use clippy_utils::diagnostics::span_lint_and_then;
use clippy_utils::ty::is_type_diagnostic_item;
use clippy_utils::{method_chain_args, return_ty};
use if_chain::if_chain;
use rustc_hir as hir;
use rustc_hir::intravisit::{self, NestedVisitorMap, Visitor};
use rustc_hir::{Expr, ImplItemKind};
use rustc_lint::{LateContext, LateLintPass};
use rustc_middle::hir::map::Map;
use rustc_middle::ty;
use rustc_session::{declare_lint_pass, declare_tool_lint};
use rustc_span::{sym, Span};

declare_clippy_lint! {
    /// **What it does:** Checks for functions of type Result that contain `expect()` or `unwrap()`
    ///
    /// **Why is this bad?** These functions promote recoverable errors to non-recoverable errors which may be undesirable in code bases which wish to avoid panics.
    ///
    /// **Known problems:** This can cause false positives in functions that handle both recoverable and non recoverable errors.
    ///
    /// **Example:**
    /// Before:
    /// ```rust
    /// fn divisible_by_3(i_str: String) -> Result<(), String> {
    ///     let i = i_str
    ///         .parse::<i32>()
    ///         .expect("cannot divide the input by three");
    ///
    ///     if i % 3 != 0 {
    ///         Err("Number is not divisible by 3")?
    ///     }
    ///
    ///     Ok(())
    /// }
    /// ```
    ///
    /// After:
    /// ```rust
    /// fn divisible_by_3(i_str: String) -> Result<(), String> {
    ///     let i = i_str
    ///         .parse::<i32>()
    ///         .map_err(|e| format!("cannot divide the input by three: {}", e))?;
    ///
    ///     if i % 3 != 0 {
    ///         Err("Number is not divisible by 3")?
    ///     }
    ///
    ///     Ok(())
    /// }
    /// ```
    pub UNWRAP_IN_RESULT,
    restriction,
    "functions of type `Result<..>` or `Option`<...> that contain `expect()` or `unwrap()`"
}

declare_lint_pass!(UnwrapInResult=> [UNWRAP_IN_RESULT]);

impl<'tcx> LateLintPass<'tcx> for UnwrapInResult {
    fn check_impl_item(&mut self, cx: &LateContext<'tcx>, impl_item: &'tcx hir::ImplItem<'_>) {
        if_chain! {
            // first check if it's a method or function
            if let hir::ImplItemKind::Fn(ref _signature, _) = impl_item.kind;
            // checking if its return type is `result` or `option`
            if is_type_diagnostic_item(cx, return_ty(cx, impl_item.hir_id()), sym::result_type)
                || is_type_diagnostic_item(cx, return_ty(cx, impl_item.hir_id()), sym::option_type);
            then {
                lint_impl_body(cx, impl_item.span, impl_item);
            }
        }
    }
}

struct FindExpectUnwrap<'a, 'tcx> {
    lcx: &'a LateContext<'tcx>,
    typeck_results: &'tcx ty::TypeckResults<'tcx>,
    result: Vec<Span>,
}

impl<'a, 'tcx> Visitor<'tcx> for FindExpectUnwrap<'a, 'tcx> {
    type Map = Map<'tcx>;

    fn visit_expr(&mut self, expr: &'tcx Expr<'_>) {
        // check for `expect`
        if let Some(arglists) = method_chain_args(expr, &["expect"]) {
            let reciever_ty = self.typeck_results.expr_ty(&arglists[0][0]).peel_refs();
            if is_type_diagnostic_item(self.lcx, reciever_ty, sym::option_type)
                || is_type_diagnostic_item(self.lcx, reciever_ty, sym::result_type)
            {
                self.result.push(expr.span);
            }
        }

        // check for `unwrap`
        if let Some(arglists) = method_chain_args(expr, &["unwrap"]) {
            let reciever_ty = self.typeck_results.expr_ty(&arglists[0][0]).peel_refs();
            if is_type_diagnostic_item(self.lcx, reciever_ty, sym::option_type)
                || is_type_diagnostic_item(self.lcx, reciever_ty, sym::result_type)
            {
                self.result.push(expr.span);
            }
        }

        // and check sub-expressions
        intravisit::walk_expr(self, expr);
    }

    fn nested_visit_map(&mut self) -> NestedVisitorMap<Self::Map> {
        NestedVisitorMap::None
    }
}

fn lint_impl_body<'tcx>(cx: &LateContext<'tcx>, impl_span: Span, impl_item: &'tcx hir::ImplItem<'_>) {
    if_chain! {

        if let ImplItemKind::Fn(_, body_id) = impl_item.kind;
        then {
            let body = cx.tcx.hir().body(body_id);
            let mut fpu = FindExpectUnwrap {
                lcx: cx,
                typeck_results: cx.tcx.typeck(impl_item.def_id),
                result: Vec::new(),
            };
            fpu.visit_expr(&body.value);

            // if we've found one, lint
            if  !fpu.result.is_empty()  {
                span_lint_and_then(
                    cx,
                    UNWRAP_IN_RESULT,
                    impl_span,
                    "used unwrap or expect in a function that returns result or option",
                    move |diag| {
                        diag.help(
                            "unwrap and expect should not be used in a function that returns result or option" );
                        diag.span_note(fpu.result, "potential non-recoverable error(s)");
                    });
            }
        }
    }
}