frontend 0.4.1

rustc's frontend with no LLVM and no std: parsing through MIR, as a library
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//! A pass that inserts the `ConstEvalCounter` instruction into any blocks that have a back edge
//! (thus indicating there is a loop in the CFG), or whose terminator is a function call.

// `#![no_std]`: these arrive with the standard prelude and name no path, so a `std::`
// search cannot see them - and a `#[derive]` can use them without the name appearing
// in this file at all, which is why they are not trimmed by inspection.
use alloc::borrow::ToOwned;
use alloc::boxed::Box;
use alloc::format;
use alloc::string::{String, ToString};
use alloc::vec;
use alloc::vec::Vec;

use crate::rustc_data_structures::graph::dominators::Dominators;
use crate::rustc_middle::mir::{
    BasicBlock, BasicBlockData, Body, Statement, StatementKind, TerminatorKind,
};
use crate::rustc_middle::ty::TyCtxt;
use tracing::instrument;

use crate::rustc_mir_transform::PassPolicy;

pub(super) struct CtfeLimit;

impl<'tcx> crate::rustc_mir_transform::MirPass<'tcx> for CtfeLimit {
    #[instrument(skip(self, _tcx, body))]
    fn run_pass(&self, _tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
        let doms = body.basic_blocks.dominators();
        let indices: Vec<BasicBlock> = body
            .basic_blocks
            .iter_enumerated()
            .filter_map(|(node, node_data)| {
                if matches!(node_data.terminator().kind, TerminatorKind::Call { .. } | TerminatorKind::TailCall { .. })
                    // Back edges in a CFG indicate loops
                    || has_back_edge(doms, node, node_data)
                {
                    Some(node)
                } else {
                    None
                }
            })
            .collect();

        let basic_blocks = body.basic_blocks.as_mut_preserves_cfg();
        for index in indices {
            let bbdata = &mut basic_blocks[index];
            let source_info = bbdata.terminator().source_info;
            bbdata.statements.push(Statement::new(source_info, StatementKind::ConstEvalCounter));
        }
    }

    fn policy(&self, _sess: &crate::rustc_session::Session) -> PassPolicy {
        // This is part of CTFE diagnostics rather than an optimization.
        PassPolicy::optional_non_optimization(true)
    }
}

fn has_back_edge(
    doms: &Dominators<BasicBlock>,
    node: BasicBlock,
    node_data: &BasicBlockData<'_>,
) -> bool {
    if !doms.is_reachable(node) {
        return false;
    }
    // Check if any of the dominators of the node are also the node's successor.
    node_data.terminator().successors().any(|succ| doms.dominates(succ, node))
}