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

[RalfJung]

Context: rust-lang/unsafe-code-guidelines#552

This experiments with a suggestion by @RustyYato to stop considering repr(C) types as 1-ZST for the purpose of repr(transparent). If we go with rust-lang/rfcs#3845 (or another approach for fixing repr(C)), they will anyway not be ZST on all targets any more, so this removes a portability hazard. Furthermore, zero-sized repr(C) structs may have to be treated as non-ZST for the win64 ABI (at least that's what gcc/clang do), so allowing them to be ignored in repr(transparent) types is not entirely coherent.

Turns out we already have an FCW for repr(transparent), namely #78586. This extends that lint to also check for repr(C) -- without actually updating the wording so the errors you get make little sense for now. Before landing we need to decide if these should be the same lint or not and then adjust the wording accordingly. Note that some types are only rejected if we check for both non-exhaustive structs and repr(C) structs so it's not entierly clear how to disentangle this into separate lints.

The 2nd commit makes the lint a hard error so that we can crater this. I think crater will mostly find errors related to the non-exhaustive check, not the new repr(C) check, but as it's hard to disentangle them let's just see what happens before making the lint setup more complicated.

[rustbot]

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[RalfJung]

@bors try

[rust-log-analyzer]

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:

- error: `extern` block uses type `Foo`, which is not FFI-safe
-   --> $DIR/lint-ctypes.rs:47:28
+ error: zero-sized fields in `repr(transparent)` cannot contain external non-exhaustive types
+   --> $DIR/lint-ctypes.rs:41:38
3    |
- LL |     pub fn ptr_type1(size: *const Foo);
-    |                            ^^^^^^^^^^ not FFI-safe
+ LL | pub struct TransparentCustomZst(i32, ZeroSize);
+    |                                      ^^^^^^^^
6    |
-    = 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
---
- LL | #![deny(improper_ctypes)]
-    |         ^^^^^^^^^^^^^^^
+    = note: this struct contains `ZeroSize`, which is marked with `#[repr(C)]`, and makes it not a breaking change to become non-zero-sized in the future.
19 
- error: `extern` block uses type `Foo`, which is not FFI-safe
-   --> $DIR/lint-ctypes.rs:48:28
-    |
- LL |     pub fn ptr_type2(size: *const Foo);
-    |                            ^^^^^^^^^^ not FFI-safe
-    |
---
-    |
- LL | pub struct Foo;
-    | ^^^^^^^^^^^^^^
- 
- error: `extern` block uses type `((),)`, which is not FFI-safe
-   --> $DIR/lint-ctypes.rs:50:25
-    |
- LL |     pub fn ptr_tuple(p: *const ((),));
-    |                         ^^^^^^^^^^^^ 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
-   --> $DIR/lint-ctypes.rs:51:26
-    |
- LL |     pub fn slice_type(p: &[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
-   --> $DIR/lint-ctypes.rs:52:24
-    |
- LL |     pub 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` block uses type `Box<u32>`, which is not FFI-safe
-   --> $DIR/lint-ctypes.rs:53:24
-    |
- LL |     pub fn box_type(p: 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
-   --> $DIR/lint-ctypes.rs:55:25
-    |
- LL |     pub 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` block uses type `dyn Bar`, which is not FFI-safe
-   --> $DIR/lint-ctypes.rs:56:26
-    |
- LL |     pub fn trait_type(p: &dyn Bar);
-    |                          ^^^^^^^^ not FFI-safe
-    |
-    = note: trait objects have no C equivalent
- 
- error: `extern` block uses type `(i32, i32)`, which is not FFI-safe
-   --> $DIR/lint-ctypes.rs:57:26
-    |
- LL |     pub fn tuple_type(p: (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
-   --> $DIR/lint-ctypes.rs:58:27
-    |
- LL |     pub fn tuple_type2(p: I32Pair);
-    |                           ^^^^^^^ 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
-   --> $DIR/lint-ctypes.rs:59:25
-    |
- LL |     pub 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
-   --> $DIR/lint-ctypes.rs:21:1
-    |
- LL | pub struct ZeroSize;
-    | ^^^^^^^^^^^^^^^^^^^
- 
- error: `extern` block uses type `ZeroSizeWithPhantomData`, which is not FFI-safe
-   --> $DIR/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
-   --> $DIR/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
-   --> $DIR/lint-ctypes.rs:63:12
-    |
- LL |         -> ::std::marker::PhantomData<bool>;
-    |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ not FFI-safe
-    |
-    = note: composed only of `PhantomData`
- 
- error: `extern` block uses type `fn()`, which is not FFI-safe
-   --> $DIR/lint-ctypes.rs:64:23
-    |
- LL |     pub 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` block uses type `fn()`, which is not FFI-safe
-   --> $DIR/lint-ctypes.rs:65:24
-    |
- LL |     pub 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` block uses type `Box<u32>`, which is not FFI-safe
-   --> $DIR/lint-ctypes.rs:66:28
-    |
- LL |     pub fn fn_contained(p: RustBadRet);
-    |                            ^^^^^^^^^^ 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
---
-    |
-    = 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
-   --> $DIR/lint-ctypes.rs:68:30
-    |
- LL |     pub fn transparent_fn(p: TransparentBadFn);
-    |                              ^^^^^^^^^^^^^^^^ 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
-   --> $DIR/lint-ctypes.rs:69:27
-    |
- LL |     pub fn raw_array(arr: [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
-   --> $DIR/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
-   --> $DIR/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
+ error: aborting due to 1 previous error
213 
214 

Note: some mismatched output was normalized before being compared
-   --> /checkout/tests/ui/lint/improper-ctypes/lint-ctypes.rs:41:38
+ error: zero-sized fields in `repr(transparent)` cannot contain external non-exhaustive types
+   --> $DIR/lint-ctypes.rs:41:38
+ LL | pub struct TransparentCustomZst(i32, ZeroSize);
+    |                                      ^^^^^^^^
+    = note: this struct contains `ZeroSize`, which is marked with `#[repr(C)]`, and makes it not a breaking change to become non-zero-sized in the future.
+ error: aborting due to 1 previous error


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 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 -------------------------------
error: zero-sized fields in `repr(transparent)` cannot contain external non-exhaustive types
##[error]  --> /checkout/tests/ui/lint/improper-ctypes/lint-ctypes.rs:41:38
   |
LL | pub struct TransparentCustomZst(i32, ZeroSize);
   |                                      ^^^^^^^^
   |
   = note: this struct contains `ZeroSize`, which is marked with `#[repr(C)]`, and makes it not a breaking change to become non-zero-sized in the future.

error: aborting due to 1 previous error
------------------------------------------

---- [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:

- error: `extern` fn uses type `[u32]`, which is not FFI-safe
-   --> $DIR/lint-fn.rs:70:33
+ error: zero-sized fields in `repr(transparent)` cannot contain external non-exhaustive types
+   --> $DIR/lint-fn.rs:57:38
3    |
- LL | pub extern "C" fn slice_type(p: &[u32]) { }
-    |                                 ^^^^^^ not FFI-safe
+ LL | pub struct TransparentCustomZst(i32, ZeroSize);
+    |                                      ^^^^^^^^
6    |
-    = help: consider using a raw pointer instead
-    = note: slices have no C equivalent
- note: the lint level is defined here
-   --> $DIR/lint-fn.rs:2:9
-    |
- LL | #![deny(improper_ctypes_definitions)]
-    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^
+    = note: this struct contains `ZeroSize`, which is marked with `#[repr(C)]`, and makes it not a breaking change to become non-zero-sized in the future.
14 
- error: `extern` fn uses type `str`, which is not FFI-safe
-   --> $DIR/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
-   --> $DIR/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
-   --> $DIR/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
-   --> $DIR/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
-   --> $DIR/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
-   --> $DIR/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
-   --> $DIR/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
-   --> $DIR/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
-   --> $DIR/lint-fn.rs:25:1
-    |
- LL | pub struct ZeroSize;
-    | ^^^^^^^^^^^^^^^^^^^
- 
- error: `extern` fn uses type `ZeroSizeWithPhantomData`, which is not FFI-safe
-   --> $DIR/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
-   --> $DIR/lint-fn.rs:60:1
-    |
- LL | pub struct ZeroSizeWithPhantomData(PhantomData<i32>);
-    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
- 
- error: `extern` fn uses type `PhantomData<bool>`, which is not FFI-safe
-   --> $DIR/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
-   --> $DIR/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
-   --> $DIR/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
-   --> $DIR/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
-   --> $DIR/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
-   --> $DIR/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
-   --> $DIR/lint-fn.rs:185:41
-    |
- LL | pub extern "C" fn used_generic5<T>() -> Vec<T> {
-    |                                         ^^^^^^ not FFI-safe
-    |
---
165 

Note: some mismatched output was normalized before being compared
-   --> /checkout/tests/ui/lint/improper-ctypes/lint-fn.rs:57:38
+ error: zero-sized fields in `repr(transparent)` cannot contain external non-exhaustive types
+   --> $DIR/lint-fn.rs:57:38
+ LL | pub struct TransparentCustomZst(i32, ZeroSize);
+    |                                      ^^^^^^^^
+    = note: this struct contains `ZeroSize`, which is marked with `#[repr(C)]`, and makes it not a breaking change to become non-zero-sized in the future.
+ error: aborting due to 1 previous error


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 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 -------------------------------
error: zero-sized fields in `repr(transparent)` cannot contain external non-exhaustive types
##[error]  --> /checkout/tests/ui/lint/improper-ctypes/lint-fn.rs:57:38
   |
LL | pub struct TransparentCustomZst(i32, ZeroSize);
   |                                      ^^^^^^^^
   |
   = note: this struct contains `ZeroSize`, which is marked with `#[repr(C)]`, and makes it not a breaking change to become non-zero-sized in the future.

error: aborting due to 1 previous error

[rust-bors]

☀️ Try build successful (CI)
Build commit: 0de49ae (0de49ae9e81f9a1e7df6f0783824ce94ed18e8a9, parent: a2db9280539229a3b8a084a09886670a57bc7e9c)

[petrochenkov]

@craterbot check

[craterbot]

👌 Experiment pr-147185 created and queued.
🤖 Automatically detected try build 0de49ae
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