rustc_mir_transform/
lower_slice_len.rs

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//! This pass lowers calls to core::slice::len to just PtrMetadata op.
//! It should run before inlining!

use rustc_hir::def_id::DefId;
use rustc_middle::mir::*;
use rustc_middle::ty::TyCtxt;

pub(super) struct LowerSliceLenCalls;

impl<'tcx> crate::MirPass<'tcx> for LowerSliceLenCalls {
    fn is_enabled(&self, sess: &rustc_session::Session) -> bool {
        sess.mir_opt_level() > 0
    }

    fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
        let language_items = tcx.lang_items();
        let Some(slice_len_fn_item_def_id) = language_items.slice_len_fn() else {
            // there is no lang item to compare to :)
            return;
        };

        // The one successor remains unchanged, so no need to invalidate
        let basic_blocks = body.basic_blocks.as_mut_preserves_cfg();
        for block in basic_blocks {
            // lower `<[_]>::len` calls
            lower_slice_len_call(block, slice_len_fn_item_def_id);
        }
    }
}

fn lower_slice_len_call<'tcx>(block: &mut BasicBlockData<'tcx>, slice_len_fn_item_def_id: DefId) {
    let terminator = block.terminator();
    if let TerminatorKind::Call {
        func,
        args,
        destination,
        target: Some(bb),
        call_source: CallSource::Normal,
        ..
    } = &terminator.kind
        // some heuristics for fast rejection
        && let [arg] = &args[..]
        && let Some((fn_def_id, _)) = func.const_fn_def()
        && fn_def_id == slice_len_fn_item_def_id
    {
        // perform modifications from something like:
        //     _5 = core::slice::<impl [u8]>::len(move _6) -> bb1
        // into:
        //     _5 = PtrMetadata(move _6)
        //     goto bb1

        // make new RValue for Len
        let r_value = Rvalue::UnaryOp(UnOp::PtrMetadata, arg.node.clone());
        let len_statement_kind = StatementKind::Assign(Box::new((*destination, r_value)));
        let add_statement =
            Statement { kind: len_statement_kind, source_info: terminator.source_info };

        // modify terminator into simple Goto
        let new_terminator_kind = TerminatorKind::Goto { target: *bb };

        block.statements.push(add_statement);
        block.terminator_mut().kind = new_terminator_kind;
    }
}