use clippy_utils::diagnostics::span_lint_and_help;
use clippy_utils::visitors::LocalUsedVisitor;
use if_chain::if_chain;
use rustc_hir::{Impl, ImplItem, ImplItemKind, ItemKind};
use rustc_lint::{LateContext, LateLintPass};
use rustc_session::{declare_lint_pass, declare_tool_lint};

declare_clippy_lint! {
    /// **What it does:** Checks methods that contain a `self` argument but don't use it
    ///
    /// **Why is this bad?** It may be clearer to define the method as an associated function instead
    /// of an instance method if it doesn't require `self`.
    ///
    /// **Known problems:** None.
    ///
    /// **Example:**
    /// ```rust,ignore
    /// struct A;
    /// impl A {
    ///     fn method(&self) {}
    /// }
    /// ```
    ///
    /// Could be written:
    ///
    /// ```rust,ignore
    /// struct A;
    /// impl A {
    ///     fn method() {}
    /// }
    /// ```
    pub UNUSED_SELF,
    pedantic,
    "methods that contain a `self` argument but don't use it"
}

declare_lint_pass!(UnusedSelf => [UNUSED_SELF]);

impl<'tcx> LateLintPass<'tcx> for UnusedSelf {
    fn check_impl_item(&mut self, cx: &LateContext<'tcx>, impl_item: &ImplItem<'_>) {
        if impl_item.span.from_expansion() {
            return;
        }
        let parent = cx.tcx.hir().get_parent_item(impl_item.hir_id());
        let parent_item = cx.tcx.hir().expect_item(parent);
        let assoc_item = cx.tcx.associated_item(impl_item.def_id);
        if_chain! {
            if let ItemKind::Impl(Impl { of_trait: None, .. }) = parent_item.kind;
            if assoc_item.fn_has_self_parameter;
            if let ImplItemKind::Fn(.., body_id) = &impl_item.kind;
            let body = cx.tcx.hir().body(*body_id);
            if let [self_param, ..] = body.params;
            let self_hir_id = self_param.pat.hir_id;
            if !LocalUsedVisitor::new(cx, self_hir_id).check_body(body);
            then {
                span_lint_and_help(
                    cx,
                    UNUSED_SELF,
                    self_param.span,
                    "unused `self` argument",
                    None,
                    "consider refactoring to a associated function",
                );
            }
        }
    }
}