clippy_lints/src/methods/flat_map_identity.rs

use clippy_utils::diagnostics::span_lint_and_sugg;
use clippy_utils::is_expr_untyped_identity_function;
use clippy_utils::res::{MaybeDef, MaybeTypeckRes};
use rustc_errors::Applicability;
use rustc_hir as hir;
use rustc_lint::LateContext;
use rustc_span::{Span, sym};

use super::FLAT_MAP_IDENTITY;

/// lint use of `flat_map` for `Iterators` where `flatten` would be sufficient
pub(super) fn check<'tcx>(
    cx: &LateContext<'tcx>,
    expr: &'tcx hir::Expr<'_>,
    flat_map_arg: &'tcx hir::Expr<'_>,
    flat_map_span: Span,
) {
    if cx.ty_based_def(expr).opt_parent(cx).is_diag_item(cx, sym::Iterator)
        && is_expr_untyped_identity_function(cx, flat_map_arg)
    {
        span_lint_and_sugg(
            cx,
            FLAT_MAP_IDENTITY,
            flat_map_span.with_hi(expr.span.hi()),
            "use of `flat_map` with an identity function",
            "try",
            "flatten()".to_string(),
            Applicability::MachineApplicable,
        );
    }
}