use crate::{lints::PtrNullChecksDiag, LateContext, LateLintPass, LintContext};
use rustc_ast::LitKind;
use rustc_hir::{BinOpKind, Expr, ExprKind, TyKind};
use rustc_session::{declare_lint, declare_lint_pass};
use rustc_span::sym;

declare_lint! {
    /// The `useless_ptr_null_checks` lint checks for useless null checks against pointers
    /// obtained from non-null types.
    ///
    /// ### Example
    ///
    /// ```rust
    /// # fn test() {}
    /// let fn_ptr: fn() = /* somehow obtained nullable function pointer */
    /// #   test;
    ///
    /// if (fn_ptr as *const ()).is_null() { /* ... */ }
    /// ```
    ///
    /// {{produces}}
    ///
    /// ### Explanation
    ///
    /// Function pointers and references are assumed to be non-null, checking them for null
    /// will always return false.
    USELESS_PTR_NULL_CHECKS,
    Warn,
    "useless checking of non-null-typed pointer"
}

declare_lint_pass!(PtrNullChecks => [USELESS_PTR_NULL_CHECKS]);

/// This function detects and returns the original expression from a series of consecutive casts,
/// ie. `(my_fn as *const _ as *mut _).cast_mut()` would return the expression for `my_fn`.
fn ptr_cast_chain<'a>(cx: &'a LateContext<'_>, mut e: &'a Expr<'a>) -> Option<&'a Expr<'a>> {
    let mut had_at_least_one_cast = false;
    loop {
        e = e.peel_blocks();
        e = if let ExprKind::Cast(expr, t) = e.kind
            && let TyKind::Ptr(_) = t.kind {
            had_at_least_one_cast = true;
            expr
        } else if let ExprKind::MethodCall(_, expr, [], _) = e.kind
            && let Some(def_id) = cx.typeck_results().type_dependent_def_id(e.hir_id)
            && matches!(cx.tcx.get_diagnostic_name(def_id), Some(sym::ptr_cast | sym::ptr_cast_mut)) {
            had_at_least_one_cast = true;
            expr
        } else if let ExprKind::Call(path, [arg]) = e.kind
            && let ExprKind::Path(ref qpath) = path.kind
            && let Some(def_id) = cx.qpath_res(qpath, path.hir_id).opt_def_id()
            && matches!(cx.tcx.get_diagnostic_name(def_id), Some(sym::ptr_from_ref | sym::ptr_from_mut)) {
            had_at_least_one_cast = true;
            arg
        } else if had_at_least_one_cast {
            return Some(e);
        } else {
            return None;
        };
    }
}

fn incorrect_check<'a>(cx: &LateContext<'a>, expr: &Expr<'_>) -> Option<PtrNullChecksDiag<'a>> {
    let expr = ptr_cast_chain(cx, expr)?;

    let orig_ty = cx.typeck_results().expr_ty(expr);
    if orig_ty.is_fn() {
        Some(PtrNullChecksDiag::FnPtr { orig_ty, label: expr.span })
    } else if orig_ty.is_ref() {
        Some(PtrNullChecksDiag::Ref { orig_ty, label: expr.span })
    } else {
        None
    }
}

impl<'tcx> LateLintPass<'tcx> for PtrNullChecks {
    fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
        match expr.kind {
            // Catching:
            // <*<const/mut> <ty>>::is_null(fn_ptr as *<const/mut> <ty>)
            ExprKind::Call(path, [arg])
                if let ExprKind::Path(ref qpath) = path.kind
                    && let Some(def_id) = cx.qpath_res(qpath, path.hir_id).opt_def_id()
                    && matches!(
                        cx.tcx.get_diagnostic_name(def_id),
                        Some(sym::ptr_const_is_null | sym::ptr_is_null)
                    )
                    && let Some(diag) = incorrect_check(cx, arg) =>
            {
                cx.emit_spanned_lint(USELESS_PTR_NULL_CHECKS, expr.span, diag)
            }

            // Catching:
            // (fn_ptr as *<const/mut> <ty>).is_null()
            ExprKind::MethodCall(_, receiver, _, _)
                if let Some(def_id) = cx.typeck_results().type_dependent_def_id(expr.hir_id)
                    && matches!(
                        cx.tcx.get_diagnostic_name(def_id),
                        Some(sym::ptr_const_is_null | sym::ptr_is_null)
                    )
                    && let Some(diag) = incorrect_check(cx, receiver) =>
            {
                cx.emit_spanned_lint(USELESS_PTR_NULL_CHECKS, expr.span, diag)
            }

            ExprKind::Binary(op, left, right) if matches!(op.node, BinOpKind::Eq) => {
                let to_check: &Expr<'_>;
                let diag: PtrNullChecksDiag<'_>;
                if let Some(ddiag) = incorrect_check(cx, left) {
                    to_check = right;
                    diag = ddiag;
                } else if let Some(ddiag) = incorrect_check(cx, right) {
                    to_check = left;
                    diag = ddiag;
                } else {
                    return;
                }

                match to_check.kind {
                    // Catching:
                    // (fn_ptr as *<const/mut> <ty>) == (0 as <ty>)
                    ExprKind::Cast(cast_expr, _)
                        if let ExprKind::Lit(spanned) = cast_expr.kind
                            && let LitKind::Int(v, _) = spanned.node && v == 0 =>
                    {
                        cx.emit_spanned_lint(USELESS_PTR_NULL_CHECKS, expr.span, diag)
                    },

                    // Catching:
                    // (fn_ptr as *<const/mut> <ty>) == std::ptr::null()
                    ExprKind::Call(path, [])
                        if let ExprKind::Path(ref qpath) = path.kind
                            && let Some(def_id) = cx.qpath_res(qpath, path.hir_id).opt_def_id()
                            && let Some(diag_item) = cx.tcx.get_diagnostic_name(def_id)
                            && (diag_item == sym::ptr_null || diag_item == sym::ptr_null_mut) =>
                    {
                        cx.emit_spanned_lint(USELESS_PTR_NULL_CHECKS, expr.span, diag)
                    },

                    _ => {},
                }
            }
            _ => {}
        }
    }
}
