Ability to omit `: ()` on const item declarations #149738

GrigorenkoPV · 2025-12-07T18:17:17Z

Tracking issue: Tracking Issue for item-level const blocks #149226
Based on top of const blocks as a mod item #149174
- So the actual diff is GrigorenkoPV/rust@const_block_item...const_unit

As suggested in #149174 (comment), this PR adds an ability to omit : () from const item declarations. If the type is omitted, it defaults to (). This is similar to how function declarations treat -> ().

Implementation details:

The gate is #![feature(const_items_unit_type_default)] with the same tracking issue as for #![feature(const_block_items)]
ast::ConstItem's ty is changed from Box<ast::Ty> into ast::FnRetTy, which is like an Option<Box<Ty>> but with a Span for the None variant.
During parsing:
- static items with unspecified type still give an error, just as before
- const items with an unspecified type no longer give the previous error, but instead demand that you have the new feature enabled and fallback to : ().
During AST->HIR lowering:
- const items with no type specified in AST get a () in HIR
rustfmt support included

TODO:

Sadly, it seemes like it undoes the internal compiler error: missing type for const item #69396. Maybe instead of falling back to : (), fallback to : _ (as it was before) when the feature is not enabled?
fix the tests that started failing
make a separate tracking issue or update Tracking Issue for item-level const blocks #149226
is the feature gate name ok?
do not reuse FnRetTy or rename it?
do the "unspecified means ()" not during AST->HIR, but later on?
allow to omit : () for static items too? for consistency. despite static: () being kinda useless.
better diagnostic messages?

GrigorenkoPV · 2025-12-07T18:18:51Z

CC @joshtriplett as the lang champion

rust-log-analyzer · 2025-12-07T20:19:21Z

The job aarch64-gnu-llvm-20-1 failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

+ LL | const C2 = 42;
+    |         ^
+    |
+    = note: see issue #149226 <https://github.com/rust-lang/rust/issues/149226> for more information
+    = help: add `#![feature(const_items_unit_type_default)]` to the crate attributes to enable
+    = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+    = help: consider specifying the type explicitly
+ 
+ error[E0658]: omitting type on const item declaration is experimental
2   --> $DIR/const-no-type.rs:33:8
3    |
4 LL | const C = 42;

-    |        ^ help: provide a type for the constant: `: i32`
+    |        ^
+    |
+    = note: see issue #149226 <https://github.com/rust-lang/rust/issues/149226> for more information
+    = help: add `#![feature(const_items_unit_type_default)]` to the crate attributes to enable
+    = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+    = help: consider specifying the type explicitly
6 
- error: missing type for `const` item
+ error[E0658]: omitting type on const item declaration is experimental
8   --> $DIR/const-no-type.rs:38:8
9    |
10 LL | const D = &&42;

-    |        ^ help: provide a type for the constant: `: &&i32`
+    |        ^
+    |
+    = note: see issue #149226 <https://github.com/rust-lang/rust/issues/149226> for more information
+    = help: add `#![feature(const_items_unit_type_default)]` to the crate attributes to enable
+    = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+    = help: consider specifying the type explicitly
12 
13 error: missing type for `static` item
14   --> $DIR/const-no-type.rs:43:9

22 LL | static mut SM = "abc";
23    |              ^ help: provide a type for the static variable: `: &str`
24 
- error: missing type for `const` item
-   --> $DIR/const-no-type.rs:14:9
+ error[E0308]: mismatched types
+   --> $DIR/const-no-type.rs:33:11
---
+   --> $DIR/const-no-type.rs:38:11
30    |
- help: provide a type for the item
-    |
- LL | const C2: <type> = 42;
-    |         ++++++++
+ LL | const D = &&42;
+    |           ^^^^ expected `()`, found `&&{integer}`
35 
36 error: missing type for `static` item
37   --> $DIR/const-no-type.rs:20:10

55 LL | static mut SM2: <type> = "abc";
56    |               ++++++++
57 
- error: aborting due to 7 previous errors
+ error: aborting due to 9 previous errors
59 
---
+ LL | const C2 = 42;
+    |         ^
+    |
+    = note: see issue #149226 <https://github.com/rust-lang/rust/issues/149226> for more information
+    = help: add `#![feature(const_items_unit_type_default)]` to the crate attributes to enable
+    = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+    = help: consider specifying the type explicitly
+ 
+ error[E0658]: omitting type on const item declaration is experimental
+    |        ^
+    |
+    = note: see issue #149226 <https://github.com/rust-lang/rust/issues/149226> for more information
+    = help: add `#![feature(const_items_unit_type_default)]` to the crate attributes to enable
+    = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+    = help: consider specifying the type explicitly
+ error[E0658]: omitting type on const item declaration is experimental
+    |        ^
+    |
+    = note: see issue #149226 <https://github.com/rust-lang/rust/issues/149226> for more information
+    = help: add `#![feature(const_items_unit_type_default)]` to the crate attributes to enable
+    = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+    = help: consider specifying the type explicitly
+ error[E0308]: mismatched types
+   --> $DIR/const-no-type.rs:33:11
+ LL | const C = 42;
+    |           ^^ expected `()`, found integer
+ 
+ error[E0308]: mismatched types
+   --> $DIR/const-no-type.rs:38:11
+ LL | const D = &&42;
+    |           ^^^^ expected `()`, found `&&{integer}`
+ error: aborting due to 9 previous errors
+ Some errors have detailed explanations: E0308, E0658.
+ For more information about an error, try `rustc --explain E0308`.


The actual stderr differed from the expected stderr
To update references, rerun the tests and pass the `--bless` flag
To only update this specific test, also pass `--test-args suggestions/const-no-type.rs`

error: 1 errors occurred comparing output.
status: exit status: 1
command: env -u RUSTC_LOG_COLOR RUSTC_ICE="0" RUST_BACKTRACE="short" "/checkout/obj/build/aarch64-unknown-linux-gnu/stage2/bin/rustc" "/checkout/tests/ui/suggestions/const-no-type.rs" "-Zthreads=1" "-Zsimulate-remapped-rust-src-base=/rustc/FAKE_PREFIX" "-Ztranslate-remapped-path-to-local-path=no" "-Z" "ignore-directory-in-diagnostics-source-blocks=/cargo" "-Z" "ignore-directory-in-diagnostics-source-blocks=/checkout/vendor" "--sysroot" "/checkout/obj/build/aarch64-unknown-linux-gnu/stage2" "--target=aarch64-unknown-linux-gnu" "--check-cfg" "cfg(test,FALSE)" "--error-format" "json" "--json" "future-incompat" "-Ccodegen-units=1" "-Zui-testing" "-Zdeduplicate-diagnostics=no" "-Zwrite-long-types-to-disk=no" "-Cstrip=debuginfo" "--emit" "metadata" "-C" "prefer-dynamic" "--out-dir" "/checkout/obj/build/aarch64-unknown-linux-gnu/test/ui/suggestions/const-no-type" "-A" "unused" "-W" "unused_attributes" "-A" "internal_features" "-A" "unused_parens" "-A" "unused_braces" "-Crpath" "-Cdebuginfo=0" "-Lnative=/checkout/obj/build/aarch64-unknown-linux-gnu/native/rust-test-helpers"
stdout: none
--- stderr -------------------------------
error[E0658]: omitting type on const item declaration is experimental
##[error]  --> /checkout/tests/ui/suggestions/const-no-type.rs:14:9
   |
LL | const C2 = 42;
   |         ^
   |
   = note: see issue #149226 <https://github.com/rust-lang/rust/issues/149226> for more information
   = help: add `#![feature(const_items_unit_type_default)]` to the crate attributes to enable
   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
   = help: consider specifying the type explicitly

error[E0658]: omitting type on const item declaration is experimental
##[error]  --> /checkout/tests/ui/suggestions/const-no-type.rs:33:8
   |
LL | const C = 42;
   |        ^
   |
   = note: see issue #149226 <https://github.com/rust-lang/rust/issues/149226> for more information
   = help: add `#![feature(const_items_unit_type_default)]` to the crate attributes to enable
   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
   = help: consider specifying the type explicitly

error[E0658]: omitting type on const item declaration is experimental
##[error]  --> /checkout/tests/ui/suggestions/const-no-type.rs:38:8
   |
LL | const D = &&42;
   |        ^
   |
   = note: see issue #149226 <https://github.com/rust-lang/rust/issues/149226> for more information
   = help: add `#![feature(const_items_unit_type_default)]` to the crate attributes to enable
   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
   = help: consider specifying the type explicitly

error: missing type for `static` item
##[error]  --> /checkout/tests/ui/suggestions/const-no-type.rs:43:9
   |
LL | static S = Vec::<String>::new();
   |         ^ help: provide a type for the static variable: `: Vec<String>`

error: missing type for `static mut` item
##[error]  --> /checkout/tests/ui/suggestions/const-no-type.rs:48:14
   |
LL | static mut SM = "abc";
   |              ^ help: provide a type for the static variable: `: &str`

error[E0308]: mismatched types
##[error]  --> /checkout/tests/ui/suggestions/const-no-type.rs:33:11
   |
LL | const C = 42;
   |           ^^ expected `()`, found integer

error[E0308]: mismatched types
##[error]  --> /checkout/tests/ui/suggestions/const-no-type.rs:38:11
   |
LL | const D = &&42;
   |           ^^^^ expected `()`, found `&&{integer}`

error: missing type for `static` item
##[error]  --> /checkout/tests/ui/suggestions/const-no-type.rs:20:10
   |
LL | static S2 = "abc";
   |          ^
   |
help: provide a type for the item
   |
LL | static S2: <type> = "abc";
   |          ++++++++

error: missing type for `static mut` item
##[error]  --> /checkout/tests/ui/suggestions/const-no-type.rs:26:15
   |
LL | static mut SM2 = "abc";
   |               ^
   |
help: provide a type for the item
   |
LL | static mut SM2: <type> = "abc";
   |               ++++++++

error: aborting due to 9 previous errors

Some errors have detailed explanations: E0308, E0658.
---
---- [ui] tests/ui/suggestions/unnamable-types.rs stdout ----

error: Error: expected failure status (Some(1)) but received status Some(101).
status: exit status: 101
command: env -u RUSTC_LOG_COLOR RUSTC_ICE="0" RUST_BACKTRACE="short" "/checkout/obj/build/aarch64-unknown-linux-gnu/stage2/bin/rustc" "/checkout/tests/ui/suggestions/unnamable-types.rs" "-Zthreads=1" "-Zsimulate-remapped-rust-src-base=/rustc/FAKE_PREFIX" "-Ztranslate-remapped-path-to-local-path=no" "-Z" "ignore-directory-in-diagnostics-source-blocks=/cargo" "-Z" "ignore-directory-in-diagnostics-source-blocks=/checkout/vendor" "--sysroot" "/checkout/obj/build/aarch64-unknown-linux-gnu/stage2" "--target=aarch64-unknown-linux-gnu" "--check-cfg" "cfg(test,FALSE)" "--error-format" "json" "--json" "future-incompat" "-Ccodegen-units=1" "-Zui-testing" "-Zdeduplicate-diagnostics=no" "-Zwrite-long-types-to-disk=no" "-Cstrip=debuginfo" "--emit" "metadata" "-C" "prefer-dynamic" "--out-dir" "/checkout/obj/build/aarch64-unknown-linux-gnu/test/ui/suggestions/unnamable-types" "-A" "unused" "-W" "unused_attributes" "-A" "internal_features" "-A" "unused_parens" "-A" "unused_braces" "-Crpath" "-Cdebuginfo=0" "-Lnative=/checkout/obj/build/aarch64-unknown-linux-gnu/native/rust-test-helpers"
stdout: none
--- stderr -------------------------------
error[E0658]: omitting type on const item declaration is experimental
##[error]  --> /checkout/tests/ui/suggestions/unnamable-types.rs:6:8
   |
LL | const A = 5;
   |        ^
   |
   = note: see issue #149226 <https://github.com/rust-lang/rust/issues/149226> for more information
   = help: add `#![feature(const_items_unit_type_default)]` to the crate attributes to enable
   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
   = help: consider specifying the type explicitly

error[E0658]: omitting type on const item declaration is experimental
##[error]  --> /checkout/tests/ui/suggestions/unnamable-types.rs:23:8
   |
LL | const D = S { t: { let i = 0; move || -> i32 { i } } };
   |        ^
   |
   = note: see issue #149226 <https://github.com/rust-lang/rust/issues/149226> for more information
   = help: add `#![feature(const_items_unit_type_default)]` to the crate attributes to enable
   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
   = help: consider specifying the type explicitly

error[E0658]: omitting type on const item declaration is experimental
##[error]  --> /checkout/tests/ui/suggestions/unnamable-types.rs:29:8
   |
LL | const E = foo;
   |        ^
   |
   = note: see issue #149226 <https://github.com/rust-lang/rust/issues/149226> for more information
   = help: add `#![feature(const_items_unit_type_default)]` to the crate attributes to enable
   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
   = help: consider specifying the type explicitly

error[E0658]: omitting type on const item declaration is experimental
##[error]  --> /checkout/tests/ui/suggestions/unnamable-types.rs:32:8
   |
LL | const F = S { t: foo };
   |        ^
   |
   = note: see issue #149226 <https://github.com/rust-lang/rust/issues/149226> for more information
   = help: add `#![feature(const_items_unit_type_default)]` to the crate attributes to enable
   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
   = help: consider specifying the type explicitly

error[E0658]: omitting type on const item declaration is experimental
##[error]  --> /checkout/tests/ui/suggestions/unnamable-types.rs:37:8
   |
LL | const G = #[coroutine] || -> i32 { yield 0; return 1; };
   |        ^
   |
   = note: see issue #149226 <https://github.com/rust-lang/rust/issues/149226> for more information
   = help: add `#![feature(const_items_unit_type_default)]` to the crate attributes to enable
   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
   = help: consider specifying the type explicitly

error[E0121]: the placeholder `_` is not allowed within types on item signatures for static variables
##[error]  --> /checkout/tests/ui/suggestions/unnamable-types.rs:10:11
   |
LL | static B: _ = "abc";
   |           ^ not allowed in type signatures
   |
help: replace this with a fully-specified type
   |
LL - static B: _ = "abc";
LL + static B: &str = "abc";
   |

error[E0121]: the placeholder `_` is not allowed within types on item signatures for constants
##[error]  --> /checkout/tests/ui/suggestions/unnamable-types.rs:17:10
   |
LL | const C: _ = || 42;
   |          ^ not allowed in type signatures
   |
note: however, the inferred type `{[email protected]:17:14}` cannot be named
  --> /checkout/tests/ui/suggestions/unnamable-types.rs:17:14
   |
LL | const C: _ = || 42;
   |              ^^^^^

error[E0308]: mismatched types
##[error]  --> /checkout/tests/ui/suggestions/unnamable-types.rs:6:11
   |
LL | const A = 5;
   |           ^ expected `()`, found integer

error[E0308]: mismatched types
##[error]  --> /checkout/tests/ui/suggestions/unnamable-types.rs:23:11
   |
LL | const D = S { t: { let i = 0; move || -> i32 { i } } };
   |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `()`, found `S<{[email protected]:23:31}>`
   |
   = note: expected unit type `()`
                 found struct `S<{closure@/checkout/tests/ui/suggestions/unnamable-types.rs:23:31: 23:45}>`

error[E0308]: mismatched types
##[error]  --> /checkout/tests/ui/suggestions/unnamable-types.rs:29:11
   |
LL | const E = foo;
   |           ^^^ expected `()`, found fn item
   |
   = note: expected unit type `()`
                found fn item `fn() -> i32 {foo}`

error[E0308]: mismatched types
##[error]  --> /checkout/tests/ui/suggestions/unnamable-types.rs:32:11
   |
LL | const F = S { t: foo };
   |           ^^^^^^^^^^^^ expected `()`, found `S<fn() -> i32 {foo}>`
   |
   = note: expected unit type `()`
                 found struct `S<fn() -> i32 {foo}>`


thread 'rustc' (156713) panicked at /checkout/compiler/rustc_hir/src/hir.rs:1353:18:
can't get the span of an arbitrary parsed attribute: Parsed(Coroutine(/checkout/tests/ui/suggestions/unnamable-types.rs:37:11: 37:23 (#0)))
stack backtrace:
   0: __rustc::rust_begin_unwind
   1: core::panicking::panic_fmt
   2: <rustc_hir_typeck::fn_ctxt::FnCtxt>::precedence::{closure#0}
   3: <rustc_hir::hir::Expr>::precedence
   4: <rustc_hir_typeck::fn_ctxt::FnCtxt>::suggest_deref_ref_or_into
   5: <rustc_hir_typeck::fn_ctxt::FnCtxt>::emit_type_mismatch_suggestions
   6: <rustc_hir_typeck::fn_ctxt::FnCtxt>::emit_coerce_suggestions
   7: <rustc_hir_typeck::fn_ctxt::FnCtxt>::demand_coerce_diag
   8: <rustc_hir_typeck::fn_ctxt::FnCtxt>::check_expr_coercible_to_type
   9: rustc_hir_typeck::typeck_with_inspect
      [... omitted 3 frames ...]
  10: rustc_mir_build::thir::pattern::check_match::check_match
      [... omitted 3 frames ...]
  11: rustc_middle::query::inner::query_ensure_error_guaranteed::<rustc_data_structures::vec_cache::VecCache<rustc_span::def_id::LocalDefId, rustc_middle::query::erase::Erased<[u8; 1]>, rustc_query_system::dep_graph::graph::DepNodeIndex>, ()>
  12: rustc_mir_build::builder::build_mir
  13: rustc_mir_transform::mir_built
      [... omitted 3 frames ...]
  14: rustc_mir_transform::trivial_const::trivial_const_provider
      [... omitted 3 frames ...]
  15: rustc_const_eval::const_eval::eval_queries::eval_to_const_value_raw_provider
      [... omitted 3 frames ...]
  16: <rustc_middle::ty::context::TyCtxt>::par_hir_body_owners::<rustc_hir_analysis::check_crate::{closure#2}>::{closure#0}
  17: rustc_data_structures::sync::parallel::par_for_each_in::<&rustc_span::def_id::LocalDefId, &[rustc_span::def_id::LocalDefId], <rustc_middle::ty::context::TyCtxt>::par_hir_body_owners<rustc_hir_analysis::check_crate::{closure#2}>::{closure#0}>
  18: rustc_hir_analysis::check_crate
  19: rustc_interface::passes::analysis
      [... omitted 3 frames ...]
  20: <std::thread::local::LocalKey<core::cell::Cell<*const ()>>>::with::<rustc_middle::ty::context::tls::enter_context<<rustc_middle::ty::context::GlobalCtxt>::enter<rustc_interface::passes::create_and_enter_global_ctxt<core::option::Option<rustc_interface::queries::Linker>, rustc_driver_impl::run_compiler::{closure#0}::{closure#2}>::{closure#2}::{closure#0}, core::option::Option<rustc_interface::queries::Linker>>::{closure#1}, core::option::Option<rustc_interface::queries::Linker>>::{closure#0}, core::option::Option<rustc_interface::queries::Linker>>
  21: <rustc_middle::ty::context::TyCtxt>::create_global_ctxt::<core::option::Option<rustc_interface::queries::Linker>, rustc_interface::passes::create_and_enter_global_ctxt<core::option::Option<rustc_interface::queries::Linker>, rustc_driver_impl::run_compiler::{closure#0}::{closure#2}>::{closure#2}::{closure#0}>
  22: <rustc_interface::passes::create_and_enter_global_ctxt<core::option::Option<rustc_interface::queries::Linker>, rustc_driver_impl::run_compiler::{closure#0}::{closure#2}>::{closure#2} as core::ops::function::FnOnce<(&rustc_session::session::Session, rustc_middle::ty::context::CurrentGcx, alloc::sync::Arc<rustc_data_structures::jobserver::Proxy>, &std::sync::once_lock::OnceLock<rustc_middle::ty::context::GlobalCtxt>, &rustc_data_structures::sync::worker_local::WorkerLocal<rustc_middle::arena::Arena>, &rustc_data_structures::sync::worker_local::WorkerLocal<rustc_hir::Arena>, rustc_driver_impl::run_compiler::{closure#0}::{closure#2})>>::call_once::{shim:vtable#0}
  23: <alloc::boxed::Box<dyn for<'a> core::ops::function::FnOnce<(&'a rustc_session::session::Session, rustc_middle::ty::context::CurrentGcx, alloc::sync::Arc<rustc_data_structures::jobserver::Proxy>, &'a std::sync::once_lock::OnceLock<rustc_middle::ty::context::GlobalCtxt<'a>>, &'a rustc_data_structures::sync::worker_local::WorkerLocal<rustc_middle::arena::Arena<'a>>, &'a rustc_data_structures::sync::worker_local::WorkerLocal<rustc_hir::Arena<'a>>, rustc_driver_impl::run_compiler::{closure#0}::{closure#2}), Output = core::option::Option<rustc_interface::queries::Linker>>> as core::ops::function::FnOnce<(&rustc_session::session::Session, rustc_middle::ty::context::CurrentGcx, alloc::sync::Arc<rustc_data_structures::jobserver::Proxy>, &std::sync::once_lock::OnceLock<rustc_middle::ty::context::GlobalCtxt>, &rustc_data_structures::sync::worker_local::WorkerLocal<rustc_middle::arena::Arena>, &rustc_data_structures::sync::worker_local::WorkerLocal<rustc_hir::Arena>, rustc_driver_impl::run_compiler::{closure#0}::{closure#2})>>::call_once
  24: rustc_interface::passes::create_and_enter_global_ctxt::<core::option::Option<rustc_interface::queries::Linker>, rustc_driver_impl::run_compiler::{closure#0}::{closure#2}>
  25: <scoped_tls::ScopedKey<rustc_span::SessionGlobals>>::set::<rustc_interface::util::run_in_thread_with_globals<rustc_interface::util::run_in_thread_pool_with_globals<rustc_interface::interface::run_compiler<(), rustc_driver_impl::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}::{closure#0}, ()>
  26: rustc_span::create_session_globals_then::<(), rustc_interface::util::run_in_thread_with_globals<rustc_interface::util::run_in_thread_pool_with_globals<rustc_interface::interface::run_compiler<(), rustc_driver_impl::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}::{closure#0}>
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

---
note: please make sure that you have updated to the latest nightly

note: rustc 1.94.0-nightly (b087a12cf 2025-12-07) running on aarch64-unknown-linux-gnu

note: compiler flags: -Z threads=1 -Z simulate-remapped-rust-src-base=/rustc/FAKE_PREFIX -Z translate-remapped-path-to-local-path=no -Z ignore-directory-in-diagnostics-source-blocks=/cargo -Z ignore-directory-in-diagnostics-source-blocks=/checkout/vendor -C codegen-units=1 -Z ui-testing -Z deduplicate-diagnostics=no -Z write-long-types-to-disk=no -C strip=debuginfo -C prefer-dynamic -C rpath -C debuginfo=0

query stack during panic:
#0 [typeck] type-checking `G`
#1 [check_match] match-checking `G`
#2 [mir_built] building MIR for `G`
#3 [trivial_const] checking if `G` is a trivial const
#4 [eval_to_const_value_raw] simplifying constant for the type system `G`
#5 [analysis] running analysis passes on crate `unnamable_types`
end of query stack
error: aborting due to 11 previous errors

Some errors have detailed explanations: E0121, E0308, E0658.
For more information about an error, try `rustc --explain E0121`.
---
- error: missing type for `const` item
+ error[E0658]: omitting type on const item declaration is experimental
2   --> $DIR/issue-100164.rs:3:10
3    |
4 LL | const _A: = 123;

-    |          ^ help: provide a type for the constant: `i32`
+    |          ^
+    |
+    = note: see issue #149226 <https://github.com/rust-lang/rust/issues/149226> for more information
+    = help: add `#![feature(const_items_unit_type_default)]` to the crate attributes to enable
+    = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+    = help: consider specifying the type explicitly
6 
- error: missing type for `const` item
+ error[E0658]: omitting type on const item declaration is experimental
8   --> $DIR/issue-100164.rs:7:14
9    |
10 LL |     const _B: = 123;

-    |              ^ help: provide a type for the constant: `i32`
+    |              ^
+    |
+    = note: see issue #149226 <https://github.com/rust-lang/rust/issues/149226> for more information
+    = help: add `#![feature(const_items_unit_type_default)]` to the crate attributes to enable
+    = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+    = help: consider specifying the type explicitly
12 
- error: aborting due to 2 previous errors
+ error[E0308]: mismatched types
+   --> $DIR/issue-100164.rs:3:13
+    |
+ LL | const _A: = 123;
+    |             ^^^ expected `()`, found integer
14 
+ error[E0308]: mismatched types
+   --> $DIR/issue-100164.rs:7:17
+    |
+ LL |     const _B: = 123;
+    |                 ^^^ expected `()`, found integer
+ 
+ error: aborting due to 4 previous errors
+ 
+ Some errors have detailed explanations: E0308, E0658.
---
+ error[E0658]: omitting type on const item declaration is experimental
+    |          ^
+    |
+    = note: see issue #149226 <https://github.com/rust-lang/rust/issues/149226> for more information
+    = help: add `#![feature(const_items_unit_type_default)]` to the crate attributes to enable
+    = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+    = help: consider specifying the type explicitly
+ error[E0658]: omitting type on const item declaration is experimental
+    |              ^
+    |
+    = note: see issue #149226 <https://github.com/rust-lang/rust/issues/149226> for more information
+    = help: add `#![feature(const_items_unit_type_default)]` to the crate attributes to enable
+    = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+    = help: consider specifying the type explicitly
+ error[E0308]: mismatched types
+   --> $DIR/issue-100164.rs:3:13
+    |
+ LL | const _A: = 123;
+    |             ^^^ expected `()`, found integer
+ error[E0308]: mismatched types
+   --> $DIR/issue-100164.rs:7:17
+    |
+ LL |     const _B: = 123;
+    |                 ^^^ expected `()`, found integer
+ 
+ error: aborting due to 4 previous errors
+ 
+ Some errors have detailed explanations: E0308, E0658.
---
diff of fixed:

1 //@ run-rustfix
2 
- const _A: i32 = 123;
+ const _A: = 123;
4 //~^ ERROR: missing type for `const` item
5 
6 fn main() {

-     const _B: i32 = 123;
+     const _B: = 123;
8     //~^ ERROR: missing type for `const` item
9 }
10 


The actual fixed differed from the expected fixed
To update references, rerun the tests and pass the `--bless` flag
To only update this specific test, also pass `--test-args typeck/issue-100164.rs`

error: 2 errors occurred comparing output.
status: exit status: 1
command: env -u RUSTC_LOG_COLOR RUSTC_ICE="0" RUST_BACKTRACE="short" "/checkout/obj/build/aarch64-unknown-linux-gnu/stage2/bin/rustc" "/checkout/tests/ui/typeck/issue-100164.rs" "-Zthreads=1" "-Zsimulate-remapped-rust-src-base=/rustc/FAKE_PREFIX" "-Ztranslate-remapped-path-to-local-path=no" "-Z" "ignore-directory-in-diagnostics-source-blocks=/cargo" "-Z" "ignore-directory-in-diagnostics-source-blocks=/checkout/vendor" "--sysroot" "/checkout/obj/build/aarch64-unknown-linux-gnu/stage2" "--target=aarch64-unknown-linux-gnu" "--check-cfg" "cfg(test,FALSE)" "--error-format" "json" "--json" "future-incompat" "-Ccodegen-units=1" "-Zui-testing" "-Zdeduplicate-diagnostics=no" "-Zwrite-long-types-to-disk=no" "-Cstrip=debuginfo" "--emit" "metadata" "-C" "prefer-dynamic" "--out-dir" "/checkout/obj/build/aarch64-unknown-linux-gnu/test/ui/typeck/issue-100164" "-A" "unused" "-W" "unused_attributes" "-A" "internal_features" "-A" "unused_parens" "-A" "unused_braces" "-Crpath" "-Cdebuginfo=0" "-Lnative=/checkout/obj/build/aarch64-unknown-linux-gnu/native/rust-test-helpers"
stdout: none
--- stderr -------------------------------
error[E0658]: omitting type on const item declaration is experimental
##[error]  --> /checkout/tests/ui/typeck/issue-100164.rs:3:10
   |
LL | const _A: = 123;
   |          ^
   |
   = note: see issue #149226 <https://github.com/rust-lang/rust/issues/149226> for more information
   = help: add `#![feature(const_items_unit_type_default)]` to the crate attributes to enable
   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
   = help: consider specifying the type explicitly

error[E0658]: omitting type on const item declaration is experimental
##[error]  --> /checkout/tests/ui/typeck/issue-100164.rs:7:14
   |
LL |     const _B: = 123;
   |              ^
   |
   = note: see issue #149226 <https://github.com/rust-lang/rust/issues/149226> for more information
   = help: add `#![feature(const_items_unit_type_default)]` to the crate attributes to enable
   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
   = help: consider specifying the type explicitly

error[E0308]: mismatched types
##[error]  --> /checkout/tests/ui/typeck/issue-100164.rs:3:13
   |
LL | const _A: = 123;
   |             ^^^ expected `()`, found integer

error[E0308]: mismatched types
##[error]  --> /checkout/tests/ui/typeck/issue-100164.rs:7:17
   |
LL |     const _B: = 123;
   |                 ^^^ expected `()`, found integer

error: aborting due to 4 previous errors

Some errors have detailed explanations: E0308, E0658.

bors · 2025-12-12T11:10:35Z

☔ The latest upstream changes (presumably #149891) made this pull request unmergeable. Please resolve the merge conflicts.

rust-bors · 2026-01-02T16:32:36Z

☔ The latest upstream changes made this pull request unmergeable. Please resolve the merge conflicts.

GrigorenkoPV added 5 commits December 6, 2025 22:19

Sugar for const _: () =

31cf103

separate ast item for const block items

8971bb3

Audit AllowConstBlockItems

635f0bc

rustfmt: support const block items

cffeea9

const_block_items: tests

16b9a8a

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const_items_unit_type_default: initial implementation

83dbb0c

rustbot removed the T-rustdoc Relevant to the rustdoc team, which will review and decide on the PR/issue. label Dec 7, 2025

GrigorenkoPV added 4 commits December 7, 2025 22:20

const_items_unit_type_default: add tests

d1e4d9a

const_items_unit_type_default: bless tests

1b220ea

const_items_unit_type_default: rustfmt support

e9badd6

const_items_unit_type_default: unbreak clippy

3ecc105

GrigorenkoPV force-pushed the const_unit branch from 939012d to 3ecc105 Compare December 7, 2025 19:34

rustbot added the T-clippy Relevant to the Clippy team. label Dec 7, 2025

GrigorenkoPV mentioned this pull request Dec 8, 2025

Improve diagnostics for const a: = expr; #100168

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Ability to omit `: ()` on const item declarations #149738

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Ability to omit `: ()` on const item declarations #149738

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