clippy_lints/src/question_mark.rs

use crate::manual_let_else::MANUAL_LET_ELSE;
use crate::question_mark_used::QUESTION_MARK_USED;
use clippy_config::Conf;
use clippy_config::types::MatchLintBehaviour;
use clippy_utils::diagnostics::span_lint_and_sugg;
use clippy_utils::msrvs::{self, Msrv};
use clippy_utils::res::{MaybeDef, MaybeQPath, MaybeResPath};
use clippy_utils::source::{snippet_with_applicability, snippet_with_context};
use clippy_utils::sugg::Sugg;
use clippy_utils::ty::{implements_trait, is_copy};
use clippy_utils::usage::local_used_after_expr;
use clippy_utils::{
    eq_expr_value, fn_def_id_with_node_args, higher, is_else_clause, is_in_const_context, is_lint_allowed,
    pat_and_expr_can_be_question_mark, peel_blocks, peel_blocks_with_stmt, span_contains_cfg, span_contains_comment,
    sym,
};
use rustc_errors::Applicability;
use rustc_hir::LangItem::{self, OptionNone, OptionSome, ResultErr, ResultOk};
use rustc_hir::def::Res;
use rustc_hir::{
    Arm, BindingMode, Block, Body, ByRef, Expr, ExprKind, FnRetTy, HirId, LetStmt, MatchSource, Mutability, Node, Pat,
    PatKind, PathSegment, QPath, Stmt, StmtKind,
};
use rustc_lint::{LateContext, LateLintPass};
use rustc_middle::ty::{self, Ty};
use rustc_session::impl_lint_pass;
use rustc_span::symbol::Symbol;

declare_clippy_lint! {
    /// ### What it does
    /// Checks for expressions that could be replaced by the `?` operator.
    ///
    /// ### Why is this bad?
    /// Using the `?` operator is shorter and more idiomatic.
    ///
    /// ### Example
    /// ```ignore
    /// if option.is_none() {
    ///     return None;
    /// }
    /// ```
    ///
    /// Could be written:
    ///
    /// ```ignore
    /// option?;
    /// ```
    #[clippy::version = "pre 1.29.0"]
    pub QUESTION_MARK,
    style,
    "checks for expressions that could be replaced by the `?` operator"
}

impl_lint_pass!(QuestionMark => [MANUAL_LET_ELSE, QUESTION_MARK]);

pub struct QuestionMark {
    pub(crate) msrv: Msrv,
    pub(crate) matches_behaviour: MatchLintBehaviour,
    /// Keeps track of how many try blocks we are in at any point during linting.
    /// This allows us to answer the question "are we inside of a try block"
    /// very quickly, without having to walk up the parent chain, by simply checking
    /// if it is greater than zero.
    /// As for why we need this in the first place: <https://github.com/rust-lang/rust-clippy/issues/8628>
    try_block_depth_stack: Vec<u32>,
    /// Keeps track of the number of inferred return type closures we are inside, to avoid problems
    /// with the `Err(x.into())` expansion being ambiguous.
    inferred_ret_closure_stack: u16,
}

impl QuestionMark {
    pub fn new(conf: &'static Conf) -> Self {
        Self {
            msrv: conf.msrv,
            matches_behaviour: conf.matches_for_let_else,
            try_block_depth_stack: Vec::new(),
            inferred_ret_closure_stack: 0,
        }
    }
}

enum IfBlockType<'hir> {
    /// An `if x.is_xxx() { a } else { b } ` expression.
    ///
    /// Contains: `caller (x), caller_type, call_sym (is_xxx), if_then (a), if_else (b)`
    IfIs(&'hir Expr<'hir>, Ty<'hir>, Symbol, &'hir Expr<'hir>),
    /// An `if let Xxx(a) = b { c } else { d }` expression.
    ///
    /// Contains: `let_pat_qpath (Xxx), let_pat_type, let_pat_sym (a), let_expr (b), if_then (c),
    /// if_else (d)`
    IfLet(
        Res,
        Ty<'hir>,
        Symbol,
        &'hir Expr<'hir>,
        &'hir Expr<'hir>,
        Option<&'hir Expr<'hir>>,
    ),
}

fn find_let_else_ret_expression<'hir>(block: &'hir Block<'hir>) -> Option<&'hir Expr<'hir>> {
    if let Block {
        stmts: [],
        expr: Some(els),
        ..
    } = block
    {
        Some(els)
    } else if let [stmt] = block.stmts
        && let StmtKind::Semi(expr) = stmt.kind
        && let ExprKind::Ret(..) = expr.kind
    {
        Some(expr)
    } else {
        None
    }
}

fn check_let_some_else_return_none(cx: &LateContext<'_>, stmt: &Stmt<'_>) {
    /// Make sure the init expr implements try trait so a valid suggestion could be given.
    ///
    /// Because the init expr could have the type of `&Option<T>` which does not implements `Try`.
    ///
    /// NB: This conveniently prevents the cause of
    /// issue [#12412](https://github.com/rust-lang/rust-clippy/issues/12412),
    /// since accessing an `Option` field from a borrowed struct requires borrow, such as
    /// `&some_struct.opt`, which is type of `&Option`. And we can't suggest `&some_struct.opt?`
    /// or `(&some_struct.opt)?` since the first one has different semantics and the later does
    /// not implements `Try`.
    fn init_expr_can_use_question_mark(cx: &LateContext<'_>, init_expr: &Expr<'_>) -> bool {
        let init_ty = cx.typeck_results().expr_ty_adjusted(init_expr);
        cx.tcx
            .lang_items()
            .try_trait()
            .is_some_and(|did| implements_trait(cx, init_ty, did, &[]))
    }

    if let StmtKind::Let(LetStmt {
        pat,
        init: Some(init_expr),
        els: Some(els),
        ..
    }) = stmt.kind
        && init_expr_can_use_question_mark(cx, init_expr)
        && let Some(ret) = find_let_else_ret_expression(els)
        && let Some(inner_pat) = pat_and_expr_can_be_question_mark(cx, pat, ret)
        && !span_contains_comment(cx, els.span)
        && !span_contains_cfg(cx, els.span)
    {
        let mut applicability = Applicability::MaybeIncorrect;
        let init_expr_str =
            Sugg::hir_with_context(cx, init_expr, stmt.span.ctxt(), "..", &mut applicability).maybe_paren();
        // Take care when binding is `ref`
        let sugg = if let PatKind::Binding(
            BindingMode(ByRef::Yes(_, ref_mutability), binding_mutability),
            _hir_id,
            ident,
            subpattern,
        ) = inner_pat.kind
        {
            let (from_method, replace_to) = match ref_mutability {
                Mutability::Mut => (".as_mut()", "&mut "),
                Mutability::Not => (".as_ref()", "&"),
            };

            let mutability_str = match binding_mutability {
                Mutability::Mut => "mut ",
                Mutability::Not => "",
            };

            // Handle subpattern (@ subpattern)
            let maybe_subpattern = match subpattern {
                Some(Pat {
                    kind: PatKind::Binding(BindingMode(ByRef::Yes(..), _), _, subident, None),
                    ..
                }) => {
                    // avoid `&ref`
                    // note that, because you can't have aliased, mutable references, we don't have to worry about
                    // the outer and inner mutability being different
                    format!(" @ {subident}")
                },
                Some(subpattern) => {
                    let substr = snippet_with_applicability(cx, subpattern.span, "..", &mut applicability);
                    format!(" @ {replace_to}{substr}")
                },
                None => String::new(),
            };

            format!("let {mutability_str}{ident}{maybe_subpattern} = {init_expr_str}{from_method}?;")
        } else {
            let receiver_str = snippet_with_applicability(cx, inner_pat.span, "..", &mut applicability);
            format!("let {receiver_str} = {init_expr_str}?;")
        };
        span_lint_and_sugg(
            cx,
            QUESTION_MARK,
            stmt.span,
            "this `let...else` may be rewritten with the `?` operator",
            "replace it with",
            sugg,
            applicability,
        );
    }
}

fn is_early_return(smbl: Symbol, cx: &LateContext<'_>, if_block: &IfBlockType<'_>) -> bool {
    match *if_block {
        IfBlockType::IfIs(caller, caller_ty, call_sym, if_then) => {
            // If the block could be identified as `if x.is_none()/is_err()`,
            // we then only need to check the if_then return to see if it is none/err.
            caller_ty.is_diag_item(cx, smbl)
                && expr_return_none_or_err(smbl, cx, if_then, caller, None)
                && match smbl {
                    sym::Option => call_sym == sym::is_none,
                    sym::Result => call_sym == sym::is_err,
                    _ => false,
                }
        },
        IfBlockType::IfLet(res, let_expr_ty, let_pat_sym, let_expr, if_then, if_else) => {
            let_expr_ty.is_diag_item(cx, smbl)
                && match smbl {
                    sym::Option => {
                        // We only need to check `if let Some(x) = option` not `if let None = option`,
                        // because the later one will be suggested as `if option.is_none()` thus causing conflict.
                        res.ctor_parent(cx).is_lang_item(cx, OptionSome)
                            && if_else.is_some()
                            && expr_return_none_or_err(smbl, cx, if_else.unwrap(), let_expr, None)
                    },
                    sym::Result => {
                        (res.ctor_parent(cx).is_lang_item(cx, ResultOk)
                            && if_else.is_some()
                            && expr_return_none_or_err(smbl, cx, if_else.unwrap(), let_expr, Some(let_pat_sym)))
                            || res.ctor_parent(cx).is_lang_item(cx, ResultErr)
                                && expr_return_none_or_err(smbl, cx, if_then, let_expr, Some(let_pat_sym))
                                && if_else.is_none()
                    },
                    _ => false,
                }
        },
    }
}

fn expr_return_none_or_err(
    smbl: Symbol,
    cx: &LateContext<'_>,
    expr: &Expr<'_>,
    cond_expr: &Expr<'_>,
    err_sym: Option<Symbol>,
) -> bool {
    match peel_blocks_with_stmt(expr).kind {
        ExprKind::Ret(Some(ret_expr)) => expr_return_none_or_err(smbl, cx, ret_expr, cond_expr, err_sym),
        ExprKind::Path(ref qpath) => match smbl {
            sym::Option => cx
                .qpath_res(qpath, expr.hir_id)
                .ctor_parent(cx)
                .is_lang_item(cx, OptionNone),
            sym::Result => expr.res_local_id().is_some() && expr.res_local_id() == cond_expr.res_local_id(),
            _ => false,
        },
        ExprKind::Call(call_expr, [arg]) => {
            if smbl == sym::Result
                && let ExprKind::Path(QPath::Resolved(_, path)) = &call_expr.kind
                && let Some(segment) = path.segments.first()
                && let Some(err_sym) = err_sym
                && let ExprKind::Path(QPath::Resolved(_, arg_path)) = &arg.kind
                && let Some(PathSegment { ident, .. }) = arg_path.segments.first()
            {
                return segment.ident.name == sym::Err && err_sym == ident.name;
            }
            false
        },
        _ => false,
    }
}

/// Checks if the given expression on the given context matches the following structure:
///
/// ```ignore
/// if option.is_none() {
///     return None;
/// }
/// ```
///
/// ```ignore
/// if result.is_err() {
///     return result;
/// }
/// ```
///
/// If it matches, it will suggest to use the `?` operator instead
fn check_is_none_or_err_and_early_return<'tcx>(cx: &LateContext<'tcx>, expr: &Expr<'tcx>) {
    if let Some(higher::If { cond, then, r#else }) = higher::If::hir(expr)
        && !is_else_clause(cx.tcx, expr)
        && let ExprKind::MethodCall(segment, caller, [], _) = &cond.kind
        && let caller_ty = cx.typeck_results().expr_ty(caller)
        && let if_block = IfBlockType::IfIs(caller, caller_ty, segment.ident.name, then)
        && (is_early_return(sym::Option, cx, &if_block) || is_early_return(sym::Result, cx, &if_block))
    {
        let mut applicability = Applicability::MachineApplicable;
        let receiver_str = snippet_with_context(cx, caller.span, expr.span.ctxt(), "..", &mut applicability).0;
        let by_ref = !cx.type_is_copy_modulo_regions(caller_ty)
            && !matches!(caller.kind, ExprKind::Call(..) | ExprKind::MethodCall(..));
        let sugg = if let Some(else_inner) = r#else {
            if eq_expr_value(cx, caller, peel_blocks(else_inner)) {
                format!("Some({receiver_str}?)")
            } else {
                return;
            }
        } else {
            format!("{receiver_str}{}?;", if by_ref { ".as_ref()" } else { "" })
        };

        span_lint_and_sugg(
            cx,
            QUESTION_MARK,
            expr.span,
            "this block may be rewritten with the `?` operator",
            "replace it with",
            sugg,
            applicability,
        );
    }
}

#[derive(Clone, Copy, Debug)]
enum TryMode {
    Result,
    Option,
}

fn find_try_mode<'tcx>(cx: &LateContext<'tcx>, scrutinee: &Expr<'tcx>) -> Option<TryMode> {
    let scrutinee_ty = cx.typeck_results().expr_ty_adjusted(scrutinee);
    let ty::Adt(scrutinee_adt_def, _) = scrutinee_ty.kind() else {
        return None;
    };

    match cx.tcx.get_diagnostic_name(scrutinee_adt_def.did())? {
        sym::Result => Some(TryMode::Result),
        sym::Option => Some(TryMode::Option),
        _ => None,
    }
}

// Check that `pat` is `{ctor_lang_item}(val)`, returning `val`.
fn extract_ctor_call<'a, 'tcx>(
    cx: &LateContext<'tcx>,
    expected_ctor: LangItem,
    pat: &'a Pat<'tcx>,
) -> Option<&'a Pat<'tcx>> {
    if let PatKind::TupleStruct(variant_path, [val_binding], _) = &pat.kind
        && cx
            .qpath_res(variant_path, pat.hir_id)
            .ctor_parent(cx)
            .is_lang_item(cx, expected_ctor)
    {
        Some(val_binding)
    } else {
        None
    }
}

// Extracts the local ID of a plain `val` pattern.
fn extract_binding_pat(pat: &Pat<'_>) -> Option<HirId> {
    if let PatKind::Binding(BindingMode::NONE, binding, _, None) = pat.kind {
        Some(binding)
    } else {
        None
    }
}

fn check_arm_is_some_or_ok<'tcx>(cx: &LateContext<'tcx>, mode: TryMode, arm: &Arm<'tcx>) -> bool {
    let happy_ctor = match mode {
        TryMode::Result => ResultOk,
        TryMode::Option => OptionSome,
    };

    // Check for `Ok(val)` or `Some(val)`
    if arm.guard.is_none()
        && let Some(val_binding) = extract_ctor_call(cx, happy_ctor, arm.pat)
        // Extract out `val`
        && let Some(binding) = extract_binding_pat(val_binding)
        // Check body is just `=> val`
        && peel_blocks(arm.body).res_local_id() == Some(binding)
    {
        true
    } else {
        false
    }
}

fn check_arm_is_none_or_err<'tcx>(cx: &LateContext<'tcx>, mode: TryMode, arm: &Arm<'tcx>) -> bool {
    if arm.guard.is_some() {
        return false;
    }

    let arm_body = peel_blocks(arm.body);
    match mode {
        TryMode::Result => {
            // Check that pat is Err(val)
            if let Some(ok_pat) = extract_ctor_call(cx, ResultErr, arm.pat)
                && let Some(ok_val) = extract_binding_pat(ok_pat)
                // check `=> return Err(...)`
                && let ExprKind::Ret(Some(wrapped_ret_expr)) = arm_body.kind
                && let ExprKind::Call(ok_ctor, [ret_expr]) = wrapped_ret_expr.kind
                && ok_ctor.res(cx).ctor_parent(cx).is_lang_item(cx, ResultErr)
                // check if `...` is `val` from binding or `val.into()`
                && is_local_or_local_into(cx, ret_expr, ok_val)
            {
                true
            } else {
                false
            }
        },
        TryMode::Option => {
            // Check the pat is `None`
            if arm.pat.res(cx).ctor_parent(cx).is_lang_item(cx, OptionNone)
                // Check `=> return None`
                && let ExprKind::Ret(Some(ret_expr)) = arm_body.kind
                && ret_expr.res(cx).ctor_parent(cx).is_lang_item(cx, OptionNone)
                && !ret_expr.span.from_expansion()
            {
                true
            } else {
                false
            }
        },
    }
}

/// Check if `expr` is `val` or `val.into()`
fn is_local_or_local_into(cx: &LateContext<'_>, expr: &Expr<'_>, val: HirId) -> bool {
    let is_into_call = fn_def_id_with_node_args(cx, expr)
        .and_then(|(fn_def_id, _)| cx.tcx.trait_of_assoc(fn_def_id))
        .is_some_and(|trait_def_id| cx.tcx.is_diagnostic_item(sym::Into, trait_def_id));
    match expr.kind {
        ExprKind::MethodCall(_, recv, [], _) | ExprKind::Call(_, [recv]) => {
            is_into_call && recv.res_local_id() == Some(val)
        },
        _ => expr.res_local_id() == Some(val),
    }
}

fn check_arms_are_try<'tcx>(cx: &LateContext<'tcx>, mode: TryMode, arm1: &Arm<'tcx>, arm2: &Arm<'tcx>) -> bool {
    (check_arm_is_some_or_ok(cx, mode, arm1) && check_arm_is_none_or_err(cx, mode, arm2))
        || (check_arm_is_some_or_ok(cx, mode, arm2) && check_arm_is_none_or_err(cx, mode, arm1))
}

fn check_if_try_match<'tcx>(cx: &LateContext<'tcx>, expr: &Expr<'tcx>) {
    if let ExprKind::Match(scrutinee, [arm1, arm2], MatchSource::Normal | MatchSource::Postfix) = expr.kind
        && !expr.span.from_expansion()
        && let Some(mode) = find_try_mode(cx, scrutinee)
        && !span_contains_cfg(cx, expr.span)
        && check_arms_are_try(cx, mode, arm1, arm2)
    {
        let mut applicability = Applicability::MachineApplicable;
        let snippet = snippet_with_applicability(cx, scrutinee.span.source_callsite(), "..", &mut applicability);

        span_lint_and_sugg(
            cx,
            QUESTION_MARK,
            expr.span,
            "this `match` expression can be replaced with `?`",
            "try instead",
            snippet.into_owned() + "?",
            applicability,
        );
    }
}

fn check_if_let_some_or_err_and_early_return<'tcx>(cx: &LateContext<'tcx>, expr: &Expr<'tcx>) {
    if let Some(higher::IfLet {
        let_pat,
        let_expr,
        if_then,
        if_else,
        ..
    }) = higher::IfLet::hir(cx, expr)
        && let PatKind::TupleStruct(ref path1, [field], ddpos) = let_pat.kind
        && ddpos.as_opt_usize().is_none()
        && let PatKind::Binding(BindingMode(by_ref, _), bind_id, ident, None) = field.kind
        && let caller_ty = cx.typeck_results().expr_ty(let_expr)
        && let if_block = IfBlockType::IfLet(
            cx.qpath_res(path1, let_pat.hir_id),
            caller_ty,
            ident.name,
            let_expr,
            if_then,
            if_else,
        )
        && ((is_early_return(sym::Option, cx, &if_block) && peel_blocks(if_then).res_local_id() == Some(bind_id))
            || is_early_return(sym::Result, cx, &if_block))
        && if_else
            .map(|e| eq_expr_value(cx, let_expr, peel_blocks(e)))
            .is_none_or(|e| !e)
    {
        if !is_copy(cx, caller_ty)
            && let Some(hir_id) = let_expr.res_local_id()
            && local_used_after_expr(cx, hir_id, expr)
        {
            return;
        }

        let mut applicability = Applicability::MachineApplicable;
        let receiver_str = snippet_with_applicability(cx, let_expr.span, "..", &mut applicability);
        let parent = cx.tcx.parent_hir_node(expr.hir_id);
        let requires_semi = matches!(parent, Node::Stmt(_)) || cx.typeck_results().expr_ty(expr).is_unit();
        let method_call_str = match by_ref {
            ByRef::Yes(_, Mutability::Mut) => ".as_mut()",
            ByRef::Yes(_, Mutability::Not) => ".as_ref()",
            ByRef::No => "",
        };

        let mut sugg = format!(
            "{receiver_str}{method_call_str}?{}",
            if requires_semi { ";" } else { "" }
        );
        if is_else_clause(cx.tcx, expr) || (requires_semi && !matches!(parent, Node::Stmt(_) | Node::Block(_))) {
            sugg = format!("{{ {sugg} }}");
        }

        span_lint_and_sugg(
            cx,
            QUESTION_MARK,
            expr.span,
            "this block may be rewritten with the `?` operator",
            "replace it with",
            sugg,
            applicability,
        );
    }
}

impl QuestionMark {
    fn inside_try_block(&self) -> bool {
        self.try_block_depth_stack.last() > Some(&0)
    }
}

fn is_try_block(cx: &LateContext<'_>, bl: &Block<'_>) -> bool {
    if let Some(expr) = bl.expr
        && let ExprKind::Call(callee, [_]) = expr.kind
        && let ExprKind::Path(qpath) = callee.kind
        && cx.tcx.qpath_is_lang_item(qpath, LangItem::TryTraitFromOutput)
    {
        true
    } else {
        false
    }
}

fn is_inferred_ret_closure(expr: &Expr<'_>) -> bool {
    let ExprKind::Closure(closure) = expr.kind else {
        return false;
    };

    match closure.fn_decl.output {
        FnRetTy::Return(ret_ty) => ret_ty.is_suggestable_infer_ty(),
        FnRetTy::DefaultReturn(_) => true,
    }
}

impl<'tcx> LateLintPass<'tcx> for QuestionMark {
    fn check_stmt(&mut self, cx: &LateContext<'tcx>, stmt: &'tcx Stmt<'_>) {
        if !is_lint_allowed(cx, QUESTION_MARK_USED, stmt.hir_id) || !self.msrv.meets(cx, msrvs::QUESTION_MARK_OPERATOR)
        {
            return;
        }

        if !self.inside_try_block() && !is_in_const_context(cx) {
            check_let_some_else_return_none(cx, stmt);
        }
        self.check_manual_let_else(cx, stmt);
    }

    fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
        if is_inferred_ret_closure(expr) {
            self.inferred_ret_closure_stack += 1;
            return;
        }

        if !self.inside_try_block()
            && !is_in_const_context(cx)
            && is_lint_allowed(cx, QUESTION_MARK_USED, expr.hir_id)
            && self.msrv.meets(cx, msrvs::QUESTION_MARK_OPERATOR)
        {
            check_is_none_or_err_and_early_return(cx, expr);
            check_if_let_some_or_err_and_early_return(cx, expr);

            if self.inferred_ret_closure_stack == 0 {
                check_if_try_match(cx, expr);
            }
        }
    }

    fn check_expr_post(&mut self, _: &LateContext<'tcx>, expr: &'tcx Expr<'tcx>) {
        if is_inferred_ret_closure(expr) {
            self.inferred_ret_closure_stack -= 1;
        }
    }

    fn check_block(&mut self, cx: &LateContext<'tcx>, block: &'tcx Block<'tcx>) {
        if is_try_block(cx, block) {
            *self
                .try_block_depth_stack
                .last_mut()
                .expect("blocks are always part of bodies and must have a depth") += 1;
        }
    }

    fn check_body(&mut self, _: &LateContext<'tcx>, _: &Body<'tcx>) {
        self.try_block_depth_stack.push(0);
    }

    fn check_body_post(&mut self, _: &LateContext<'tcx>, _: &Body<'tcx>) {
        self.try_block_depth_stack.pop();
    }

    fn check_block_post(&mut self, cx: &LateContext<'tcx>, block: &'tcx Block<'tcx>) {
        if is_try_block(cx, block) {
            *self
                .try_block_depth_stack
                .last_mut()
                .expect("blocks are always part of bodies and must have a depth") -= 1;
        }
    }
}