use clippy_utils::diagnostics::span_lint_and_then;
use if_chain::if_chain;
use rustc_ast::{Item, ItemKind, UseTreeKind};
use rustc_errors::Applicability;
use rustc_lint::{EarlyContext, EarlyLintPass};
use rustc_session::{declare_lint_pass, declare_tool_lint};
use rustc_span::symbol::kw;

declare_clippy_lint! {
    /// **What it does:** Checks for imports ending in `::{self}`.
    ///
    /// **Why is this bad?** In most cases, this can be written much more cleanly by omitting `::{self}`.
    ///
    /// **Known problems:** Removing `::{self}` will cause any non-module items at the same path to also be imported.
    /// This might cause a naming conflict (https://github.com/rust-lang/rustfmt/issues/3568). This lint makes no attempt
    /// to detect this scenario and that is why it is a restriction lint.
    ///
    /// **Example:**
    ///
    /// ```rust
    /// use std::io::{self};
    /// ```
    /// Use instead:
    /// ```rust
    /// use std::io;
    /// ```
    pub UNNECESSARY_SELF_IMPORTS,
    restriction,
    "imports ending in `::{self}`, which can be omitted"
}

declare_lint_pass!(UnnecessarySelfImports => [UNNECESSARY_SELF_IMPORTS]);

impl EarlyLintPass for UnnecessarySelfImports {
    fn check_item(&mut self, cx: &EarlyContext<'_>, item: &Item) {
        if_chain! {
            if let ItemKind::Use(use_tree) = &item.kind;
            if let UseTreeKind::Nested(nodes) = &use_tree.kind;
            if let [(self_tree, _)] = &**nodes;
            if let [self_seg] = &*self_tree.prefix.segments;
            if self_seg.ident.name == kw::SelfLower;
            if let Some(last_segment) = use_tree.prefix.segments.last();

            then {
                span_lint_and_then(
                    cx,
                    UNNECESSARY_SELF_IMPORTS,
                    item.span,
                    "import ending with `::{self}`",
                    |diag| {
                        diag.span_suggestion(
                            last_segment.span().with_hi(item.span.hi()),
                            "consider omitting `::{self}`",
                            format!(
                                "{}{};",
                                last_segment.ident,
                                if let UseTreeKind::Simple(Some(alias), ..) = self_tree.kind { format!(" as {}", alias) } else { String::new() },
                            ),
                            Applicability::MaybeIncorrect,
                        );
                        diag.note("this will slightly change semantics; any non-module items at the same path will also be imported");
                    },
                );
            }
        }
    }
}