1 use clippy_utils::diagnostics::span_lint_and_help;
2 use clippy_utils::ty::{implements_trait, is_type_diagnostic_item};
3 use clippy_utils::{get_trait_def_id, paths, return_ty, trait_ref_of_method};
4 use if_chain::if_chain;
5 use rustc_hir::{ImplItem, ImplItemKind};
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 /// ### Known problems
18 /// None
19 ///
20 /// ### Example
21 /// ```rust
22 /// // Bad
23 /// pub struct A;
24 ///
25 /// impl A {
26 /// pub fn to_string(&self) -> String {
27 /// "I am A".to_string()
28 /// }
29 /// }
30 /// ```
31 ///
32 /// ```rust
33 /// // Good
34 /// use std::fmt;
35 ///
36 /// pub struct A;
37 ///
38 /// impl fmt::Display for A {
39 /// fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
40 /// write!(f, "I am A")
41 /// }
42 /// }
43 /// ```
44 pub INHERENT_TO_STRING,
45 style,
46 "type implements inherent method `to_string()`, but should instead implement the `Display` trait"
47 }
48
49 declare_clippy_lint! {
50 /// ### What it does
51 /// 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.
52 ///
53 /// ### Why is this bad?
54 /// 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`.
55 ///
56 /// ### Known problems
57 /// None
58 ///
59 /// ### Example
60 /// ```rust
61 /// // Bad
62 /// use std::fmt;
63 ///
64 /// pub struct A;
65 ///
66 /// impl A {
67 /// pub fn to_string(&self) -> String {
68 /// "I am A".to_string()
69 /// }
70 /// }
71 ///
72 /// impl fmt::Display for A {
73 /// fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
74 /// write!(f, "I am A, too")
75 /// }
76 /// }
77 /// ```
78 ///
79 /// ```rust
80 /// // Good
81 /// use std::fmt;
82 ///
83 /// pub struct A;
84 ///
85 /// impl fmt::Display for A {
86 /// fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
87 /// write!(f, "I am A")
88 /// }
89 /// }
90 /// ```
91 pub INHERENT_TO_STRING_SHADOW_DISPLAY,
92 correctness,
93 "type implements inherent method `to_string()`, which gets shadowed by the implementation of the `Display` trait"
94 }
95
96 declare_lint_pass!(InherentToString => [INHERENT_TO_STRING, INHERENT_TO_STRING_SHADOW_DISPLAY]);
97
98 impl<'tcx> LateLintPass<'tcx> for InherentToString {
check_impl_item(&mut self, cx: &LateContext<'tcx>, impl_item: &'tcx ImplItem<'_>)99 fn check_impl_item(&mut self, cx: &LateContext<'tcx>, impl_item: &'tcx ImplItem<'_>) {
100 if impl_item.span.from_expansion() {
101 return;
102 }
103
104 if_chain! {
105 // Check if item is a method, called to_string and has a parameter 'self'
106 if let ImplItemKind::Fn(ref signature, _) = impl_item.kind;
107 if impl_item.ident.name.as_str() == "to_string";
108 let decl = &signature.decl;
109 if decl.implicit_self.has_implicit_self();
110 if decl.inputs.len() == 1;
111 if impl_item.generics.params.is_empty();
112
113 // Check if return type is String
114 if is_type_diagnostic_item(cx, return_ty(cx, impl_item.hir_id()), sym::String);
115
116 // Filters instances of to_string which are required by a trait
117 if trait_ref_of_method(cx, impl_item.hir_id()).is_none();
118
119 then {
120 show_lint(cx, impl_item);
121 }
122 }
123 }
124 }
125
show_lint(cx: &LateContext<'_>, item: &ImplItem<'_>)126 fn show_lint(cx: &LateContext<'_>, item: &ImplItem<'_>) {
127 let display_trait_id = get_trait_def_id(cx, &paths::DISPLAY_TRAIT).expect("Failed to get trait ID of `Display`!");
128
129 // Get the real type of 'self'
130 let self_type = cx.tcx.fn_sig(item.def_id).input(0);
131 let self_type = self_type.skip_binder().peel_refs();
132
133 // Emit either a warning or an error
134 if implements_trait(cx, self_type, display_trait_id, &[]) {
135 span_lint_and_help(
136 cx,
137 INHERENT_TO_STRING_SHADOW_DISPLAY,
138 item.span,
139 &format!(
140 "type `{}` implements inherent method `to_string(&self) -> String` which shadows the implementation of `Display`",
141 self_type
142 ),
143 None,
144 &format!("remove the inherent method from type `{}`", self_type),
145 );
146 } else {
147 span_lint_and_help(
148 cx,
149 INHERENT_TO_STRING,
150 item.span,
151 &format!(
152 "implementation of inherent method `to_string(&self) -> String` for type `{}`",
153 self_type
154 ),
155 None,
156 &format!("implement trait `Display` for type `{}` instead", self_type),
157 );
158 }
159 }
160