1 //! This pass lowers calls to core::slice::len to just Len op.
2 //! It should run before inlining!
5 use rustc_hir::def_id::DefId;
6 use rustc_index::vec::IndexVec;
7 use rustc_middle::mir::*;
8 use rustc_middle::ty::{self, TyCtxt};
10 pub struct LowerSliceLenCalls;
12 impl<'tcx> MirPass<'tcx> for LowerSliceLenCalls {
13 fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
14 lower_slice_len_calls(tcx, body)
18 pub fn lower_slice_len_calls<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
19 let language_items = tcx.lang_items();
20 let Some(slice_len_fn_item_def_id) = language_items.slice_len_fn() else {
21 // there is no language item to compare to :)
25 let (basic_blocks, local_decls) = body.basic_blocks_and_local_decls_mut();
27 for block in basic_blocks {
28 // lower `<[_]>::len` calls
29 lower_slice_len_call(tcx, block, &*local_decls, slice_len_fn_item_def_id);
33 struct SliceLenPatchInformation<'tcx> {
34 add_statement: Statement<'tcx>,
35 new_terminator_kind: TerminatorKind<'tcx>,
38 fn lower_slice_len_call<'tcx>(
40 block: &mut BasicBlockData<'tcx>,
41 local_decls: &IndexVec<Local, LocalDecl<'tcx>>,
42 slice_len_fn_item_def_id: DefId,
44 let mut patch_found: Option<SliceLenPatchInformation<'_>> = None;
46 let terminator = block.terminator();
47 match &terminator.kind {
48 TerminatorKind::Call {
51 destination: Some((dest, bb)),
56 // some heuristics for fast rejection
60 let arg = match args[0].place() {
64 let func_ty = func.ty(local_decls, tcx);
65 match func_ty.kind() {
66 ty::FnDef(fn_def_id, _) if fn_def_id == &slice_len_fn_item_def_id => {
67 // perform modifications
68 // from something like `_5 = core::slice::<impl [u8]>::len(move _6) -> bb1`
69 // into `_5 = Len(*_6)
72 // make new RValue for Len
73 let deref_arg = tcx.mk_place_deref(arg);
74 let r_value = Rvalue::Len(deref_arg);
75 let len_statement_kind = StatementKind::Assign(Box::new((*dest, r_value)));
77 Statement { kind: len_statement_kind, source_info: terminator.source_info };
79 // modify terminator into simple Goto
80 let new_terminator_kind = TerminatorKind::Goto { target: *bb };
82 let patch = SliceLenPatchInformation { add_statement, new_terminator_kind };
84 patch_found = Some(patch);
92 if let Some(SliceLenPatchInformation { add_statement, new_terminator_kind }) = patch_found {
93 block.statements.push(add_statement);
94 block.terminator_mut().kind = new_terminator_kind;