[crater only] complain about uninhabited types being ignored in repr(transparent) #147589

RalfJung · 2025-10-11T12:59:41Z

RalfJung · 2025-10-11T12:59:57Z

[crater only] complain about uninhabited types being ignored in repr(transparent)

rust-log-analyzer · 2025-10-11T13:36:42Z

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)

---- [ui] tests/ui/lint/improper-ctypes/lint-ctypes.rs stdout ----
Saved the actual stderr to `/checkout/obj/build/aarch64-unknown-linux-gnu/test/ui/lint/improper-ctypes/lint-ctypes/lint-ctypes.stderr`
diff of stderr:

+ warning: zero-sized fields in `repr(transparent)` cannot contain external non-exhaustive types
+   --> $DIR/lint-ctypes.rs:41:38
+    |
+ LL | pub struct TransparentCustomZst(i32, ZeroSize);
+    |                                      ^^^^^^^^
+    |
+    = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+    = note: for more information, see issue #78586 <https://github.com/rust-lang/rust/issues/78586>
+    = note: this field contains `ZeroSize`, which is marked with `#[repr(C)]`, and makes it not a breaking change to become non-zero-sized in the future.
+    = note: `#[warn(repr_transparent_external_private_fields)]` (part of `#[warn(future_incompatible)]`) on by default
+ 
1 error: `extern` block uses type `Foo`, which is not FFI-safe
2   --> $DIR/lint-ctypes.rs:47:28
3    |

209    = help: consider adding a `#[repr(C)]`, `#[repr(transparent)]`, or integer `#[repr(...)]` attribute to this enum
210    = note: enum has no representation hint
211 
- error: aborting due to 21 previous errors
+ error: aborting due to 21 previous errors; 1 warning emitted
213 
214 

Note: some mismatched output was normalized before being compared
-   --> /checkout/tests/ui/lint/improper-ctypes/lint-ctypes.rs:41:38
+ warning: zero-sized fields in `repr(transparent)` cannot contain external non-exhaustive types
+   --> $DIR/lint-ctypes.rs:41:38
+    |
+ LL | pub struct TransparentCustomZst(i32, ZeroSize);
+    |                                      ^^^^^^^^
+    |
+    = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+    = note: for more information, see issue #78586 <https://github.com/rust-lang/rust/issues/78586>
+    = note: this field contains `ZeroSize`, which is marked with `#[repr(C)]`, and makes it not a breaking change to become non-zero-sized in the future.
---
To only update this specific test, also pass `--test-args lint/improper-ctypes/lint-ctypes.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/lint/improper-ctypes/lint-ctypes.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/lint/improper-ctypes/lint-ctypes" "-A" "unused" "-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 -------------------------------
warning: zero-sized fields in `repr(transparent)` cannot contain external non-exhaustive types
##[warning]  --> /checkout/tests/ui/lint/improper-ctypes/lint-ctypes.rs:41:38
   |
LL | pub struct TransparentCustomZst(i32, ZeroSize);
   |                                      ^^^^^^^^
   |
   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
   = note: for more information, see issue #78586 <https://github.com/rust-lang/rust/issues/78586>
   = note: this field contains `ZeroSize`, which is marked with `#[repr(C)]`, and makes it not a breaking change to become non-zero-sized in the future.
   = note: `#[warn(repr_transparent_external_private_fields)]` (part of `#[warn(future_incompatible)]`) on by default

error: `extern` block uses type `Foo`, which is not FFI-safe
##[error]  --> /checkout/tests/ui/lint/improper-ctypes/lint-ctypes.rs:47:28
   |
LL |     pub fn ptr_type1(size: *const Foo); //~ ERROR: uses type `Foo`
   |                            ^^^^^^^^^^ not FFI-safe
   |
   = help: consider adding a `#[repr(C)]` or `#[repr(transparent)]` attribute to this struct
   = note: this struct has unspecified layout
note: the type is defined here
  --> /checkout/tests/ui/lint/improper-ctypes/lint-ctypes.rs:25:1
---
   |
LL | #![deny(improper_ctypes)]
   |         ^^^^^^^^^^^^^^^

error: `extern` block uses type `Foo`, which is not FFI-safe
##[error]  --> /checkout/tests/ui/lint/improper-ctypes/lint-ctypes.rs:48:28
   |
LL |     pub fn ptr_type2(size: *const Foo); //~ ERROR: uses type `Foo`
   |                            ^^^^^^^^^^ not FFI-safe
   |
   = help: consider adding a `#[repr(C)]` or `#[repr(transparent)]` attribute to this struct
   = note: this struct has unspecified layout
note: the type is defined here
  --> /checkout/tests/ui/lint/improper-ctypes/lint-ctypes.rs:25:1
   |
LL | pub struct Foo;
   | ^^^^^^^^^^^^^^

error: `extern` block uses type `((),)`, which is not FFI-safe
##[error]  --> /checkout/tests/ui/lint/improper-ctypes/lint-ctypes.rs:50:25
   |
LL |     pub fn ptr_tuple(p: *const ((),)); //~ ERROR: uses type `((),)`
   |                         ^^^^^^^^^^^^ not FFI-safe
   |
   = help: consider using a struct instead
   = note: tuples have unspecified layout

error: `extern` block uses type `[u32]`, which is not FFI-safe
##[error]  --> /checkout/tests/ui/lint/improper-ctypes/lint-ctypes.rs:51:26
   |
LL |     pub fn slice_type(p: &[u32]); //~ ERROR: uses type `[u32]`
   |                          ^^^^^^ not FFI-safe
   |
   = help: consider using a raw pointer instead
   = note: slices have no C equivalent

error: `extern` block uses type `str`, which is not FFI-safe
##[error]  --> /checkout/tests/ui/lint/improper-ctypes/lint-ctypes.rs:52:24
   |
LL |     pub fn str_type(p: &str); //~ ERROR: uses type `str`
   |                        ^^^^ not FFI-safe
   |
   = help: consider using `*const u8` and a length instead
   = note: string slices have no C equivalent

error: `extern` block uses type `Box<u32>`, which is not FFI-safe
##[error]  --> /checkout/tests/ui/lint/improper-ctypes/lint-ctypes.rs:53:24
   |
LL |     pub fn box_type(p: Box<u32>); //~ ERROR uses type `Box<u32>`
   |                        ^^^^^^^^ not FFI-safe
   |
   = help: consider adding a `#[repr(C)]` or `#[repr(transparent)]` attribute to this struct
   = note: this struct has unspecified layout

error: `extern` block uses type `char`, which is not FFI-safe
##[error]  --> /checkout/tests/ui/lint/improper-ctypes/lint-ctypes.rs:55:25
   |
LL |     pub fn char_type(p: char); //~ ERROR uses type `char`
   |                         ^^^^ not FFI-safe
   |
   = help: consider using `u32` or `libc::wchar_t` instead
   = note: the `char` type has no C equivalent

error: `extern` block uses type `dyn Bar`, which is not FFI-safe
##[error]  --> /checkout/tests/ui/lint/improper-ctypes/lint-ctypes.rs:56:26
   |
LL |     pub fn trait_type(p: &dyn Bar); //~ ERROR uses type `dyn Bar`
   |                          ^^^^^^^^ not FFI-safe
   |
   = note: trait objects have no C equivalent

error: `extern` block uses type `(i32, i32)`, which is not FFI-safe
##[error]  --> /checkout/tests/ui/lint/improper-ctypes/lint-ctypes.rs:57:26
   |
LL |     pub fn tuple_type(p: (i32, i32)); //~ ERROR uses type `(i32, i32)`
   |                          ^^^^^^^^^^ not FFI-safe
   |
   = help: consider using a struct instead
   = note: tuples have unspecified layout

error: `extern` block uses type `(i32, i32)`, which is not FFI-safe
##[error]  --> /checkout/tests/ui/lint/improper-ctypes/lint-ctypes.rs:58:27
   |
LL |     pub fn tuple_type2(p: I32Pair); //~ ERROR uses type `(i32, i32)`
   |                           ^^^^^^^ not FFI-safe
   |
   = help: consider using a struct instead
   = note: tuples have unspecified layout

error: `extern` block uses type `ZeroSize`, which is not FFI-safe
##[error]  --> /checkout/tests/ui/lint/improper-ctypes/lint-ctypes.rs:59:25
   |
LL |     pub fn zero_size(p: ZeroSize); //~ ERROR uses type `ZeroSize`
   |                         ^^^^^^^^ not FFI-safe
   |
   = help: consider adding a member to this struct
   = note: this struct has no fields
note: the type is defined here
  --> /checkout/tests/ui/lint/improper-ctypes/lint-ctypes.rs:21:1
   |
LL | pub struct ZeroSize;
   | ^^^^^^^^^^^^^^^^^^^

error: `extern` block uses type `ZeroSizeWithPhantomData`, which is not FFI-safe
##[error]  --> /checkout/tests/ui/lint/improper-ctypes/lint-ctypes.rs:60:33
   |
LL |     pub fn zero_size_phantom(p: ZeroSizeWithPhantomData);
   |                                 ^^^^^^^^^^^^^^^^^^^^^^^ not FFI-safe
   |
   = note: composed only of `PhantomData`
note: the type is defined here
  --> /checkout/tests/ui/lint/improper-ctypes/lint-ctypes.rs:44:1
   |
LL | pub struct ZeroSizeWithPhantomData(::std::marker::PhantomData<i32>);
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error: `extern` block uses type `PhantomData<bool>`, which is not FFI-safe
##[error]  --> /checkout/tests/ui/lint/improper-ctypes/lint-ctypes.rs:63:12
   |
LL |         -> ::std::marker::PhantomData<bool>; //~ ERROR uses type `PhantomData<bool>`
   |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ not FFI-safe
   |
   = note: composed only of `PhantomData`

error: `extern` block uses type `fn()`, which is not FFI-safe
##[error]  --> /checkout/tests/ui/lint/improper-ctypes/lint-ctypes.rs:64:23
   |
LL |     pub fn fn_type(p: RustFn); //~ ERROR uses type `fn()`
   |                       ^^^^^^ not FFI-safe
   |
   = help: consider using an `extern fn(...) -> ...` function pointer instead
   = note: this function pointer has Rust-specific calling convention

error: `extern` block uses type `fn()`, which is not FFI-safe
##[error]  --> /checkout/tests/ui/lint/improper-ctypes/lint-ctypes.rs:65:24
   |
LL |     pub fn fn_type2(p: fn()); //~ ERROR uses type `fn()`
   |                        ^^^^ not FFI-safe
   |
   = help: consider using an `extern fn(...) -> ...` function pointer instead
   = note: this function pointer has Rust-specific calling convention

error: `extern` block uses type `Box<u32>`, which is not FFI-safe
##[error]  --> /checkout/tests/ui/lint/improper-ctypes/lint-ctypes.rs:66:28
   |
LL |     pub fn fn_contained(p: RustBadRet); //~ ERROR: uses type `Box<u32>`
   |                            ^^^^^^^^^^ not FFI-safe
   |
   = help: consider adding a `#[repr(C)]` or `#[repr(transparent)]` attribute to this struct
   = note: this struct has unspecified layout

error: `extern` block uses type `str`, which is not FFI-safe
##[error]  --> /checkout/tests/ui/lint/improper-ctypes/lint-ctypes.rs:67:31
   |
LL |     pub fn transparent_str(p: TransparentStr); //~ ERROR: uses type `str`
   |                               ^^^^^^^^^^^^^^ not FFI-safe
   |
   = help: consider using `*const u8` and a length instead
   = note: string slices have no C equivalent

error: `extern` block uses type `Box<u32>`, which is not FFI-safe
##[error]  --> /checkout/tests/ui/lint/improper-ctypes/lint-ctypes.rs:68:30
   |
LL |     pub fn transparent_fn(p: TransparentBadFn); //~ ERROR: uses type `Box<u32>`
   |                              ^^^^^^^^^^^^^^^^ not FFI-safe
   |
   = help: consider adding a `#[repr(C)]` or `#[repr(transparent)]` attribute to this struct
   = note: this struct has unspecified layout

error: `extern` block uses type `[u8; 8]`, which is not FFI-safe
##[error]  --> /checkout/tests/ui/lint/improper-ctypes/lint-ctypes.rs:69:27
   |
LL |     pub fn raw_array(arr: [u8; 8]); //~ ERROR: uses type `[u8; 8]`
   |                           ^^^^^^^ not FFI-safe
   |
   = help: consider passing a pointer to the array
   = note: passing raw arrays by value is not FFI-safe

error: `extern` block uses type `Option<UnsafeCell<extern "C" fn()>>`, which is not FFI-safe
##[error]  --> /checkout/tests/ui/lint/improper-ctypes/lint-ctypes.rs:71:26
   |
LL |     pub fn no_niche_a(a: Option<UnsafeCell<extern "C" fn()>>);
   |                          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ not FFI-safe
   |
   = help: consider adding a `#[repr(C)]`, `#[repr(transparent)]`, or integer `#[repr(...)]` attribute to this enum
   = note: enum has no representation hint

error: `extern` block uses type `Option<UnsafeCell<&i32>>`, which is not FFI-safe
##[error]  --> /checkout/tests/ui/lint/improper-ctypes/lint-ctypes.rs:73:26
   |
LL |     pub fn no_niche_b(b: Option<UnsafeCell<&i32>>);
   |                          ^^^^^^^^^^^^^^^^^^^^^^^^ not FFI-safe
   |
   = help: consider adding a `#[repr(C)]`, `#[repr(transparent)]`, or integer `#[repr(...)]` attribute to this enum
   = note: enum has no representation hint

error: aborting due to 21 previous errors; 1 warning emitted
------------------------------------------

---- [ui] tests/ui/lint/improper-ctypes/lint-ctypes.rs stdout end ----
---- [ui] tests/ui/lint/improper-ctypes/lint-fn.rs stdout ----
Saved the actual stderr to `/checkout/obj/build/aarch64-unknown-linux-gnu/test/ui/lint/improper-ctypes/lint-fn/lint-fn.stderr`
diff of stderr:

+ warning: zero-sized fields in `repr(transparent)` cannot contain external non-exhaustive types
+   --> $DIR/lint-fn.rs:57:38
+    |
+ LL | pub struct TransparentCustomZst(i32, ZeroSize);
+    |                                      ^^^^^^^^
+    |
+    = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+    = note: for more information, see issue #78586 <https://github.com/rust-lang/rust/issues/78586>
+    = note: this field contains `ZeroSize`, which is marked with `#[repr(C)]`, and makes it not a breaking change to become non-zero-sized in the future.
+    = note: `#[warn(repr_transparent_external_private_fields)]` (part of `#[warn(future_incompatible)]`) on by default
+ 
1 error: `extern` fn uses type `[u32]`, which is not FFI-safe
2   --> $DIR/lint-fn.rs:70:33
3    |

160    = help: consider adding a `#[repr(C)]` or `#[repr(transparent)]` attribute to this struct
161    = note: this struct has unspecified layout
---
165 

Note: some mismatched output was normalized before being compared
-   --> /checkout/tests/ui/lint/improper-ctypes/lint-fn.rs:57:38
+ warning: zero-sized fields in `repr(transparent)` cannot contain external non-exhaustive types
+   --> $DIR/lint-fn.rs:57:38
+    |
+ LL | pub struct TransparentCustomZst(i32, ZeroSize);
+    |                                      ^^^^^^^^
+    |
+    = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+    = note: for more information, see issue #78586 <https://github.com/rust-lang/rust/issues/78586>
+    = note: this field contains `ZeroSize`, which is marked with `#[repr(C)]`, and makes it not a breaking change to become non-zero-sized in the future.
---
To only update this specific test, also pass `--test-args lint/improper-ctypes/lint-fn.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/lint/improper-ctypes/lint-fn.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/lint/improper-ctypes/lint-fn" "-A" "unused" "-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 -------------------------------
warning: zero-sized fields in `repr(transparent)` cannot contain external non-exhaustive types
##[warning]  --> /checkout/tests/ui/lint/improper-ctypes/lint-fn.rs:57:38
   |
LL | pub struct TransparentCustomZst(i32, ZeroSize);
   |                                      ^^^^^^^^
   |
   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
   = note: for more information, see issue #78586 <https://github.com/rust-lang/rust/issues/78586>
   = note: this field contains `ZeroSize`, which is marked with `#[repr(C)]`, and makes it not a breaking change to become non-zero-sized in the future.
   = note: `#[warn(repr_transparent_external_private_fields)]` (part of `#[warn(future_incompatible)]`) on by default

error: `extern` fn uses type `[u32]`, which is not FFI-safe
##[error]  --> /checkout/tests/ui/lint/improper-ctypes/lint-fn.rs:70:33
   |
LL | pub extern "C" fn slice_type(p: &[u32]) { }
   |                                 ^^^^^^ not FFI-safe
   |
   = help: consider using a raw pointer instead
   = note: slices have no C equivalent
note: the lint level is defined here
  --> /checkout/tests/ui/lint/improper-ctypes/lint-fn.rs:2:9
   |
LL | #![deny(improper_ctypes_definitions)]
   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^

error: `extern` fn uses type `str`, which is not FFI-safe
##[error]  --> /checkout/tests/ui/lint/improper-ctypes/lint-fn.rs:73:31
   |
LL | pub extern "C" fn str_type(p: &str) { }
   |                               ^^^^ not FFI-safe
   |
   = help: consider using `*const u8` and a length instead
   = note: string slices have no C equivalent

error: `extern` fn uses type `Box<[u8]>`, which is not FFI-safe
##[error]  --> /checkout/tests/ui/lint/improper-ctypes/lint-fn.rs:80:34
   |
LL | pub extern "C" fn boxed_slice(p: Box<[u8]>) { }
   |                                  ^^^^^^^^^ not FFI-safe
   |
   = note: box cannot be represented as a single pointer

error: `extern` fn uses type `Box<str>`, which is not FFI-safe
##[error]  --> /checkout/tests/ui/lint/improper-ctypes/lint-fn.rs:83:35
   |
LL | pub extern "C" fn boxed_string(p: Box<str>) { }
   |                                   ^^^^^^^^ not FFI-safe
   |
   = note: box cannot be represented as a single pointer

error: `extern` fn uses type `Box<dyn Trait>`, which is not FFI-safe
##[error]  --> /checkout/tests/ui/lint/improper-ctypes/lint-fn.rs:86:34
   |
LL | pub extern "C" fn boxed_trait(p: Box<dyn Trait>) { }
   |                                  ^^^^^^^^^^^^^^ not FFI-safe
   |
   = note: box cannot be represented as a single pointer

error: `extern` fn uses type `char`, which is not FFI-safe
##[error]  --> /checkout/tests/ui/lint/improper-ctypes/lint-fn.rs:89:32
   |
LL | pub extern "C" fn char_type(p: char) { }
   |                                ^^^^ not FFI-safe
   |
   = help: consider using `u32` or `libc::wchar_t` instead
   = note: the `char` type has no C equivalent

error: `extern` fn uses type `(i32, i32)`, which is not FFI-safe
##[error]  --> /checkout/tests/ui/lint/improper-ctypes/lint-fn.rs:92:33
   |
LL | pub extern "C" fn tuple_type(p: (i32, i32)) { }
   |                                 ^^^^^^^^^^ not FFI-safe
   |
   = help: consider using a struct instead
   = note: tuples have unspecified layout

error: `extern` fn uses type `(i32, i32)`, which is not FFI-safe
##[error]  --> /checkout/tests/ui/lint/improper-ctypes/lint-fn.rs:95:34
   |
LL | pub extern "C" fn tuple_type2(p: I32Pair) { }
   |                                  ^^^^^^^ not FFI-safe
   |
   = help: consider using a struct instead
   = note: tuples have unspecified layout

error: `extern` fn uses type `ZeroSize`, which is not FFI-safe
##[error]  --> /checkout/tests/ui/lint/improper-ctypes/lint-fn.rs:98:32
   |
LL | pub extern "C" fn zero_size(p: ZeroSize) { }
   |                                ^^^^^^^^ not FFI-safe
   |
   = help: consider adding a member to this struct
   = note: this struct has no fields
note: the type is defined here
  --> /checkout/tests/ui/lint/improper-ctypes/lint-fn.rs:25:1
   |
LL | pub struct ZeroSize;
   | ^^^^^^^^^^^^^^^^^^^

error: `extern` fn uses type `ZeroSizeWithPhantomData`, which is not FFI-safe
##[error]  --> /checkout/tests/ui/lint/improper-ctypes/lint-fn.rs:101:40
   |
LL | pub extern "C" fn zero_size_phantom(p: ZeroSizeWithPhantomData) { }
   |                                        ^^^^^^^^^^^^^^^^^^^^^^^ not FFI-safe
   |
   = note: composed only of `PhantomData`
note: the type is defined here
  --> /checkout/tests/ui/lint/improper-ctypes/lint-fn.rs:60:1
   |
LL | pub struct ZeroSizeWithPhantomData(PhantomData<i32>);
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error: `extern` fn uses type `PhantomData<bool>`, which is not FFI-safe
##[error]  --> /checkout/tests/ui/lint/improper-ctypes/lint-fn.rs:104:51
   |
LL | pub extern "C" fn zero_size_phantom_toplevel() -> PhantomData<bool> {
   |                                                   ^^^^^^^^^^^^^^^^^ not FFI-safe
   |
   = note: composed only of `PhantomData`

error: `extern` fn uses type `fn()`, which is not FFI-safe
##[error]  --> /checkout/tests/ui/lint/improper-ctypes/lint-fn.rs:109:30
   |
LL | pub extern "C" fn fn_type(p: RustFn) { }
   |                              ^^^^^^ not FFI-safe
   |
   = help: consider using an `extern fn(...) -> ...` function pointer instead
   = note: this function pointer has Rust-specific calling convention

error: `extern` fn uses type `fn()`, which is not FFI-safe
##[error]  --> /checkout/tests/ui/lint/improper-ctypes/lint-fn.rs:112:31
   |
LL | pub extern "C" fn fn_type2(p: fn()) { }
   |                               ^^^^ not FFI-safe
   |
   = help: consider using an `extern fn(...) -> ...` function pointer instead
   = note: this function pointer has Rust-specific calling convention

error: `extern` fn uses type `str`, which is not FFI-safe
##[error]  --> /checkout/tests/ui/lint/improper-ctypes/lint-fn.rs:117:38
   |
LL | pub extern "C" fn transparent_str(p: TransparentStr) { }
   |                                      ^^^^^^^^^^^^^^ not FFI-safe
   |
   = help: consider using `*const u8` and a length instead
   = note: string slices have no C equivalent

error: `extern` fn uses type `PhantomData<bool>`, which is not FFI-safe
##[error]  --> /checkout/tests/ui/lint/improper-ctypes/lint-fn.rs:169:43
   |
LL | pub extern "C" fn unused_generic2<T>() -> PhantomData<bool> {
   |                                           ^^^^^^^^^^^^^^^^^ not FFI-safe
   |
   = note: composed only of `PhantomData`

error: `extern` fn uses type `Vec<T>`, which is not FFI-safe
##[error]  --> /checkout/tests/ui/lint/improper-ctypes/lint-fn.rs:182:39
   |
LL | pub extern "C" fn used_generic4<T>(x: Vec<T>) { }
   |                                       ^^^^^^ not FFI-safe
   |
   = help: consider adding a `#[repr(C)]` or `#[repr(transparent)]` attribute to this struct
   = note: this struct has unspecified layout

error: `extern` fn uses type `Vec<T>`, which is not FFI-safe
##[error]  --> /checkout/tests/ui/lint/improper-ctypes/lint-fn.rs:185:41
   |
LL | pub extern "C" fn used_generic5<T>() -> Vec<T> {
   |                                         ^^^^^^ not FFI-safe
   |

rust-bors · 2025-10-11T15:09:37Z

☀️ Try build successful (CI)
Build commit: 54e27f5 (54e27f559f23913c1d5afb1a0f0114e9e61b81b8, parent: b3f8586fb1e4859678d6b231e780ff81801d2282)

RalfJung · 2025-10-11T15:20:35Z

@craterbot check

craterbot · 2025-10-11T15:21:04Z

👌 Experiment pr-147589 created and queued.
🤖 Automatically detected try build 54e27f5
🔍 You can check out the queue and this experiment's details.

ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

RalfJung added 7 commits September 30, 2025 12:39

repr(transparent): do not consider repr(C) types to be 1-ZST

5bd164b

for crater: make this lint a hard error

24d6a9a

also complain about uninhabited types being ignored in repr(transparent)

eee5199

use non-semver-aware uninhabited check

c1dcd7e

revert code to be more like previous structure, but with explicit types

44cb9b1

make it a lint again

b142623

remove hyper from PGO so we can crater this

0f4ab71

rust-bors bot added a commit that referenced this pull request Oct 11, 2025

Auto merge of #147589 - RalfJung:repr-transparent-uninhabited, r=<try>

54e27f5

[crater only] complain about uninhabited types being ignored in repr(transparent)

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craterbot added S-waiting-on-crater Status: Waiting on a crater run to be completed. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Oct 11, 2025

RalfJung mentioned this pull request Oct 11, 2025

Do not consider uninhabited 1-ZST "trivial" for the purpose of repr(transparent)? #147588

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[crater only] complain about uninhabited types being ignored in repr(transparent) #147589

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