rust/clippy_lints/src/if_then_some_else_none.rs

use clippy_config::msrvs::{self, Msrv};
use clippy_config::Conf;
use clippy_utils::diagnostics::span_lint_and_help;
use clippy_utils::eager_or_lazy::switch_to_eager_eval;
use clippy_utils::source::snippet_with_context;
use clippy_utils::sugg::Sugg;
use clippy_utils::{contains_return, higher, in_constant, is_else_clause, is_res_lang_ctor, path_res, peel_blocks};
use rustc_errors::Applicability;
use rustc_hir::LangItem::{OptionNone, OptionSome};
use rustc_hir::{Expr, ExprKind};
use rustc_lint::{LateContext, LateLintPass, LintContext};
use rustc_middle::lint::in_external_macro;
use rustc_session::impl_lint_pass;

declare_clippy_lint! {
    /// ### What it does
    /// Checks for if-else that could be written using either `bool::then` or `bool::then_some`.
    ///
    /// ### Why restrict this?
    /// Looks a little redundant. Using `bool::then` is more concise and incurs no loss of clarity.
    /// For simple calculations and known values, use `bool::then_some`, which is eagerly evaluated
    /// in comparison to `bool::then`.
    ///
    /// ### Example
    /// ```no_run
    /// # let v = vec![0];
    /// let a = if v.is_empty() {
    ///     println!("true!");
    ///     Some(42)
    /// } else {
    ///     None
    /// };
    /// ```
    ///
    /// Could be written:
    ///
    /// ```no_run
    /// # let v = vec![0];
    /// let a = v.is_empty().then(|| {
    ///     println!("true!");
    ///     42
    /// });
    /// ```
    #[clippy::version = "1.53.0"]
    pub IF_THEN_SOME_ELSE_NONE,
    restriction,
    "Finds if-else that could be written using either `bool::then` or `bool::then_some`"
}

pub struct IfThenSomeElseNone {
    msrv: Msrv,
}

impl IfThenSomeElseNone {
    pub fn new(conf: &'static Conf) -> Self {
        Self {
            msrv: conf.msrv.clone(),
        }
    }
}

impl_lint_pass!(IfThenSomeElseNone => [IF_THEN_SOME_ELSE_NONE]);

impl<'tcx> LateLintPass<'tcx> for IfThenSomeElseNone {
    fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'tcx>) {
        if !self.msrv.meets(msrvs::BOOL_THEN) {
            return;
        }

        if in_external_macro(cx.sess(), expr.span) {
            return;
        }

        // We only care about the top-most `if` in the chain
        if is_else_clause(cx.tcx, expr) {
            return;
        }

        // `bool::then()` and `bool::then_some()` are not const
        if in_constant(cx, expr.hir_id) {
            return;
        }

        let ctxt = expr.span.ctxt();

        if let Some(higher::If {
            cond,
            then,
            r#else: Some(els),
        }) = higher::If::hir(expr)
            && let ExprKind::Block(then_block, _) = then.kind
            && let Some(then_expr) = then_block.expr
            && let ExprKind::Call(then_call, [then_arg]) = then_expr.kind
            && then_expr.span.ctxt() == ctxt
            && is_res_lang_ctor(cx, path_res(cx, then_call), OptionSome)
            && is_res_lang_ctor(cx, path_res(cx, peel_blocks(els)), OptionNone)
            && !contains_return(then_block.stmts)
        {
            let mut app = Applicability::Unspecified;
            let cond_snip = Sugg::hir_with_context(cx, cond, expr.span.ctxt(), "[condition]", &mut app)
                .maybe_par()
                .to_string();
            let arg_snip = snippet_with_context(cx, then_arg.span, ctxt, "[body]", &mut app).0;
            let mut method_body = if then_block.stmts.is_empty() {
                arg_snip.into_owned()
            } else {
                format!("{{ /* snippet */ {arg_snip} }}")
            };
            let method_name = if switch_to_eager_eval(cx, expr) && self.msrv.meets(msrvs::BOOL_THEN_SOME) {
                "then_some"
            } else {
                method_body.insert_str(0, "|| ");
                "then"
            };

            let help =
                format!("consider using `bool::{method_name}` like: `{cond_snip}.{method_name}({method_body})`",);
            span_lint_and_help(
                cx,
                IF_THEN_SOME_ELSE_NONE,
                expr.span,
                format!("this could be simplified with `bool::{method_name}`"),
                None,
                help,
            );
        }
    }

    extract_msrv_attr!(LateContext);
}