AstGen: rework multi-object for loop

* Allow unbounded looping.
* Lower by incrementing raw pointers for each iterable rather than
  incrementing a single index variable. This elides safety checks
  without any analysis required thanks to the length assertion and
  lowers to decent machine code even in debug builds.
  - An "end" value is selected, prioritizing a counter if possible,
    falling back to a runtime calculation of ptr+len on a slice input.
* Specialize on the pattern `0..`, avoiding an unnecessary subtraction
  instruction being emitted.
* Add the `for_check_lens` ZIR instruction.

1 files changed, 15 insertions, 0 deletions

diff --git a/src/Sema.zig b/src/Sema.zig
index cf6350e35f..b5afe93511 100644
--- a/src/Sema.zig
+++ b/src/Sema.zig
@@ -1386,6 +1386,11 @@ fn analyzeBodyInner(
                 i += 1;
                 continue;
             },
+            .for_check_lens => {
+                try sema.zirForCheckLens(block, inst);
+                i += 1;
+                continue;
+            },
 
             // Special case instructions to handle comptime control flow.
             .@"break" => {
@@ -17096,6 +17101,16 @@ fn zirRestoreErrRetIndex(sema: *Sema, start_block: *Block, inst: Zir.Inst.Index)
     return sema.popErrorReturnTrace(start_block, src, operand, saved_index);
 }
 
+fn zirForCheckLens(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!void {
+    const inst_data = sema.code.instructions.items(.data)[inst].pl_node;
+    const extra = sema.code.extraData(Zir.Inst.MultiOp, inst_data.payload_index);
+    const args = sema.code.refSlice(extra.end, extra.data.operands_len);
+    const src = inst_data.src();
+
+    _ = args;
+    return sema.fail(block, src, "TODO implement zirForCheckLens", .{});
+}
+
 fn addToInferredErrorSet(sema: *Sema, uncasted_operand: Air.Inst.Ref) !void {
     assert(sema.fn_ret_ty.zigTypeTag() == .ErrorUnion);