rust/clippy_lints/src/manual_unwrap_or.rs

use clippy_utils::consts::constant_simple;
use clippy_utils::diagnostics::span_lint_and_sugg;
use clippy_utils::source::{indent_of, reindent_multiline, snippet_opt};
use clippy_utils::ty::is_type_diagnostic_item;
use clippy_utils::usage::contains_return_break_continue_macro;
use clippy_utils::{in_constant, is_lang_ctor, path_to_local_id, sugg};
use if_chain::if_chain;
use rustc_errors::Applicability;
use rustc_hir::LangItem::{OptionNone, OptionSome, ResultErr, ResultOk};
use rustc_hir::{Arm, Expr, ExprKind, PatKind};
use rustc_lint::LintContext;
use rustc_lint::{LateContext, LateLintPass};
use rustc_middle::lint::in_external_macro;
use rustc_session::{declare_lint_pass, declare_tool_lint};
use rustc_span::sym;

declare_clippy_lint! {
    /// ### What it does
    /// Finds patterns that reimplement `Option::unwrap_or` or `Result::unwrap_or`.
    ///
    /// ### Why is this bad?
    /// Concise code helps focusing on behavior instead of boilerplate.
    ///
    /// ### Example
    /// ```rust
    /// let foo: Option<i32> = None;
    /// match foo {
    ///     Some(v) => v,
    ///     None => 1,
    /// };
    /// ```
    ///
    /// Use instead:
    /// ```rust
    /// let foo: Option<i32> = None;
    /// foo.unwrap_or(1);
    /// ```
    #[clippy::version = "1.49.0"]
    pub MANUAL_UNWRAP_OR,
    complexity,
    "finds patterns that can be encoded more concisely with `Option::unwrap_or` or `Result::unwrap_or`"
}

declare_lint_pass!(ManualUnwrapOr => [MANUAL_UNWRAP_OR]);

impl LateLintPass<'_> for ManualUnwrapOr {
    fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'tcx>) {
        if in_external_macro(cx.sess(), expr.span) || in_constant(cx, expr.hir_id) {
            return;
        }
        lint_manual_unwrap_or(cx, expr);
    }
}

fn lint_manual_unwrap_or<'tcx>(cx: &LateContext<'tcx>, expr: &'tcx Expr<'tcx>) {
    fn applicable_or_arm<'a>(cx: &LateContext<'_>, arms: &'a [Arm<'a>]) -> Option<&'a Arm<'a>> {
        if_chain! {
            if arms.len() == 2;
            if arms.iter().all(|arm| arm.guard.is_none());
            if let Some((idx, or_arm)) = arms.iter().enumerate().find(|(_, arm)| {
                match arm.pat.kind {
                    PatKind::Path(ref qpath) => is_lang_ctor(cx, qpath, OptionNone),
                    PatKind::TupleStruct(ref qpath, [pat], _) =>
                        matches!(pat.kind, PatKind::Wild) && is_lang_ctor(cx, qpath, ResultErr),
                    _ => false,
                }
            });
            let unwrap_arm = &arms[1 - idx];
            if let PatKind::TupleStruct(ref qpath, [unwrap_pat], _) = unwrap_arm.pat.kind;
            if is_lang_ctor(cx, qpath, OptionSome) || is_lang_ctor(cx, qpath, ResultOk);
            if let PatKind::Binding(_, binding_hir_id, ..) = unwrap_pat.kind;
            if path_to_local_id(unwrap_arm.body, binding_hir_id);
            if cx.typeck_results().expr_adjustments(unwrap_arm.body).is_empty();
            if !contains_return_break_continue_macro(or_arm.body);
            then {
                Some(or_arm)
            } else {
                None
            }
        }
    }

    if_chain! {
        if let ExprKind::Match(scrutinee, match_arms, _) = expr.kind;
        let ty = cx.typeck_results().expr_ty(scrutinee);
        if let Some(ty_name) = if is_type_diagnostic_item(cx, ty, sym::Option) {
            Some("Option")
        } else if is_type_diagnostic_item(cx, ty, sym::Result) {
            Some("Result")
        } else {
            None
        };
        if let Some(or_arm) = applicable_or_arm(cx, match_arms);
        if let Some(or_body_snippet) = snippet_opt(cx, or_arm.body.span);
        if let Some(indent) = indent_of(cx, expr.span);
        if constant_simple(cx, cx.typeck_results(), or_arm.body).is_some();
        then {
            let reindented_or_body =
                reindent_multiline(or_body_snippet.into(), true, Some(indent));

            let suggestion = if scrutinee.span.from_expansion() {
                    // we don't want parentheses around macro, e.g. `(some_macro!()).unwrap_or(0)`
                    sugg::Sugg::hir_with_macro_callsite(cx, scrutinee, "..")
                }
                else {
                    sugg::Sugg::hir(cx, scrutinee, "..").maybe_par()
                };

            span_lint_and_sugg(
                cx,
                MANUAL_UNWRAP_OR, expr.span,
                &format!("this pattern reimplements `{}::unwrap_or`", ty_name),
                "replace with",
                format!(
                    "{}.unwrap_or({})",
                    suggestion,
                    reindented_or_body,
                ),
                Applicability::MachineApplicable,
            );
        }
    }
}