1 files changed, 99 insertions, 0 deletions
diff --git a/compiler/rustc_mir_transform/src/lower_slice_len.rs b/compiler/rustc_mir_transform/src/lower_slice_len.rs
new file mode 100644
index 000000000..2f02d00ec
--- /dev/null
+++ b/compiler/rustc_mir_transform/src/lower_slice_len.rs
@@ -0,0 +1,99 @@
+//! This pass lowers calls to core::slice::len to just Len op.
+//! It should run before inlining!
+
+use crate::MirPass;
+use rustc_hir::def_id::DefId;
+use rustc_index::vec::IndexVec;
+use rustc_middle::mir::*;
+use rustc_middle::ty::{self, TyCtxt};
+
+pub struct LowerSliceLenCalls;
+
+impl<'tcx> 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>) {
+        lower_slice_len_calls(tcx, body)
+    }
+}
+
+pub fn lower_slice_len_calls<'tcx>(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 language 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(tcx, block, &body.local_decls, slice_len_fn_item_def_id);
+    }
+}
+
+struct SliceLenPatchInformation<'tcx> {
+    add_statement: Statement<'tcx>,
+    new_terminator_kind: TerminatorKind<'tcx>,
+}
+
+fn lower_slice_len_call<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    block: &mut BasicBlockData<'tcx>,
+    local_decls: &IndexVec<Local, LocalDecl<'tcx>>,
+    slice_len_fn_item_def_id: DefId,
+) {
+    let mut patch_found: Option<SliceLenPatchInformation<'_>> = None;
+
+    let terminator = block.terminator();
+    match &terminator.kind {
+        TerminatorKind::Call {
+            func,
+            args,
+            destination,
+            target: Some(bb),
+            cleanup: None,
+            from_hir_call: true,
+            ..
+        } => {
+            // some heuristics for fast rejection
+            if args.len() != 1 {
+                return;
+            }
+            let Some(arg) = args[0].place() else { return };
+            let func_ty = func.ty(local_decls, tcx);
+            match func_ty.kind() {
+                ty::FnDef(fn_def_id, _) if 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 = Len(*_6)
+                    // goto bb1
+
+                    // make new RValue for Len
+                    let deref_arg = tcx.mk_place_deref(arg);
+                    let r_value = Rvalue::Len(deref_arg);
+                    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 };
+
+                    let patch = SliceLenPatchInformation { add_statement, new_terminator_kind };
+
+                    patch_found = Some(patch);
+                }
+                _ => {}
+            }
+        }
+        _ => {}
+    }
+
+    if let Some(SliceLenPatchInformation { add_statement, new_terminator_kind }) = patch_found {
+        block.statements.push(add_statement);
+        block.terminator_mut().kind = new_terminator_kind;
+    }
+}