rust-clippy/clippy_lints/src/loops/while_let_loop.rs

use super::WHILE_LET_LOOP;
use crate::utils::span_lint_and_sugg;
use clippy_utils::source::snippet_with_applicability;
use rustc_errors::Applicability;
use rustc_hir::{Block, Expr, ExprKind, MatchSource, StmtKind};
use rustc_lint::{LateContext, LintContext};
use rustc_middle::lint::in_external_macro;

pub(super) fn check(cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>, loop_block: &'tcx Block<'_>) {
    // extract the expression from the first statement (if any) in a block
    let inner_stmt_expr = extract_expr_from_first_stmt(loop_block);
    // or extract the first expression (if any) from the block
    if let Some(inner) = inner_stmt_expr.or_else(|| extract_first_expr(loop_block)) {
        if let ExprKind::Match(ref matchexpr, ref arms, ref source) = inner.kind {
            // ensure "if let" compatible match structure
            match *source {
                MatchSource::Normal | MatchSource::IfLetDesugar { .. } => {
                    if arms.len() == 2
                        && arms[0].guard.is_none()
                        && arms[1].guard.is_none()
                        && is_simple_break_expr(&arms[1].body)
                    {
                        if in_external_macro(cx.sess(), expr.span) {
                            return;
                        }

                        // NOTE: we used to build a body here instead of using
                        // ellipsis, this was removed because:
                        // 1) it was ugly with big bodies;
                        // 2) it was not indented properly;
                        // 3) it wasn’t very smart (see #675).
                        let mut applicability = Applicability::HasPlaceholders;
                        span_lint_and_sugg(
                            cx,
                            WHILE_LET_LOOP,
                            expr.span,
                            "this loop could be written as a `while let` loop",
                            "try",
                            format!(
                                "while let {} = {} {{ .. }}",
                                snippet_with_applicability(cx, arms[0].pat.span, "..", &mut applicability),
                                snippet_with_applicability(cx, matchexpr.span, "..", &mut applicability),
                            ),
                            applicability,
                        );
                    }
                },
                _ => (),
            }
        }
    }
}

/// If a block begins with a statement (possibly a `let` binding) and has an
/// expression, return it.
fn extract_expr_from_first_stmt<'tcx>(block: &Block<'tcx>) -> Option<&'tcx Expr<'tcx>> {
    if block.stmts.is_empty() {
        return None;
    }
    if let StmtKind::Local(ref local) = block.stmts[0].kind {
        local.init //.map(|expr| expr)
    } else {
        None
    }
}

/// If a block begins with an expression (with or without semicolon), return it.
fn extract_first_expr<'tcx>(block: &Block<'tcx>) -> Option<&'tcx Expr<'tcx>> {
    match block.expr {
        Some(ref expr) if block.stmts.is_empty() => Some(expr),
        None if !block.stmts.is_empty() => match block.stmts[0].kind {
            StmtKind::Expr(ref expr) | StmtKind::Semi(ref expr) => Some(expr),
            StmtKind::Local(..) | StmtKind::Item(..) => None,
        },
        _ => None,
    }
}

/// Returns `true` if expr contains a single break expr without destination label
/// and
/// passed expression. The expression may be within a block.
fn is_simple_break_expr(expr: &Expr<'_>) -> bool {
    match expr.kind {
        ExprKind::Break(dest, ref passed_expr) if dest.label.is_none() && passed_expr.is_none() => true,
        ExprKind::Block(ref b, _) => extract_first_expr(b).map_or(false, |subexpr| is_simple_break_expr(subexpr)),
        _ => false,
    }
}