_type, display_trait_id, &[]) {
 132         span_lint_and_help(
 133             cx,
 134             INHERENT_TO_STRING_SHADOW_DISPLAY,
 135             item.span,
 136             &format!(
 137                 "type `{self_type}` implements inherent method `to_string(&self) -> String` which shadows the implementation of `Display`"
 138             ),
 139             None,
 140             &format!("remove the inherent method from type `{self_type}`"),
 141         );
 142     } else {
 143         span_lint_and_help(
 144             cx,
 145             INHERENT_TO_STRING,
 146             item.span,
 147             &format!("implementation of inherent method `to_string(&self) -> String` for type `{self_type}`"),
 148             None,
 149             &format!("implement trait `Display` for type `{self_type}` instead"),
 150         );
 151     }
 152 }