rust-clippy/clippy_lints/src/manual_map.rs

use crate::{map_unit_fn::OPTION_MAP_UNIT_FN, matches::MATCH_AS_REF};
use clippy_utils::diagnostics::span_lint_and_sugg;
use clippy_utils::higher;
use clippy_utils::source::{snippet_with_applicability, snippet_with_context};
use clippy_utils::ty::{is_type_diagnostic_item, peel_mid_ty_refs_is_mutable};
use clippy_utils::{
    can_move_expr_to_closure, in_constant, is_else_clause, is_lang_ctor, is_lint_allowed, path_to_local_id,
    peel_hir_expr_refs,
};
use rustc_ast::util::parser::PREC_POSTFIX;
use rustc_errors::Applicability;
use rustc_hir::LangItem::{OptionNone, OptionSome};
use rustc_hir::{Arm, BindingAnnotation, Block, Expr, ExprKind, HirId, Mutability, Pat, PatKind};
use rustc_lint::{LateContext, LateLintPass, LintContext};
use rustc_middle::lint::in_external_macro;
use rustc_session::{declare_lint_pass, declare_tool_lint};
use rustc_span::{sym, SyntaxContext};

declare_clippy_lint! {
    /// ### What it does
    /// Checks for usages of `match` which could be implemented using `map`
    ///
    /// ### Why is this bad?
    /// Using the `map` method is clearer and more concise.
    ///
    /// ### Example
    /// ```rust
    /// match Some(0) {
    ///     Some(x) => Some(x + 1),
    ///     None => None,
    /// };
    /// ```
    /// Use instead:
    /// ```rust
    /// Some(0).map(|x| x + 1);
    /// ```
    pub MANUAL_MAP,
    style,
    "reimplementation of `map`"
}

declare_lint_pass!(ManualMap => [MANUAL_MAP]);

impl LateLintPass<'_> for ManualMap {
    #[allow(clippy::too_many_lines)]
    fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
        if let Some(higher::IfLet {
            let_pat,
            let_expr,
            if_then,
            if_else: Some(if_else),
        }) = higher::IfLet::hir(expr)
        {
            manage_lint(cx, expr, (&let_pat.kind, if_then), (&PatKind::Wild, if_else), let_expr);
        }

        if let ExprKind::Match(scrutinee, [then @ Arm { guard: None, .. }, r#else @ Arm { guard: None, .. }], _) =
            expr.kind
        {
            manage_lint(
                cx,
                expr,
                (&then.pat.kind, then.body),
                (&r#else.pat.kind, r#else.body),
                scrutinee,
            );
        }
    }
}

fn manage_lint<'tcx>(
    cx: &LateContext<'tcx>,
    expr: &'tcx Expr<'_>,
    then: (&'tcx PatKind<'_>, &'tcx Expr<'_>),
    r#else: (&'tcx PatKind<'_>, &'tcx Expr<'_>),
    scrut: &'tcx Expr<'_>,
) {
    if in_external_macro(cx.sess(), expr.span) || in_constant(cx, expr.hir_id) {
        return;
    }

    let (scrutinee_ty, ty_ref_count, ty_mutability) = peel_mid_ty_refs_is_mutable(cx.typeck_results().expr_ty(scrut));
    if !(is_type_diagnostic_item(cx, scrutinee_ty, sym::option_type)
        && is_type_diagnostic_item(cx, cx.typeck_results().expr_ty(expr), sym::option_type))
    {
        return;
    }

    let (then_pat, then_expr) = then;
    let (else_pat, else_expr) = r#else;

    let expr_ctxt = expr.span.ctxt();
    let (some_expr, some_pat, pat_ref_count, is_wild_none) = match (
        try_parse_pattern(cx, then_pat, expr_ctxt),
        try_parse_pattern(cx, else_pat, expr_ctxt),
    ) {
        (Some(OptionPat::Wild), Some(OptionPat::Some { pattern, ref_count })) if is_none_expr(cx, then_expr) => {
            (else_expr, pattern, ref_count, true)
        },
        (Some(OptionPat::None), Some(OptionPat::Some { pattern, ref_count })) if is_none_expr(cx, then_expr) => {
            (else_expr, pattern, ref_count, false)
        },
        (Some(OptionPat::Some { pattern, ref_count }), Some(OptionPat::Wild)) if is_none_expr(cx, else_expr) => {
            (then_expr, pattern, ref_count, true)
        },
        (Some(OptionPat::Some { pattern, ref_count }), Some(OptionPat::None)) if is_none_expr(cx, else_expr) => {
            (then_expr, pattern, ref_count, false)
        },
        _ => return,
    };

    // Top level or patterns aren't allowed in closures.
    if matches!(some_pat.kind, PatKind::Or(_)) {
        return;
    }

    let some_expr = match get_some_expr(cx, some_expr, expr_ctxt) {
        Some(expr) => expr,
        None => return,
    };

    if cx.typeck_results().expr_ty(some_expr) == cx.tcx.types.unit && !is_lint_allowed(cx, OPTION_MAP_UNIT_FN, expr.hir_id) {
        return;
    }

    // `map` won't perform any adjustments.
    if !cx.typeck_results().expr_adjustments(some_expr).is_empty() {
        return;
    }

    if !can_move_expr_to_closure(cx, some_expr) {
        return;
    }

    // Determine which binding mode to use.
    let explicit_ref = some_pat.contains_explicit_ref_binding();
    let binding_ref = explicit_ref.or_else(|| (ty_ref_count != pat_ref_count).then(|| ty_mutability));

    let as_ref_str = match binding_ref {
        Some(Mutability::Mut) => ".as_mut()",
        Some(Mutability::Not) => ".as_ref()",
        None => "",
    };

    let mut app = Applicability::MachineApplicable;

    // Remove address-of expressions from the scrutinee. Either `as_ref` will be called, or
    // it's being passed by value.
    let scrutinee = peel_hir_expr_refs(scrut).0;
    let (scrutinee_str, _) = snippet_with_context(cx, scrutinee.span, expr_ctxt, "..", &mut app);
    let scrutinee_str = if scrutinee.span.ctxt() == expr.span.ctxt() && scrutinee.precedence().order() < PREC_POSTFIX {
        format!("({})", scrutinee_str)
    } else {
        scrutinee_str.into()
    };

    let body_str = if let PatKind::Binding(annotation, id, some_binding, None) = some_pat.kind {
        match can_pass_as_func(cx, id, some_expr) {
            Some(func) if func.span.ctxt() == some_expr.span.ctxt() => {
                snippet_with_applicability(cx, func.span, "..", &mut app).into_owned()
            },
            _ => {
                if path_to_local_id(some_expr, id)
                    && !is_lint_allowed(cx, MATCH_AS_REF, expr.hir_id)
                    && binding_ref.is_some()
                {
                    return;
                }

                // `ref` and `ref mut` annotations were handled earlier.
                let annotation = if matches!(annotation, BindingAnnotation::Mutable) {
                    "mut "
                } else {
                    ""
                };
                format!(
                    "|{}{}| {}",
                    annotation,
                    some_binding,
                    snippet_with_context(cx, some_expr.span, expr_ctxt, "..", &mut app).0
                )
            },
        }
    } else if !is_wild_none && explicit_ref.is_none() {
        // TODO: handle explicit reference annotations.
        format!(
            "|{}| {}",
            snippet_with_context(cx, some_pat.span, expr_ctxt, "..", &mut app).0,
            snippet_with_context(cx, some_expr.span, expr_ctxt, "..", &mut app).0
        )
    } else {
        // Refutable bindings and mixed reference annotations can't be handled by `map`.
        return;
    };

    span_lint_and_sugg(
        cx,
        MANUAL_MAP,
        expr.span,
        "manual implementation of `Option::map`",
        "try this",
        if is_else_clause(cx.tcx, expr) {
            format!("{{ {}{}.map({}) }}", scrutinee_str, as_ref_str, body_str)
        } else {
            format!("{}{}.map({})", scrutinee_str, as_ref_str, body_str)
        },
        app,
    );
}

// Checks whether the expression could be passed as a function, or whether a closure is needed.
// Returns the function to be passed to `map` if it exists.
fn can_pass_as_func(cx: &LateContext<'tcx>, binding: HirId, expr: &'tcx Expr<'_>) -> Option<&'tcx Expr<'tcx>> {
    match expr.kind {
        ExprKind::Call(func, [arg])
            if path_to_local_id (arg, binding) && cx.typeck_results().expr_adjustments(arg).is_empty() =>
        {
            Some(func)
        },
        _ => None,
    }
}

enum OptionPat<'a> {
    Wild,
    None,
    Some {
        // The pattern contained in the `Some` tuple.
        pattern: &'a Pat<'a>,
        // The number of references before the `Some` tuple.
        // e.g. `&&Some(_)` has a ref count of 2.
        ref_count: usize,
    },
}

// Try to parse into a recognized `Option` pattern.
// i.e. `_`, `None`, `Some(..)`, or a reference to any of those.
fn try_parse_pattern(
    cx: &LateContext<'tcx>,
    pat_kind: &'tcx PatKind<'_>,
    ctxt: SyntaxContext,
) -> Option<OptionPat<'tcx>> {
    fn f(
        cx: &LateContext<'tcx>,
        pat_kind: &'tcx PatKind<'_>,
        ref_count: usize,
        ctxt: SyntaxContext,
    ) -> Option<OptionPat<'tcx>> {
        match pat_kind {
            PatKind::Wild => Some(OptionPat::Wild),
            PatKind::Ref(ref_pat, _) => f(cx, &ref_pat.kind, ref_count + 1, ctxt),
            PatKind::Path(ref qpath) if is_lang_ctor(cx, qpath, OptionNone) => Some(OptionPat::None),
            PatKind::TupleStruct(ref qpath, [pattern], _) if is_lang_ctor(cx, qpath, OptionSome) => {
                Some(OptionPat::Some { pattern, ref_count })
            },
            _ => None,
        }
    }
    f(cx, pat_kind, 0, ctxt)
}

// Checks for an expression wrapped by the `Some` constructor. Returns the contained expression.
fn get_some_expr(cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>, ctxt: SyntaxContext) -> Option<&'tcx Expr<'tcx>> {
    // TODO: Allow more complex expressions.
    match expr.kind {
        ExprKind::Call(
            Expr {
                kind: ExprKind::Path(ref qpath),
                ..
            },
            [arg],
        ) if ctxt == expr.span.ctxt() && is_lang_ctor(cx, qpath, OptionSome) => Some(arg),
        ExprKind::Block(
            Block {
                stmts: [],
                expr: Some(expr),
                ..
            },
            _,
        ) => get_some_expr(cx, expr, ctxt),
        _ => None,
    }
}

// Checks for the `None` value.
fn is_none_expr(cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) -> bool {
    match expr.kind {
        ExprKind::Path(ref qpath) => is_lang_ctor(cx, qpath, OptionNone),
        ExprKind::Block(
            Block {
                stmts: [],
                expr: Some(expr),
                ..
            },
            _,
        ) => is_none_expr(cx, expr),
        _ => false,
    }
}