Auto merge of #91512 - scottmcm:array-intoiter-advance, r=Mark-Simulacrum Override `Iterator::advance(_back)_by` for `array::IntoIter` Because I happened to notice that `nth` is currently getting codegen'd as a loop even for `Copy` types:

LLVM before and after

Rust: ```rust #[no_mangle] pub fn array_intoiter_nth(it: &mut std::array::IntoIter, n: usize) -> Option { it.nth(n) } ``` Current nightly: ```llvmir define { i32, i32 } `@array_intoiter_nth(%"core::array::iter::IntoIter"* noalias nocapture align 8 dereferenceable(416) %it, i64 %n) unnamed_addr #0 personality i32 (i32, i32, i64, %"unwind::libunwind::_Unwind_Exception"*, %"unwind::libunwind::_Unwind_Context"*)* `@rust_eh_personality` !dbg !6 { start: %_3.i.i.i4.i.i = getelementptr inbounds %"core::array::iter::IntoIter", %"core::array::iter::IntoIter"* %it, i64 0, i32 0, i32 0 %_4.i.i.i5.i.i = getelementptr inbounds %"core::array::iter::IntoIter", %"core::array::iter::IntoIter"* %it, i64 0, i32 0, i32 1 %_4.i.i.i.i.i.i = load i64, i64* %_4.i.i.i5.i.i, align 8, !alias.scope !10 %.not.i.i = icmp eq i64 %n, 0, !dbg !15 %_3.i.i.i.i.pre.i = load i64, i64* %_3.i.i.i4.i.i, align 8, !dbg !40, !alias.scope !41 br i1 %.not.i.i, label %bb4.i, label %bb4.preheader.i.i, !dbg !42 bb4.preheader.i.i: ; preds = %start %umax.i = tail call i64 `@llvm.umax.i64(i64` %_3.i.i.i.i.pre.i, i64 %_4.i.i.i.i.i.i) #3, !dbg !43 %0 = sub i64 %umax.i, %_3.i.i.i.i.pre.i, !dbg !43 br label %bb4.i.i, !dbg !43 bb4.i.i: ; preds = %bb3.i.i.i.i, %bb4.preheader.i.i %_3.i.i.i.i.i.i = phi i64 [ %2, %bb3.i.i.i.i ], [ %_3.i.i.i.i.pre.i, %bb4.preheader.i.i ], !dbg !52 %iter.sroa.0.016.i.i = phi i64 [ %1, %bb3.i.i.i.i ], [ 0, %bb4.preheader.i.i ] %1 = add nuw i64 %iter.sroa.0.016.i.i, 1, !dbg !54 %exitcond.not.i = icmp eq i64 %iter.sroa.0.016.i.i, %0, !dbg !52 br i1 %exitcond.not.i, label %core::iter::traits::iterator::Iterator::nth.exit, label %bb3.i.i.i.i, !dbg !43 bb3.i.i.i.i: ; preds = %bb4.i.i %2 = add nuw i64 %_3.i.i.i.i.i.i, 1, !dbg !63 store i64 %2, i64* %_3.i.i.i4.i.i, align 8, !dbg !66, !alias.scope !75 %exitcond.not.i.i = icmp eq i64 %1, %n, !dbg !15 br i1 %exitcond.not.i.i, label %bb4.i, label %bb4.i.i, !dbg !42 bb4.i: ; preds = %bb3.i.i.i.i, %start %_3.i.i.i.i.i = phi i64 [ %_3.i.i.i.i.pre.i, %start ], [ %2, %bb3.i.i.i.i ], !dbg !84 %3 = icmp ult i64 %_3.i.i.i.i.i, %_4.i.i.i.i.i.i, !dbg !84 br i1 %3, label %bb3.i.i.i, label %core::iter::traits::iterator::Iterator::nth.exit, !dbg !89 bb3.i.i.i: ; preds = %bb4.i %4 = add nuw i64 %_3.i.i.i.i.i, 1, !dbg !90 store i64 %4, i64* %_3.i.i.i4.i.i, align 8, !dbg !93, !alias.scope !96 %5 = getelementptr inbounds %"core::array::iter::IntoIter", %"core::array::iter::IntoIter"* %it, i64 0, i32 1, i64 %_3.i.i.i.i.i, !dbg !105 %6 = load i32, i32* %5, align 4, !dbg !131, !alias.scope !141, !noalias !144 br label %core::iter::traits::iterator::Iterator::nth.exit, !dbg !149 core::iter::traits::iterator::Iterator::nth.exit: ; preds = %bb4.i.i, %bb4.i, %bb3.i.i.i %.sroa.3.0.i = phi i32 [ %6, %bb3.i.i.i ], [ undef, %bb4.i ], [ undef, %bb4.i.i ], !dbg !40 %.sroa.0.0.i = phi i32 [ 1, %bb3.i.i.i ], [ 0, %bb4.i ], [ 0, %bb4.i.i ], !dbg !40 %7 = insertvalue { i32, i32 } undef, i32 %.sroa.0.0.i, 0, !dbg !150 %8 = insertvalue { i32, i32 } %7, i32 %.sroa.3.0.i, 1, !dbg !150 ret { i32, i32 } %8, !dbg !151 } ``` With this PR: ```llvmir define { i32, i32 } `@array_intoiter_nth(%"core::array::iter::IntoIter"* noalias nocapture align 8 dereferenceable(416) %it, i64 %n) unnamed_addr #0 personality i32 (...)* `@__CxxFrameHandler3` { start: %0 = getelementptr inbounds %"core::array::iter::IntoIter", %"core::array::iter::IntoIter"* %it, i64 0, i32 0, i32 1 %_2.i.i.i.i = load i64, i64* %0, align 8, !alias.scope !6, !noalias !13 %1 = getelementptr inbounds %"core::array::iter::IntoIter", %"core::array::iter::IntoIter"* %it, i64 0, i32 0, i32 0 %_3.i.i.i.i = load i64, i64* %1, align 8, !alias.scope !16 %2 = sub i64 %_2.i.i.i.i, %_3.i.i.i.i %3 = icmp ult i64 %2, %n %.0.sroa.speculated.i.i.i.i.i = select i1 %3, i64 %2, i64 %n %_10.i.i = add i64 %.0.sroa.speculated.i.i.i.i.i, %_3.i.i.i.i store i64 %_10.i.i, i64* %1, align 8, !alias.scope !16 %.not.i = xor i1 %3, true %4 = icmp ult i64 %_10.i.i, %_2.i.i.i.i %or.cond.i = select i1 %.not.i, i1 %4, i1 false br i1 %or.cond.i, label %bb3.i.i.i, label %_ZN4core4iter6traits8iterator8Iterator3nth17hcbc727011e9e2a3bE.exit bb3.i.i.i: ; preds = %start %5 = add nuw i64 %_10.i.i, 1 store i64 %5, i64* %1, align 8, !alias.scope !17 %6 = getelementptr inbounds %"core::array::iter::IntoIter", %"core::array::iter::IntoIter"* %it, i64 0, i32 1, i64 %_10.i.i %7 = load i32, i32* %6, align 4, !alias.scope !26, !noalias !29 br label %_ZN4core4iter6traits8iterator8Iterator3nth17hcbc727011e9e2a3bE.exit _ZN4core4iter6traits8iterator8Iterator3nth17hcbc727011e9e2a3bE.exit: ; preds = %start, %bb3.i.i.i %.sroa.3.0.i = phi i32 [ undef, %start ], [ %7, %bb3.i.i.i ] %.sroa.0.0.i = phi i32 [ 0, %start ], [ 1, %bb3.i.i.i ] %8 = insertvalue { i32, i32 } undef, i32 %.sroa.0.0.i, 0 %9 = insertvalue { i32, i32 } %8, i32 %.sroa.3.0.i, 1 ret { i32, i32 } %9 } ```

Option<&foo> does not decay to *foo in extern C mode #3

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`Option<&foo>` does not decay to `*foo` in extern C mode #3