new_without_default.rs

use clippy_utils::diagnostics::span_lint_hir_and_then;
use clippy_utils::return_ty;
use clippy_utils::source::{indent_of, reindent_multiline, snippet_with_applicability};
use clippy_utils::sugg::DiagExt;
use rustc_errors::Applicability;
use rustc_hir as hir;
use rustc_hir::attrs::AttributeKind;
use rustc_hir::{Attribute, HirIdSet};
use rustc_lint::{LateContext, LateLintPass, LintContext};
use rustc_middle::ty::AssocKind;
use rustc_session::impl_lint_pass;
use rustc_span::sym;

declare_clippy_lint! {
    /// ### What it does
    /// Checks for public types with a `pub fn new() -> Self` method and no
    /// implementation of
    /// [`Default`](https://doc.rust-lang.org/std/default/trait.Default.html).
    ///
    /// ### Why is this bad?
    /// The user might expect to be able to use
    /// [`Default`](https://doc.rust-lang.org/std/default/trait.Default.html) as the
    /// type can be constructed without arguments.
    ///
    /// ### Example
    /// ```ignore
    /// pub struct Foo(Bar);
    ///
    /// impl Foo {
    ///     pub fn new() -> Self {
    ///         Foo(Bar::new())
    ///     }
    /// }
    /// ```
    ///
    /// To fix the lint, add a `Default` implementation that delegates to `new`:
    ///
    /// ```ignore
    /// pub struct Foo(Bar);
    ///
    /// impl Default for Foo {
    ///     fn default() -> Self {
    ///         Foo::new()
    ///     }
    /// }
    /// ```
    #[clippy::version = "pre 1.29.0"]
    pub NEW_WITHOUT_DEFAULT,
    style,
    "`pub fn new() -> Self` method without `Default` implementation"
}

impl_lint_pass!(NewWithoutDefault => [NEW_WITHOUT_DEFAULT]);

#[derive(Clone, Default)]
pub struct NewWithoutDefault {
    impling_types: Option<HirIdSet>,
}

impl<'tcx> LateLintPass<'tcx> for NewWithoutDefault {
    #[expect(clippy::too_many_lines)]
    fn check_item(&mut self, cx: &LateContext<'tcx>, item: &'tcx hir::Item<'_>) {
        let hir::ItemKind::Impl(hir::Impl {
            of_trait: None,
            generics,
            self_ty: impl_self_ty,
            ..
        }) = item.kind
        else {
            return;
        };

        for assoc_item in cx
            .tcx
            .associated_items(item.owner_id.def_id)
            .filter_by_name_unhygienic(sym::new)
        {
            if let AssocKind::Fn { has_self: false, .. } = assoc_item.kind
                && let assoc_item_hir_id = cx.tcx.local_def_id_to_hir_id(assoc_item.def_id.expect_local())
                && let impl_item = cx.tcx.hir_node(assoc_item_hir_id).expect_impl_item()
                && !impl_item.span.in_external_macro(cx.sess().source_map())
                && let hir::ImplItemKind::Fn(ref sig, _) = impl_item.kind
                && let id = impl_item.owner_id
                // can't be implemented for unsafe new
                && !sig.header.is_unsafe()
                // shouldn't be implemented when it is hidden in docs
                && !cx.tcx.is_doc_hidden(impl_item.owner_id.def_id)
                // when the result of `new()` depends on a parameter we should not require
                // an impl of `Default`
                && impl_item.generics.params.is_empty()
                && sig.decl.inputs.is_empty()
                && cx.effective_visibilities.is_exported(impl_item.owner_id.def_id)
                && let self_ty = cx.tcx.type_of(item.owner_id).instantiate_identity()
                && self_ty == return_ty(cx, impl_item.owner_id)
                && let Some(default_trait_id) = cx.tcx.get_diagnostic_item(sym::Default)
            {
                if self.impling_types.is_none() {
                    let mut impls = HirIdSet::default();
                    for &d in cx.tcx.local_trait_impls(default_trait_id) {
                        let ty = cx.tcx.type_of(d).instantiate_identity();
                        if let Some(ty_def) = ty.ty_adt_def()
                            && let Some(local_def_id) = ty_def.did().as_local()
                        {
                            impls.insert(cx.tcx.local_def_id_to_hir_id(local_def_id));
                        }
                    }
                    self.impling_types = Some(impls);
                }

                // Check if a Default implementation exists for the Self type, regardless of
                // generics
                if let Some(ref impling_types) = self.impling_types
                    && let self_def = cx.tcx.type_of(item.owner_id).instantiate_identity()
                    && let Some(self_def) = self_def.ty_adt_def()
                    && let Some(self_local_did) = self_def.did().as_local()
                    && let self_id = cx.tcx.local_def_id_to_hir_id(self_local_did)
                    && impling_types.contains(&self_id)
                {
                    return;
                }

                let mut app = Applicability::MachineApplicable;
                let attrs_sugg = {
                    let mut sugg = String::new();
                    for attr in cx.tcx.hir_attrs(assoc_item_hir_id) {
                        let Attribute::Parsed(AttributeKind::CfgTrace(attrs)) = attr else {
                            // This might be some other attribute that the `impl Default` ought to inherit.
                            // But it could also be one of the many attributes that:
                            // - can't be put on an impl block -- like `#[inline]`
                            // - we can't even build a suggestion for, since `Attribute::span` may panic.
                            //
                            // Because of all that, remain on the safer side -- don't inherit this attr, and just
                            // reduce the applicability
                            app = Applicability::MaybeIncorrect;
                            continue;
                        };

                        for (_, attr_span) in attrs {
                            sugg.push_str(&snippet_with_applicability(cx.sess(), *attr_span, "_", &mut app));
                            sugg.push('\n');
                        }
                    }
                    sugg
                };
                let generics_sugg = snippet_with_applicability(cx, generics.span, "", &mut app);
                let where_clause_sugg = if generics.has_where_clause_predicates {
                    let where_clause_sugg =
                        snippet_with_applicability(cx, generics.where_clause_span, "", &mut app).to_string();
                    let mut where_clause_sugg = reindent_multiline(&where_clause_sugg, true, Some(4));
                    if impl_item.generics.has_where_clause_predicates {
                        if !where_clause_sugg.ends_with(',') {
                            where_clause_sugg.push(',');
                        }

                        let additional_where_preds =
                            snippet_with_applicability(cx, impl_item.generics.where_clause_span, "", &mut app);
                        let ident = indent_of(cx, generics.where_clause_span).unwrap_or(0);
                        // Remove the leading `where ` keyword
                        let additional_where_preds = additional_where_preds.trim_start_matches("where").trim_start();
                        where_clause_sugg.push('\n');
                        where_clause_sugg.extend(std::iter::repeat_n(' ', ident));
                        where_clause_sugg.push_str(additional_where_preds);
                    }
                    format!("\n{where_clause_sugg}\n")
                } else if impl_item.generics.has_where_clause_predicates {
                    let where_clause_sugg =
                        snippet_with_applicability(cx, impl_item.generics.where_clause_span, "", &mut app);
                    let where_clause_sugg = reindent_multiline(&where_clause_sugg, true, Some(4));
                    format!("\n{}\n", where_clause_sugg.trim_start())
                } else {
                    String::new()
                };
                let self_ty_fmt = self_ty.to_string();
                let self_type_snip = snippet_with_applicability(cx, impl_self_ty.span, &self_ty_fmt, &mut app);
                span_lint_hir_and_then(
                    cx,
                    NEW_WITHOUT_DEFAULT,
                    id.into(),
                    impl_item.span,
                    format!("you should consider adding a `Default` implementation for `{self_type_snip}`"),
                    |diag| {
                        diag.suggest_prepend_item(
                            cx,
                            item.span,
                            "try adding this",
                            &create_new_without_default_suggest_msg(
                                &attrs_sugg,
                                &self_type_snip,
                                &generics_sugg,
                                &where_clause_sugg,
                            ),
                            app,
                        );
                    },
                );
            }
        }
    }
}

fn create_new_without_default_suggest_msg(
    attrs_sugg: &str,
    self_type_snip: &str,
    generics_sugg: &str,
    where_clause_sugg: &str,
) -> String {
    #[rustfmt::skip]
    format!(
"{attrs_sugg}impl{generics_sugg} Default for {self_type_snip}{where_clause_sugg} {{
    fn default() -> Self {{
        Self::new()
    }}
}}")
}