ICE when using a const fn with an if statement as an array length

[Soveu]

Code

pub const fn is_zst<T: ?Sized>() -> usize {
    if std::mem::size_of::<T>() == 0 {
        1
    } else {
        0
    }
}

pub struct AtLeastByte<T: ?Sized> {
    value: T,
    pad: [u8; is_zst::<T>()],
}

Meta

rustc --version --verbose:

rustc 1.47.0-nightly (6c8927b0c 2020-07-26)
binary: rustc
commit-hash: 6c8927b0cf80ceee19386026cf9d7fd4fd9d486f
commit-date: 2020-07-26
host: x86_64-unknown-linux-gnu
release: 1.47.0-nightly
LLVM version: 10.0

The bug also does occur in beta version.
Stable versions do not support if statements inside const fn

Error output

error: internal compiler error: src/librustc_middle/ty/subst.rs:554:17: type parameter `T/#0` (T/0) out of range when substituting (root type=Some(fn() -> usize {is_zst::<T>})) substs=[]

thread 'rustc' panicked at 'Box<Any>', /rustc/6c8927b0cf80ceee19386026cf9d7fd4fd9d486f/src/libstd/macros.rs:13:23
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

note: the compiler unexpectedly panicked. this is a bug.

note: we would appreciate a bug report: https://github.com/rust-lang/rust/issues/new?labels=C-bug%2C+I-ICE%2C+T-compiler&template=ice.md

note: rustc 1.47.0-nightly (6c8927b0c 2020-07-26) running on x86_64-unknown-linux-gnu

error: aborting due to previous error

Backtrace

error: internal compiler error: src/librustc_middle/ty/subst.rs:554:17: type parameter `T/#0` (T/0) out of range when substituting (root type=Some(fn() -> usize {is_zst::<T>})) substs=[]

thread 'rustc' panicked at 'Box<Any>', /rustc/6c8927b0cf80ceee19386026cf9d7fd4fd9d486f/src/libstd/macros.rs:13:23
stack backtrace:
   0: backtrace::backtrace::libunwind::trace
             at /cargo/registry/src/git.luolix.top-1ecc6299db9ec823/backtrace-0.3.46/src/backtrace/libunwind.rs:86
   1: backtrace::backtrace::trace_unsynchronized
             at /cargo/registry/src/git.luolix.top-1ecc6299db9ec823/backtrace-0.3.46/src/backtrace/mod.rs:66
   2: std::sys_common::backtrace::_print_fmt
             at src/libstd/sys_common/backtrace.rs:78
   3: <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt
             at src/libstd/sys_common/backtrace.rs:59
   4: core::fmt::write
             at src/libcore/fmt/mod.rs:1117
   5: std::io::Write::write_fmt
             at src/libstd/io/mod.rs:1508
   6: std::sys_common::backtrace::_print
             at src/libstd/sys_common/backtrace.rs:62
   7: std::sys_common::backtrace::print
             at src/libstd/sys_common/backtrace.rs:49
   8: std::panicking::default_hook::{{closure}}
             at src/libstd/panicking.rs:198
   9: std::panicking::default_hook
             at src/libstd/panicking.rs:217
  10: rustc_driver::report_ice
  11: std::panicking::rust_panic_with_hook
             at src/libstd/panicking.rs:530
  12: std::panicking::begin_panic
  13: rustc_errors::HandlerInner::span_bug
  14: rustc_errors::Handler::span_bug
  15: rustc_middle::util::bug::opt_span_bug_fmt::{{closure}}
  16: rustc_middle::ty::context::tls::with_opt::{{closure}}
  17: rustc_middle::ty::context::tls::with_opt
  18: rustc_middle::util::bug::opt_span_bug_fmt
  19: rustc_middle::util::bug::span_bug_fmt
  20: <rustc_middle::ty::subst::SubstFolder as rustc_middle::ty::fold::TypeFolder>::fold_ty
  21: rustc_middle::ty::fold::TypeFoldable::fold_with
  22: rustc_middle::ty::structural_impls::<impl rustc_middle::ty::fold::TypeFoldable for &rustc_middle::ty::TyS>::super_fold_with
  23: <rustc_middle::ty::subst::SubstFolder as rustc_middle::ty::fold::TypeFolder>::fold_ty
  24: <rustc_middle::ty::subst::SubstFolder as rustc_middle::ty::fold::TypeFolder>::fold_const
  25: rustc_middle::ty::normalize_erasing_regions::<impl rustc_middle::ty::context::TyCtxt>::subst_and_normalize_erasing_regions
  26: rustc_mir::interpret::operand::<impl rustc_mir::interpret::eval_context::InterpCx<M>>::eval_operand
  27: rustc_mir::interpret::step::<impl rustc_mir::interpret::eval_context::InterpCx<M>>::run
  28: rustc_mir::const_eval::eval_queries::const_eval_raw_provider
  29: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::const_eval_raw>::compute
  30: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  31: rustc_data_structures::stack::ensure_sufficient_stack
  32: rustc_query_system::query::plumbing::get_query_impl
  33: rustc_mir::const_eval::eval_queries::const_eval_validated_provider
  34: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::const_eval_validated>::compute
  35: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  36: rustc_data_structures::stack::ensure_sufficient_stack
  37: rustc_query_system::query::plumbing::get_query_impl
  38: rustc_mir::const_eval::eval_queries::const_eval_validated_provider
  39: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::const_eval_validated>::compute
  40: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  41: rustc_data_structures::stack::ensure_sufficient_stack
  42: rustc_query_system::query::plumbing::get_query_impl
  43: rustc_middle::mir::interpret::queries::<impl rustc_middle::ty::context::TyCtxt>::const_eval_global_id
  44: rustc_middle::mir::interpret::queries::<impl rustc_middle::ty::context::TyCtxt>::const_eval_resolve
  45: <rustc_trait_selection::traits::project::AssocTypeNormalizer as rustc_middle::ty::fold::TypeFolder>::fold_const
  46: rustc_middle::ty::structural_impls::<impl rustc_middle::ty::fold::TypeFoldable for &rustc_middle::ty::TyS>::super_fold_with
  47: <rustc_trait_selection::traits::project::AssocTypeNormalizer as rustc_middle::ty::fold::TypeFolder>::fold_ty
  48: rustc_data_structures::stack::ensure_sufficient_stack
  49: rustc_trait_selection::traits::project::normalize
  50: <rustc_infer::infer::InferCtxt as rustc_trait_selection::infer::InferCtxtExt>::partially_normalize_associated_types_in
  51: rustc_typeck::check::FnCtxt::normalize_associated_types_in
  52: rustc_typeck::check::wfcheck::<impl rustc_typeck::check::FnCtxt>::non_enum_variant::{{closure}}
  53: <core::iter::adapters::Map<I,F> as core::iter::traits::iterator::Iterator>::fold
  54: rustc_infer::infer::InferCtxtBuilder::enter
  55: rustc_typeck::check::wfcheck::check_item_well_formed
  56: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::check_item_well_formed>::compute
  57: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  58: rustc_data_structures::stack::ensure_sufficient_stack
  59: rustc_query_system::query::plumbing::get_query_impl
  60: rustc_query_system::query::plumbing::ensure_query_impl
  61: <std::panic::AssertUnwindSafe<F> as core::ops::function::FnOnce<()>>::call_once
  62: <std::panic::AssertUnwindSafe<F> as core::ops::function::FnOnce<()>>::call_once
  63: rustc_hir::hir::Crate::par_visit_all_item_likes
  64: rustc_session::session::Session::track_errors
  65: rustc_typeck::check_crate
  66: rustc_interface::passes::analysis
  67: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::analysis>::compute
  68: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  69: rustc_data_structures::stack::ensure_sufficient_stack
  70: rustc_query_system::query::plumbing::get_query_impl
  71: rustc_middle::ty::context::tls::enter_global
  72: rustc_interface::queries::<impl rustc_interface::interface::Compiler>::enter
  73: rustc_span::with_source_map
  74: rustc_interface::interface::create_compiler_and_run
  75: scoped_tls::ScopedKey<T>::set
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

note: the compiler unexpectedly panicked. this is a bug.

note: we would appreciate a bug report: https://github.com/rust-lang/rust/issues/new?labels=C-bug%2C+I-ICE%2C+T-compiler&template=ice.md

note: rustc 1.47.0-nightly (6c8927b0c 2020-07-26) running on x86_64-unknown-linux-gnu

query stack during panic:
#0 [const_eval_raw] const-evaluating `AtLeastByte::pad::{{constant}}#0`
#1 [const_eval_validated] const-evaluating + checking `AtLeastByte::pad::{{constant}}#0`
#2 [const_eval_validated] const-evaluating + checking `AtLeastByte::pad::{{constant}}#0`
#3 [check_item_well_formed] checking that `AtLeastByte` is well-formed
#4 [analysis] running analysis passes on this crate
end of query stack
error: aborting due to previous error

[lcnr]

This is a lot more related to const generics, see #71922, #67945 and #56445

[varkor]

This errors correctly with min_const_generics, so hopefully this will be addressed by stabilisation.

[JohnTitor]

Triage:

The current output with the latest nightly

error: generic parameters may not be used in const operations
  --> /home/runner/work/glacier/glacier/ices/75153.rs:11:24
   |
11 |     pad: [u8; is_zst::<T>()],
   |                        ^ cannot perform const operation using `T`
   |
   = note: type parameters may not be used in const expressions
   = help: use `#![feature(const_generics)]` and `#![feature(const_evaluatable_checked)]` to allow generic const expressions

error[E0277]: the size for values of type `T` cannot be known at compilation time
  --> /home/runner/work/glacier/glacier/ices/75153.rs:10:12
   |
9  | pub struct AtLeastByte<T: ?Sized> {
   |                        - this type parameter needs to be `Sized`
10 |     value: T,
   |            ^ doesn't have a size known at compile-time
   |
   = note: only the last field of a struct may have a dynamically sized type
   = help: change the field's type to have a statically known size
help: borrowed types always have a statically known size
   |
10 |     value: &T,
   |            ^
help: the `Box` type always has a statically known size and allocates its contents in the heap
   |
10 |     value: Box<T>,
   |            ^^^^ ^

error: aborting due to 2 previous errors

For more information about this error, try `rustc --explain E0277`.

with #![feature(const_generics)]

error[E0277]: the size for values of type `T` cannot be known at compilation time
   --> src/main.rs:4:28
    |
3   | pub const fn is_zst<T: ?Sized>() -> usize {
    |                     - this type parameter needs to be `Sized`
4   |     if std::mem::size_of::<T>() == 0 {
    |                            ^ doesn't have a size known at compile-time

error[E0277]: the size for values of type `T` cannot be known at compilation time
  --> src/main.rs:12:12
   |
11 | pub struct AtLeastByte<T: ?Sized> {
   |                        - this type parameter needs to be `Sized`
12 |     value: T,
   |            ^ doesn't have a size known at compile-time
   |
   = note: only the last field of a struct may have a dynamically sized type
   = help: change the field's type to have a statically known size
help: borrowed types always have a statically known size
   |
12 |     value: &T,
   |            ^
help: the `Box` type always has a statically known size and allocates its contents in the heap
   |
12 |     value: Box<T>,
   |            ^^^^ ^

error: aborting due to 2 previous errors; 1 warning emitted

Neither is ICE.

[lcnr]

will require feature(const_evaluatable_checked) to work, closing without adding a test as we already have some similar ones

[estebank]

For future reference, you don't need/want T: ?Sized.