rust-clippy/clippy_lints/src/inherent_to_string.rs

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use clippy_utils::diagnostics::span_lint_and_help;
use clippy_utils::ty::{implements_trait, is_type_diagnostic_item};
use clippy_utils::{get_trait_def_id, paths, return_ty, trait_ref_of_method};
use if_chain::if_chain;
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use rustc_hir::{ImplItem, ImplItemKind};
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use rustc_lint::{LateContext, LateLintPass};
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use rustc_session::{declare_lint_pass, declare_tool_lint};
use rustc_span::sym;
declare_clippy_lint! {
/// ### What it does
/// Checks for the definition of inherent methods with a signature of `to_string(&self) -> String`.
///
/// ### Why is this bad?
/// 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.
///
/// ### Known problems
/// None
///
/// ### Example
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/// ```rust
/// // Bad
/// pub struct A;
///
/// impl A {
/// pub fn to_string(&self) -> String {
/// "I am A".to_string()
/// }
/// }
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/// ```
///
/// ```rust
/// // Good
/// use std::fmt;
///
/// pub struct A;
///
/// impl fmt::Display for A {
/// fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
/// write!(f, "I am A")
/// }
/// }
/// ```
pub INHERENT_TO_STRING,
style,
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"type implements inherent method `to_string()`, but should instead implement the `Display` trait"
}
declare_clippy_lint! {
/// ### What it does
/// 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.
///
/// ### Why is this bad?
/// 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`.
///
/// ### Known problems
/// None
///
/// ### Example
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/// ```rust
/// // Bad
/// use std::fmt;
///
/// pub struct A;
///
/// impl A {
/// pub fn to_string(&self) -> String {
/// "I am A".to_string()
/// }
/// }
///
/// impl fmt::Display for A {
/// fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
/// write!(f, "I am A, too")
/// }
/// }
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/// ```
///
/// ```rust
/// // Good
/// use std::fmt;
///
/// pub struct A;
///
/// impl fmt::Display for A {
/// fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
/// write!(f, "I am A")
/// }
/// }
/// ```
pub INHERENT_TO_STRING_SHADOW_DISPLAY,
correctness,
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"type implements inherent method `to_string()`, which gets shadowed by the implementation of the `Display` trait"
}
declare_lint_pass!(InherentToString => [INHERENT_TO_STRING, INHERENT_TO_STRING_SHADOW_DISPLAY]);
impl<'tcx> LateLintPass<'tcx> for InherentToString {
fn check_impl_item(&mut self, cx: &LateContext<'tcx>, impl_item: &'tcx ImplItem<'_>) {
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if impl_item.span.from_expansion() {
return;
}
if_chain! {
// Check if item is a method, called to_string and has a parameter 'self'
if let ImplItemKind::Fn(ref signature, _) = impl_item.kind;
if impl_item.ident.name.as_str() == "to_string";
let decl = &signature.decl;
if decl.implicit_self.has_implicit_self();
if decl.inputs.len() == 1;
if impl_item.generics.params.is_empty();
// Check if return type is String
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if is_type_diagnostic_item(cx, return_ty(cx, impl_item.hir_id()), sym::String);
// Filters instances of to_string which are required by a trait
if trait_ref_of_method(cx, impl_item.hir_id()).is_none();
then {
show_lint(cx, impl_item);
}
}
}
}
fn show_lint(cx: &LateContext<'_>, item: &ImplItem<'_>) {
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let display_trait_id = get_trait_def_id(cx, &paths::DISPLAY_TRAIT).expect("Failed to get trait ID of `Display`!");
// Get the real type of 'self'
let self_type = cx.tcx.fn_sig(item.def_id).input(0);
let self_type = self_type.skip_binder().peel_refs();
// Emit either a warning or an error
if implements_trait(cx, self_type, display_trait_id, &[]) {
span_lint_and_help(
cx,
INHERENT_TO_STRING_SHADOW_DISPLAY,
item.span,
&format!(
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"type `{}` implements inherent method `to_string(&self) -> String` which shadows the implementation of `Display`",
self_type.to_string()
),
None,
&format!("remove the inherent method from type `{}`", self_type.to_string()),
);
} else {
span_lint_and_help(
cx,
INHERENT_TO_STRING,
item.span,
&format!(
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"implementation of inherent method `to_string(&self) -> String` for type `{}`",
self_type.to_string()
),
None,
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&format!("implement trait `Display` for type `{}` instead", self_type.to_string()),
);
}
}