//! Lint on if expressions with an else if, but without a final else branch.

use clippy_utils::diagnostics::span_lint_and_help;
use rustc_ast::ast::{Expr, ExprKind};
use rustc_lint::{EarlyContext, EarlyLintPass};
use rustc_middle::lint::in_external_macro;
use rustc_session::{declare_lint_pass, declare_tool_lint};

declare_clippy_lint! {
    /// ### What it does
    /// Checks for usage of if expressions with an `else if` branch,
    /// but without a final `else` branch.
    ///
    /// ### Why is this bad?
    /// Some coding guidelines require this (e.g., MISRA-C:2004 Rule 14.10).
    ///
    /// ### Example
    /// ```rust
    /// # fn a() {}
    /// # fn b() {}
    /// # let x: i32 = 1;
    /// if x.is_positive() {
    ///     a();
    /// } else if x.is_negative() {
    ///     b();
    /// }
    /// ```
    ///
    /// Could be written:
    ///
    /// ```rust
    /// # fn a() {}
    /// # fn b() {}
    /// # let x: i32 = 1;
    /// if x.is_positive() {
    ///     a();
    /// } else if x.is_negative() {
    ///     b();
    /// } else {
    ///     // We don't care about zero.
    /// }
    /// ```
    pub ELSE_IF_WITHOUT_ELSE,
    restriction,
    "`if` expression with an `else if`, but without a final `else` branch"
}

declare_lint_pass!(ElseIfWithoutElse => [ELSE_IF_WITHOUT_ELSE]);

impl EarlyLintPass for ElseIfWithoutElse {
    fn check_expr(&mut self, cx: &EarlyContext<'_>, mut item: &Expr) {
        if in_external_macro(cx.sess, item.span) {
            return;
        }

        while let ExprKind::If(_, _, Some(ref els)) = item.kind {
            if let ExprKind::If(_, _, None) = els.kind {
                span_lint_and_help(
                    cx,
                    ELSE_IF_WITHOUT_ELSE,
                    els.span,
                    "`if` expression with an `else if`, but without a final `else`",
                    None,
                    "add an `else` block here",
                );
            }

            item = els;
        }
    }
}