clippy_lints/src/methods/map_identity.rs

use clippy_utils::diagnostics::span_lint_and_then;
use clippy_utils::res::{MaybeDef, MaybeTypeckRes};
use clippy_utils::source::snippet_with_applicability;
use clippy_utils::ty::is_copy;
use clippy_utils::{is_expr_untyped_identity_function, is_mutable, path_to_local_with_projections};
use rustc_errors::Applicability;
use rustc_hir::{self as hir, ExprKind, Node, PatKind};
use rustc_lint::{LateContext, LintContext};
use rustc_span::{Span, Symbol, sym};

use super::MAP_IDENTITY;

const MSG: &str = "unnecessary map of the identity function";

pub(super) fn check(
    cx: &LateContext<'_>,
    expr: &hir::Expr<'_>,
    caller: &hir::Expr<'_>,
    map_arg: &hir::Expr<'_>,
    name: Symbol,
    _map_span: Span,
) {
    let caller_ty = cx.typeck_results().expr_ty(caller);

    if (cx.ty_based_def(expr).opt_parent(cx).is_diag_item(cx, sym::Iterator)
        || caller_ty.is_diag_item(cx, sym::Result)
        || caller_ty.is_diag_item(cx, sym::Option))
        && is_expr_untyped_identity_function(cx, map_arg)
        && let Some(call_span) = expr.span.trim_start(caller.span)
    {
        span_lint_and_then(cx, MAP_IDENTITY, call_span, MSG, |diag| {
            let main_sugg = (call_span, String::new());
            let mut app = if is_copy(cx, caller_ty) {
                // there is technically a behavioral change here for `Copy` iterators, where
                // `iter.map(|x| x).next()` would mutate a temporary copy of the iterator and
                // changing it to `iter.next()` mutates iter directly
                Applicability::Unspecified
            } else {
                Applicability::MachineApplicable
            };

            let needs_to_be_mutable = cx.typeck_results().expr_ty_adjusted(expr).is_mutable_ptr();
            if needs_to_be_mutable && !is_mutable(cx, caller) {
                if let Some(hir_id) = path_to_local_with_projections(caller)
                    && let Node::Pat(pat) = cx.tcx.hir_node(hir_id)
                    && let PatKind::Binding(_, _, ident, _) = pat.kind
                {
                    // We can reach the binding -- suggest making it mutable
                    let suggs = vec![main_sugg, (ident.span.shrink_to_lo(), String::from("mut "))];

                    let ident = snippet_with_applicability(cx.sess(), ident.span, "_", &mut app);

                    diag.multipart_suggestion(
                        format!("remove the call to `{name}`, and make `{ident}` mutable"),
                        suggs,
                        app,
                    );
                } else {
                    // If we can't make the binding mutable, prevent the suggestion from being automatically applied,
                    // and add a complementary help message.
                    app = Applicability::Unspecified;

                    let method_requiring_mut = if let Node::Expr(expr) = cx.tcx.parent_hir_node(expr.hir_id)
                        && let ExprKind::MethodCall(method, ..) = expr.kind
                    {
                        Some(method.ident)
                    } else {
                        None
                    };

                    diag.span_suggestion(main_sugg.0, format!("remove the call to `{name}`"), main_sugg.1, app);

                    let note = if let Some(method_requiring_mut) = method_requiring_mut {
                        format!("this must be made mutable to use `{method_requiring_mut}`")
                    } else {
                        "this must be made mutable".to_string()
                    };
                    diag.span_note(caller.span, note);
                }
            } else {
                diag.span_suggestion(main_sugg.0, format!("remove the call to `{name}`"), main_sugg.1, app);
            }
        });
    }
}