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

use super::{make_iterator_snippet, IncrementVisitor, InitializeVisitor, EXPLICIT_COUNTER_LOOP};
use clippy_utils::diagnostics::{span_lint_and_sugg, span_lint_and_then};
use clippy_utils::source::snippet_with_applicability;
use clippy_utils::{get_enclosing_block, is_integer_const};
use if_chain::if_chain;
use rustc_errors::Applicability;
use rustc_hir::intravisit::{walk_block, walk_expr};
use rustc_hir::{Expr, Pat};
use rustc_lint::LateContext;
use rustc_middle::ty::{self, Ty, UintTy};

// To trigger the EXPLICIT_COUNTER_LOOP lint, a variable must be
// incremented exactly once in the loop body, and initialized to zero
// at the start of the loop.
pub(super) fn check<'tcx>(
    cx: &LateContext<'tcx>,
    pat: &'tcx Pat<'_>,
    arg: &'tcx Expr<'_>,
    body: &'tcx Expr<'_>,
    expr: &'tcx Expr<'_>,
) {
    // Look for variables that are incremented once per loop iteration.
    let mut increment_visitor = IncrementVisitor::new(cx);
    walk_expr(&mut increment_visitor, body);

    // For each candidate, check the parent block to see if
    // it's initialized to zero at the start of the loop.
    if let Some(block) = get_enclosing_block(cx, expr.hir_id) {
        for id in increment_visitor.into_results() {
            let mut initialize_visitor = InitializeVisitor::new(cx, expr, id);
            walk_block(&mut initialize_visitor, block);

            if_chain! {
                if let Some((name, ty, initializer)) = initialize_visitor.get_result();
                if is_integer_const(cx, initializer, 0);
                then {
                    let mut applicability = Applicability::MachineApplicable;

                    let int_name = match ty.map(Ty::kind) {
                        // usize or inferred
                        Some(ty::Uint(UintTy::Usize)) | None => {
                            span_lint_and_sugg(
                                cx,
                                EXPLICIT_COUNTER_LOOP,
                                expr.span.with_hi(arg.span.hi()),
                                &format!("the variable `{}` is used as a loop counter", name),
                                "consider using",
                                format!(
                                    "for ({}, {}) in {}.enumerate()",
                                    name,
                                    snippet_with_applicability(cx, pat.span, "item", &mut applicability),
                                    make_iterator_snippet(cx, arg, &mut applicability),
                                ),
                                applicability,
                            );
                            return;
                        }
                        Some(ty::Int(int_ty)) => int_ty.name_str(),
                        Some(ty::Uint(uint_ty)) => uint_ty.name_str(),
                        _ => return,
                    };

                    span_lint_and_then(
                        cx,
                        EXPLICIT_COUNTER_LOOP,
                        expr.span.with_hi(arg.span.hi()),
                        &format!("the variable `{}` is used as a loop counter", name),
                        |diag| {
                            diag.span_suggestion(
                                expr.span.with_hi(arg.span.hi()),
                                "consider using",
                                format!(
                                    "for ({}, {}) in (0_{}..).zip({})",
                                    name,
                                    snippet_with_applicability(cx, pat.span, "item", &mut applicability),
                                    int_name,
                                    make_iterator_snippet(cx, arg, &mut applicability),
                                ),
                                applicability,
                            );

                            diag.note(&format!(
                                "`{}` is of type `{}`, making it ineligible for `Iterator::enumerate`",
                                name, int_name
                            ));
                        },
                    );
                }
            }
        }
    }
}