compiler/rustc_mir_transform/src/coverage/mod.rs

mod counters;
mod graph;
mod mappings;
pub(super) mod query;
mod spans;
#[cfg(test)]
mod tests;
mod unexpand;

use rustc_hir as hir;
use rustc_hir::intravisit::{Visitor, walk_expr};
use rustc_middle::hir::nested_filter;
use rustc_middle::mir::coverage::{CoverageKind, FunctionCoverageInfo, Mapping, MappingKind};
use rustc_middle::mir::{self, BasicBlock, Statement, StatementKind, TerminatorKind};
use rustc_middle::ty::TyCtxt;
use rustc_span::Span;
use rustc_span::def_id::LocalDefId;
use tracing::{debug, debug_span, trace};

use crate::coverage::counters::BcbCountersData;
use crate::coverage::graph::CoverageGraph;
use crate::coverage::mappings::ExtractedMappings;

/// Inserts `StatementKind::Coverage` statements that either instrument the binary with injected
/// counters, via intrinsic `llvm.instrprof.increment`, and/or inject metadata used during codegen
/// to construct the coverage map.
pub(super) struct InstrumentCoverage;

impl<'tcx> crate::MirPass<'tcx> for InstrumentCoverage {
    fn is_enabled(&self, sess: &rustc_session::Session) -> bool {
        sess.instrument_coverage()
    }

    fn run_pass(&self, tcx: TyCtxt<'tcx>, mir_body: &mut mir::Body<'tcx>) {
        let mir_source = mir_body.source;

        // This pass runs after MIR promotion, but before promoted MIR starts to
        // be transformed, so it should never see promoted MIR.
        assert!(mir_source.promoted.is_none());

        let def_id = mir_source.def_id().expect_local();

        if !tcx.is_eligible_for_coverage(def_id) {
            trace!("InstrumentCoverage skipped for {def_id:?} (not eligible)");
            return;
        }

        // An otherwise-eligible function is still skipped if its start block
        // is known to be unreachable.
        match mir_body.basic_blocks[mir::START_BLOCK].terminator().kind {
            TerminatorKind::Unreachable => {
                trace!("InstrumentCoverage skipped for unreachable `START_BLOCK`");
                return;
            }
            _ => {}
        }

        instrument_function_for_coverage(tcx, mir_body);
    }

    fn is_required(&self) -> bool {
        false
    }
}

fn instrument_function_for_coverage<'tcx>(tcx: TyCtxt<'tcx>, mir_body: &mut mir::Body<'tcx>) {
    let def_id = mir_body.source.def_id();
    let _span = debug_span!("instrument_function_for_coverage", ?def_id).entered();

    let hir_info = extract_hir_info(tcx, def_id.expect_local());

    // Build the coverage graph, which is a simplified view of the MIR control-flow
    // graph that ignores some details not relevant to coverage instrumentation.
    let graph = CoverageGraph::from_mir(mir_body);

    ////////////////////////////////////////////////////
    // Extract coverage spans and other mapping info from MIR.
    let extracted_mappings =
        mappings::extract_all_mapping_info_from_mir(tcx, mir_body, &hir_info, &graph);

    let mappings = create_mappings(&extracted_mappings);
    if mappings.is_empty() {
        // No spans could be converted into valid mappings, so skip this function.
        debug!("no spans could be converted into valid mappings; skipping");
        return;
    }

    // Use the coverage graph to prepare intermediate data that will eventually
    // be used to assign physical counters and counter expressions to points in
    // the control-flow graph.
    let BcbCountersData { node_flow_data, priority_list } =
        counters::prepare_bcb_counters_data(&graph);

    // Inject coverage statements into MIR.
    inject_coverage_statements(mir_body, &graph);

    mir_body.function_coverage_info = Some(Box::new(FunctionCoverageInfo {
        function_source_hash: hir_info.function_source_hash,

        node_flow_data,
        priority_list,

        mappings,
    }));
}

/// For each coverage span extracted from MIR, create a corresponding mapping.
///
/// FIXME(Zalathar): This used to be where BCBs in the extracted mappings were
/// resolved to a `CovTerm`. But that is now handled elsewhere, so this
/// function can potentially be simplified even further.
fn create_mappings(extracted_mappings: &ExtractedMappings) -> Vec<Mapping> {
    // Fully destructure the mappings struct to make sure we don't miss any kinds.
    let ExtractedMappings { code_mappings, branch_pairs } = extracted_mappings;
    let mut mappings = Vec::new();

    mappings.extend(code_mappings.iter().map(
        // Ordinary code mappings are the simplest kind.
        |&mappings::CodeMapping { span, bcb }| {
            let kind = MappingKind::Code { bcb };
            Mapping { kind, span }
        },
    ));

    mappings.extend(branch_pairs.iter().map(
        |&mappings::BranchPair { span, true_bcb, false_bcb }| {
            let kind = MappingKind::Branch { true_bcb, false_bcb };
            Mapping { kind, span }
        },
    ));

    mappings
}

/// Inject any necessary coverage statements into MIR, so that they influence codegen.
fn inject_coverage_statements<'tcx>(mir_body: &mut mir::Body<'tcx>, graph: &CoverageGraph) {
    for (bcb, data) in graph.iter_enumerated() {
        let target_bb = data.leader_bb();
        inject_statement(mir_body, CoverageKind::VirtualCounter { bcb }, target_bb);
    }
}

fn inject_statement(mir_body: &mut mir::Body<'_>, counter_kind: CoverageKind, bb: BasicBlock) {
    debug!("  injecting statement {counter_kind:?} for {bb:?}");
    let data = &mut mir_body[bb];
    let source_info = data.terminator().source_info;
    let statement = Statement::new(source_info, StatementKind::Coverage(counter_kind));
    data.statements.insert(0, statement);
}

/// Function information extracted from HIR by the coverage instrumentor.
#[derive(Debug)]
struct ExtractedHirInfo {
    function_source_hash: u64,
    is_async_fn: bool,
    /// The span of the function's signature, if available.
    /// Must have the same context and filename as the body span.
    fn_sig_span: Option<Span>,
    body_span: Span,
    /// "Holes" are regions within the function body (or its expansions) that
    /// should not be included in coverage spans for this function
    /// (e.g. closures and nested items).
    hole_spans: Vec<Span>,
}

fn extract_hir_info<'tcx>(tcx: TyCtxt<'tcx>, def_id: LocalDefId) -> ExtractedHirInfo {
    // FIXME(#79625): Consider improving MIR to provide the information needed, to avoid going back
    // to HIR for it.

    // HACK: For synthetic MIR bodies (async closures), use the def id of the HIR body.
    if tcx.is_synthetic_mir(def_id) {
        return extract_hir_info(tcx, tcx.local_parent(def_id));
    }

    let hir_node = tcx.hir_node_by_def_id(def_id);
    let fn_body_id = hir_node.body_id().expect("HIR node is a function with body");
    let hir_body = tcx.hir_body(fn_body_id);

    let maybe_fn_sig = hir_node.fn_sig();
    let is_async_fn = maybe_fn_sig.is_some_and(|fn_sig| fn_sig.header.is_async());

    let mut body_span = hir_body.value.span;

    use hir::{Closure, Expr, ExprKind, Node};
    // Unexpand a closure's body span back to the context of its declaration.
    // This helps with closure bodies that consist of just a single bang-macro,
    // and also with closure bodies produced by async desugaring.
    if let Node::Expr(&Expr { kind: ExprKind::Closure(&Closure { fn_decl_span, .. }), .. }) =
        hir_node
    {
        body_span = body_span.find_ancestor_in_same_ctxt(fn_decl_span).unwrap_or(body_span);
    }

    // The actual signature span is only used if it has the same context and
    // filename as the body, and precedes the body.
    let fn_sig_span = maybe_fn_sig.map(|fn_sig| fn_sig.span).filter(|&fn_sig_span| {
        let source_map = tcx.sess.source_map();
        let file_idx = |span: Span| source_map.lookup_source_file_idx(span.lo());

        fn_sig_span.eq_ctxt(body_span)
            && fn_sig_span.hi() <= body_span.lo()
            && file_idx(fn_sig_span) == file_idx(body_span)
    });

    let function_source_hash = hash_mir_source(tcx, hir_body);

    let hole_spans = extract_hole_spans_from_hir(tcx, hir_body);

    ExtractedHirInfo { function_source_hash, is_async_fn, fn_sig_span, body_span, hole_spans }
}

fn hash_mir_source<'tcx>(tcx: TyCtxt<'tcx>, hir_body: &'tcx hir::Body<'tcx>) -> u64 {
    // FIXME(cjgillot) Stop hashing HIR manually here.
    let owner = hir_body.id().hir_id.owner;
    tcx.hir_owner_nodes(owner).opt_hash_including_bodies.unwrap().to_smaller_hash().as_u64()
}

fn extract_hole_spans_from_hir<'tcx>(tcx: TyCtxt<'tcx>, hir_body: &hir::Body<'tcx>) -> Vec<Span> {
    struct HolesVisitor<'tcx> {
        tcx: TyCtxt<'tcx>,
        hole_spans: Vec<Span>,
    }

    impl<'tcx> Visitor<'tcx> for HolesVisitor<'tcx> {
        /// We have special handling for nested items, but we still want to
        /// traverse into nested bodies of things that are not considered items,
        /// such as "anon consts" (e.g. array lengths).
        type NestedFilter = nested_filter::OnlyBodies;

        fn maybe_tcx(&mut self) -> TyCtxt<'tcx> {
            self.tcx
        }

        /// We override `visit_nested_item` instead of `visit_item` because we
        /// only need the item's span, not the item itself.
        fn visit_nested_item(&mut self, id: hir::ItemId) -> Self::Result {
            let span = self.tcx.def_span(id.owner_id.def_id);
            self.visit_hole_span(span);
            // Having visited this item, we don't care about its children,
            // so don't call `walk_item`.
        }

        // We override `visit_expr` instead of the more specific expression
        // visitors, so that we have direct access to the expression span.
        fn visit_expr(&mut self, expr: &'tcx hir::Expr<'tcx>) {
            match expr.kind {
                hir::ExprKind::Closure(_) | hir::ExprKind::ConstBlock(_) => {
                    self.visit_hole_span(expr.span);
                    // Having visited this expression, we don't care about its
                    // children, so don't call `walk_expr`.
                }

                // For other expressions, recursively visit as normal.
                _ => walk_expr(self, expr),
            }
        }
    }
    impl HolesVisitor<'_> {
        fn visit_hole_span(&mut self, hole_span: Span) {
            self.hole_spans.push(hole_span);
        }
    }

    let mut visitor = HolesVisitor { tcx, hole_spans: vec![] };

    visitor.visit_body(hir_body);
    visitor.hole_spans
}