rustc_mir_transform/
lower_slice_len.rs

1//! This pass lowers calls to core::slice::len to just PtrMetadata op.
2//! It should run before inlining!
3
4use rustc_hir::def_id::DefId;
5use rustc_middle::mir::*;
6use rustc_middle::ty::TyCtxt;
7
8use crate::PassPolicy;
9
10pub(super) struct LowerSliceLenCalls;
11
12impl<'tcx> crate::MirPass<'tcx> for LowerSliceLenCalls {
13    fn policy(&self, sess: &rustc_session::Session) -> PassPolicy {
14        PassPolicy::optimization(sess.mir_opt_level() > 0)
15    }
16
17    fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
18        let language_items = tcx.lang_items();
19        let Some(slice_len_fn_item_def_id) = language_items.slice_len_fn() else {
20            // there is no lang item to compare to :)
21            return;
22        };
23
24        // The one successor remains unchanged, so no need to invalidate
25        let basic_blocks = body.basic_blocks.as_mut_preserves_cfg();
26        for block in basic_blocks {
27            // lower `<[_]>::len` calls
28            lower_slice_len_call(block, slice_len_fn_item_def_id);
29        }
30    }
31}
32
33fn lower_slice_len_call<'tcx>(block: &mut BasicBlockData<'tcx>, slice_len_fn_item_def_id: DefId) {
34    let terminator = block.terminator();
35    if let TerminatorKind::Call {
36        func,
37        args,
38        destination,
39        target: Some(bb),
40        call_source: CallSource::Normal,
41        ..
42    } = &terminator.kind
43        // some heuristics for fast rejection
44        && let [arg] = &args[..]
45        && let Some((fn_def_id, _)) = func.const_fn_def()
46        && fn_def_id == slice_len_fn_item_def_id
47    {
48        // perform modifications from something like:
49        //     _5 = core::slice::<impl [u8]>::len(move _6) -> bb1
50        // into:
51        //     _5 = PtrMetadata(move _6)
52        //     goto bb1
53
54        // make new RValue for Len
55        let r_value = Rvalue::UnaryOp(UnOp::PtrMetadata, arg.node.clone());
56        let len_statement_kind = StatementKind::Assign(Box::new((*destination, r_value)));
57        let add_statement = Statement::new(terminator.source_info, len_statement_kind);
58
59        // modify terminator into simple Goto
60        let new_terminator_kind = TerminatorKind::Goto { target: *bb };
61
62        block.statements.push(add_statement);
63        block.terminator_mut().kind = new_terminator_kind;
64    }
65}
rustc_mir_transform/lower_slice_len.rs

rustc_mir_transform/
lower_slice_len.rs
