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Auto merge of #97825 - Dylan-DPC:rollup-ya51k1k, r=Dylan-DPC

Browse Source 
Rollup of 5 pull requests

Successful merges:

 - #97058 (Various refactors to the incr comp workproduct handling)
 - #97301 (Allow unstable items to be re-exported unstably without requiring the feature be enabled)
 - #97738 (Fix ICEs from zsts within unsized types with non-zero offsets)
 - #97771 (Remove SIGIO reference on Haiku)
 - #97808 (Add some unstable target features for the wasm target codegen)

Failed merges:

r? `@ghost`
`@rustbot` modify labels: rollup
This commit is contained in:
bors 2022-06-07 11:08:58 +00:00
commit 7fe2c4b00d
18 changed files with 284 additions and 115 deletions
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33
compiler/rustc_codegen_cranelift/src/driver/aot.rs
View File

@ -66,11 +66,7 @@ fn emit_module(
let work_product = if backend_config.disable_incr_cache {
None
} else {
rustc_incremental::copy_cgu_workproduct_to_incr_comp_cache_dir(
tcx.sess,
&name,
&Some(tmp_file.clone()),
)
rustc_incremental::copy_cgu_workproduct_to_incr_comp_cache_dir(tcx.sess, &name, &tmp_file)
};
ModuleCodegenResult(
@ -84,21 +80,16 @@ fn reuse_workproduct_for_cgu(
cgu: &CodegenUnit<'_>,
work_products: &mut FxHashMap<WorkProductId, WorkProduct>,
) -> CompiledModule {
let mut object = None;
let work_product = cgu.work_product(tcx);
if let Some(saved_file) = &work_product.saved_file {
let obj_out =
tcx.output_filenames(()).temp_path(OutputType::Object, Some(cgu.name().as_str()));
object = Some(obj_out.clone());
let source_file = rustc_incremental::in_incr_comp_dir_sess(&tcx.sess, &saved_file);
if let Err(err) = rustc_fs_util::link_or_copy(&source_file, &obj_out) {
tcx.sess.err(&format!(
"unable to copy {} to {}: {}",
source_file.display(),
obj_out.display(),
err
));
}
let work_product = cgu.previous_work_product(tcx);
let obj_out = tcx.output_filenames(()).temp_path(OutputType::Object, Some(cgu.name().as_str()));
let source_file = rustc_incremental::in_incr_comp_dir_sess(&tcx.sess, &work_product.saved_file);
if let Err(err) = rustc_fs_util::link_or_copy(&source_file, &obj_out) {
tcx.sess.err(&format!(
"unable to copy {} to {}: {}",
source_file.display(),
obj_out.display(),
err
));
}
work_products.insert(cgu.work_product_id(), work_product);
@ -106,7 +97,7 @@ fn reuse_workproduct_for_cgu(
CompiledModule {
name: cgu.name().to_string(),
kind: ModuleKind::Regular,
object,
object: Some(obj_out),
dwarf_object: None,
bytecode: None,
}

56
compiler/rustc_codegen_ssa/src/back/write.rs
View File

@ -494,12 +494,12 @@ fn copy_all_cgu_workproducts_to_incr_comp_cache_dir(
let _timer = sess.timer("copy_all_cgu_workproducts_to_incr_comp_cache_dir");
for module in compiled_modules.modules.iter().filter(|m| m.kind == ModuleKind::Regular) {
let path = module.object.as_ref().cloned();
if let Some((id, product)) =
copy_cgu_workproduct_to_incr_comp_cache_dir(sess, &module.name, &path)
{
work_products.insert(id, product);
if let Some(path) = &module.object {
if let Some((id, product)) =
copy_cgu_workproduct_to_incr_comp_cache_dir(sess, &module.name, path)
{
work_products.insert(id, product);
}
}
}
@ -853,35 +853,31 @@ fn execute_copy_from_cache_work_item<B: ExtraBackendMethods>(
module: CachedModuleCodegen,
module_config: &ModuleConfig,
) -> WorkItemResult<B> {
let incr_comp_session_dir = cgcx.incr_comp_session_dir.as_ref().unwrap();
let mut object = None;
if let Some(saved_file) = module.source.saved_file {
let obj_out = cgcx.output_filenames.temp_path(OutputType::Object, Some(&module.name));
object = Some(obj_out.clone());
let source_file = in_incr_comp_dir(&incr_comp_session_dir, &saved_file);
debug!(
"copying pre-existing module `{}` from {:?} to {}",
module.name,
source_file,
obj_out.display()
);
if let Err(err) = link_or_copy(&source_file, &obj_out) {
let diag_handler = cgcx.create_diag_handler();
diag_handler.err(&format!(
"unable to copy {} to {}: {}",
source_file.display(),
obj_out.display(),
err
));
}
}
assert!(module_config.emit_obj != EmitObj::None);
assert_eq!(object.is_some(), module_config.emit_obj != EmitObj::None);
let incr_comp_session_dir = cgcx.incr_comp_session_dir.as_ref().unwrap();
let obj_out = cgcx.output_filenames.temp_path(OutputType::Object, Some(&module.name));
let source_file = in_incr_comp_dir(&incr_comp_session_dir, &module.source.saved_file);
debug!(
"copying pre-existing module `{}` from {:?} to {}",
module.name,
source_file,
obj_out.display()
);
if let Err(err) = link_or_copy(&source_file, &obj_out) {
let diag_handler = cgcx.create_diag_handler();
diag_handler.err(&format!(
"unable to copy {} to {}: {}",
source_file.display(),
obj_out.display(),
err
));
}
WorkItemResult::Compiled(CompiledModule {
name: module.name,
kind: ModuleKind::Regular,
object,
object: Some(obj_out),
dwarf_object: None,
bytecode: None,
})

9
compiler/rustc_codegen_ssa/src/base.rs
View File

@ -216,11 +216,12 @@ pub fn unsize_ptr<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
let mut result = None;
for i in 0..src_layout.fields.count() {
let src_f = src_layout.field(bx.cx(), i);
assert_eq!(src_layout.fields.offset(i).bytes(), 0);
assert_eq!(dst_layout.fields.offset(i).bytes(), 0);
if src_f.is_zst() {
continue;
}
assert_eq!(src_layout.fields.offset(i).bytes(), 0);
assert_eq!(dst_layout.fields.offset(i).bytes(), 0);
assert_eq!(src_layout.size, src_f.size);
let dst_f = dst_layout.field(bx.cx(), i);
@ -716,7 +717,7 @@ pub fn codegen_crate<B: ExtraBackendMethods>(
&ongoing_codegen.coordinator_send,
CachedModuleCodegen {
name: cgu.name().to_string(),
source: cgu.work_product(tcx),
source: cgu.previous_work_product(tcx),
},
);
true
@ -727,7 +728,7 @@ pub fn codegen_crate<B: ExtraBackendMethods>(
&ongoing_codegen.coordinator_send,
CachedModuleCodegen {
name: cgu.name().to_string(),
source: cgu.work_product(tcx),
source: cgu.previous_work_product(tcx),
},
);
true

3
compiler/rustc_codegen_ssa/src/target_features.rs
View File

@ -246,6 +246,9 @@
("simd128", None),
("atomics", Some(sym::wasm_target_feature)),
("nontrapping-fptoint", Some(sym::wasm_target_feature)),
("bulk-memory", Some(sym::wasm_target_feature)),
("mutable-globals", Some(sym::wasm_target_feature)),
("reference-types", Some(sym::wasm_target_feature)),
];
const BPF_ALLOWED_FEATURES: &[(&str, Option<Symbol>)] = &[("alu32", Some(sym::bpf_target_feature))];

18
compiler/rustc_incremental/src/persist/load.rs
View File

@ -162,18 +162,16 @@ pub fn load_dep_graph(sess: &Session) -> DepGraphFuture {
for swp in work_products {
let mut all_files_exist = true;
if let Some(ref file_name) = swp.work_product.saved_file {
let path = in_incr_comp_dir_sess(sess, file_name);
if !path.exists() {
all_files_exist = false;
let path = in_incr_comp_dir_sess(sess, &swp.work_product.saved_file);
if !path.exists() {
all_files_exist = false;
if sess.opts.debugging_opts.incremental_info {
eprintln!(
"incremental: could not find file for work \
if sess.opts.debugging_opts.incremental_info {
eprintln!(
"incremental: could not find file for work \
product: {}",
path.display()
);
}
path.display()
);
}
}

9
compiler/rustc_incremental/src/persist/save.rs
View File

@ -107,11 +107,7 @@ pub fn save_work_product_index(
for (id, wp) in previous_work_products.iter() {
if !new_work_products.contains_key(id) {
work_product::delete_workproduct_files(sess, wp);
debug_assert!(
wp.saved_file.as_ref().map_or(true, |file_name| {
!in_incr_comp_dir_sess(sess, &file_name).exists()
})
);
debug_assert!(!in_incr_comp_dir_sess(sess, &wp.saved_file).exists());
}
}
@ -119,8 +115,7 @@ pub fn save_work_product_index(
debug_assert!({
new_work_products
.iter()
.flat_map(|(_, wp)| wp.saved_file.iter())
.map(|name| in_incr_comp_dir_sess(sess, name))
.map(|(_, wp)| in_incr_comp_dir_sess(sess, &wp.saved_file))
.all(|path| path.exists())
});
}

52
compiler/rustc_incremental/src/persist/work_product.rs
View File

@ -7,34 +7,30 @@
use rustc_middle::dep_graph::{WorkProduct, WorkProductId};
use rustc_session::Session;
use std::fs as std_fs;
use std::path::PathBuf;
use std::path::Path;
/// Copies a CGU work product to the incremental compilation directory, so next compilation can find and reuse it.
pub fn copy_cgu_workproduct_to_incr_comp_cache_dir(
sess: &Session,
cgu_name: &str,
path: &Option<PathBuf>,
path: &Path,
) -> Option<(WorkProductId, WorkProduct)> {
debug!("copy_cgu_workproduct_to_incr_comp_cache_dir({:?},{:?})", cgu_name, path);
sess.opts.incremental.as_ref()?;
let saved_file = if let Some(path) = path {
let file_name = format!("{}.o", cgu_name);
let path_in_incr_dir = in_incr_comp_dir_sess(sess, &file_name);
match link_or_copy(path, &path_in_incr_dir) {
Ok(_) => Some(file_name),
Err(err) => {
sess.warn(&format!(
"error copying object file `{}` to incremental directory as `{}`: {}",
path.display(),
path_in_incr_dir.display(),
err
));
return None;
}
let file_name = format!("{}.o", cgu_name);
let path_in_incr_dir = in_incr_comp_dir_sess(sess, &file_name);
let saved_file = match link_or_copy(path, &path_in_incr_dir) {
Ok(_) => file_name,
Err(err) => {
sess.warn(&format!(
"error copying object file `{}` to incremental directory as `{}`: {}",
path.display(),
path_in_incr_dir.display(),
err
));
return None;
}
} else {
None
};
let work_product = WorkProduct { cgu_name: cgu_name.to_string(), saved_file };
@ -45,17 +41,15 @@ pub fn copy_cgu_workproduct_to_incr_comp_cache_dir(
/// Removes files for a given work product.
pub fn delete_workproduct_files(sess: &Session, work_product: &WorkProduct) {
if let Some(ref file_name) = work_product.saved_file {
let path = in_incr_comp_dir_sess(sess, file_name);
match std_fs::remove_file(&path) {
Ok(()) => {}
Err(err) => {
sess.warn(&format!(
"file-system error deleting outdated file `{}`: {}",
path.display(),
err
));
}
let path = in_incr_comp_dir_sess(sess, &work_product.saved_file);
match std_fs::remove_file(&path) {
Ok(()) => {}
Err(err) => {
sess.warn(&format!(
"file-system error deleting outdated file `{}`: {}",
path.display(),
err
));
}
}
}

77
compiler/rustc_middle/src/middle/stability.rs
View File

@ -9,10 +9,9 @@
use rustc_data_structures::fx::FxHashMap;
use rustc_errors::{Applicability, Diagnostic};
use rustc_feature::GateIssue;
use rustc_hir as hir;
use rustc_hir::def::DefKind;
use rustc_hir::def_id::{DefId, LocalDefId};
use rustc_hir::{self, HirId};
use rustc_hir::{self as hir, HirId};
use rustc_middle::ty::print::with_no_trimmed_paths;
use rustc_session::lint::builtin::{DEPRECATED, DEPRECATED_IN_FUTURE, SOFT_UNSTABLE};
use rustc_session::lint::{BuiltinLintDiagnostics, Level, Lint, LintBuffer};
@ -306,6 +305,14 @@ fn suggestion_for_allocator_api(
None
}
/// An override option for eval_stability.
pub enum AllowUnstable {
/// Don't emit an unstable error for the item
Yes,
/// Handle the item normally
No,
}
impl<'tcx> TyCtxt<'tcx> {
/// Evaluates the stability of an item.
///
@ -322,6 +329,28 @@ pub fn eval_stability(
id: Option<HirId>,
span: Span,
method_span: Option<Span>,
) -> EvalResult {
self.eval_stability_allow_unstable(def_id, id, span, method_span, AllowUnstable::No)
}
/// Evaluates the stability of an item.
///
/// Returns `EvalResult::Allow` if the item is stable, or unstable but the corresponding
/// `#![feature]` has been provided. Returns `EvalResult::Deny` which describes the offending
/// unstable feature otherwise.
///
/// If `id` is `Some(_)`, this function will also check if the item at `def_id` has been
/// deprecated. If the item is indeed deprecated, we will emit a deprecation lint attached to
/// `id`.
///
/// Pass `AllowUnstable::Yes` to `allow_unstable` to force an unstable item to be allowed. Deprecation warnings will be emitted normally.
pub fn eval_stability_allow_unstable(
self,
def_id: DefId,
id: Option<HirId>,
span: Span,
method_span: Option<Span>,
allow_unstable: AllowUnstable,
) -> EvalResult {
// Deprecated attributes apply in-crate and cross-crate.
if let Some(id) = id {
@ -419,6 +448,10 @@ pub fn eval_stability(
}
}
if matches!(allow_unstable, AllowUnstable::Yes) {
return EvalResult::Allow;
}
let suggestion = suggestion_for_allocator_api(self, def_id, span, feature);
EvalResult::Deny { feature, reason, issue, suggestion, is_soft }
}
@ -445,11 +478,38 @@ pub fn check_stability(
span: Span,
method_span: Option<Span>,
) {
self.check_optional_stability(def_id, id, span, method_span, |span, def_id| {
// The API could be uncallable for other reasons, for example when a private module
// was referenced.
self.sess.delay_span_bug(span, &format!("encountered unmarked API: {:?}", def_id));
})
self.check_stability_allow_unstable(def_id, id, span, method_span, AllowUnstable::No)
}
/// Checks if an item is stable or error out.
///
/// If the item defined by `def_id` is unstable and the corresponding `#![feature]` does not
/// exist, emits an error.
///
/// This function will also check if the item is deprecated.
/// If so, and `id` is not `None`, a deprecated lint attached to `id` will be emitted.
///
/// Pass `AllowUnstable::Yes` to `allow_unstable` to force an unstable item to be allowed. Deprecation warnings will be emitted normally.
pub fn check_stability_allow_unstable(
self,
def_id: DefId,
id: Option<HirId>,
span: Span,
method_span: Option<Span>,
allow_unstable: AllowUnstable,
) {
self.check_optional_stability(
def_id,
id,
span,
method_span,
allow_unstable,
|span, def_id| {
// The API could be uncallable for other reasons, for example when a private module
// was referenced.
self.sess.delay_span_bug(span, &format!("encountered unmarked API: {:?}", def_id));
},
)
}
/// Like `check_stability`, except that we permit items to have custom behaviour for
@ -462,6 +522,7 @@ pub fn check_optional_stability(
id: Option<HirId>,
span: Span,
method_span: Option<Span>,
allow_unstable: AllowUnstable,
unmarked: impl FnOnce(Span, DefId),
) {
let soft_handler = |lint, span, msg: &_| {
@ -469,7 +530,7 @@ pub fn check_optional_stability(
lint.build(msg).emit();
})
};
match self.eval_stability(def_id, id, span, method_span) {
match self.eval_stability_allow_unstable(def_id, id, span, method_span, allow_unstable) {
EvalResult::Allow => {}
EvalResult::Deny { feature, reason, issue, suggestion, is_soft } => report_unstable(
self.sess,

2
compiler/rustc_middle/src/mir/mono.rs
View File

@ -336,7 +336,7 @@ pub fn work_product_id(&self) -> WorkProductId {
WorkProductId::from_cgu_name(self.name().as_str())
}
pub fn work_product(&self, tcx: TyCtxt<'_>) -> WorkProduct {
pub fn previous_work_product(&self, tcx: TyCtxt<'_>) -> WorkProduct {
let work_product_id = self.work_product_id();
tcx.dep_graph
.previous_work_product(&work_product_id)

40
compiler/rustc_passes/src/stability.rs
View File

@ -9,10 +9,10 @@
use rustc_hir::def_id::{LocalDefId, CRATE_DEF_ID};
use rustc_hir::hir_id::CRATE_HIR_ID;
use rustc_hir::intravisit::{self, Visitor};
use rustc_hir::{FieldDef, Generics, HirId, Item, TraitRef, Ty, TyKind, Variant};
use rustc_hir::{FieldDef, Generics, HirId, Item, ItemKind, TraitRef, Ty, TyKind, Variant};
use rustc_middle::hir::nested_filter;
use rustc_middle::middle::privacy::AccessLevels;
use rustc_middle::middle::stability::{DeprecationEntry, Index};
use rustc_middle::middle::stability::{AllowUnstable, DeprecationEntry, Index};
use rustc_middle::ty::{self, query::Providers, TyCtxt};
use rustc_session::lint;
use rustc_session::lint::builtin::{INEFFECTIVE_UNSTABLE_TRAIT_IMPL, USELESS_DEPRECATED};
@ -807,12 +807,46 @@ fn visit_item(&mut self, item: &'tcx hir::Item<'tcx>) {
fn visit_path(&mut self, path: &'tcx hir::Path<'tcx>, id: hir::HirId) {
if let Some(def_id) = path.res.opt_def_id() {
let method_span = path.segments.last().map(|s| s.ident.span);
self.tcx.check_stability(def_id, Some(id), path.span, method_span)
self.tcx.check_stability_allow_unstable(
def_id,
Some(id),
path.span,
method_span,
if is_unstable_reexport(self.tcx, id) {
AllowUnstable::Yes
} else {
AllowUnstable::No
},
)
}
intravisit::walk_path(self, path)
}
}
/// Check whether a path is a `use` item that has been marked as unstable.
///
/// See issue #94972 for details on why this is a special case
fn is_unstable_reexport<'tcx>(tcx: TyCtxt<'tcx>, id: hir::HirId) -> bool {
// Get the LocalDefId so we can lookup the item to check the kind.
let Some(def_id) = tcx.hir().opt_local_def_id(id) else { return false; };
let Some(stab) = tcx.stability().local_stability(def_id) else {
return false;
};
if stab.level.is_stable() {
// The re-export is not marked as unstable, don't override
return false;
}
// If this is a path that isn't a use, we don't need to do anything special
if !matches!(tcx.hir().item(hir::ItemId { def_id }).kind, ItemKind::Use(..)) {
return false;
}
true
}
struct CheckTraitImplStable<'tcx> {
tcx: TyCtxt<'tcx>,
fully_stable: bool,

1
compiler/rustc_query_system/src/dep_graph/dep_node.rs
View File

@ -164,7 +164,6 @@ pub struct WorkProductId {
impl WorkProductId {
pub fn from_cgu_name(cgu_name: &str) -> WorkProductId {
let mut hasher = StableHasher::new();
cgu_name.len().hash(&mut hasher);
cgu_name.hash(&mut hasher);
WorkProductId { hash: hasher.finish() }
}

2
compiler/rustc_query_system/src/dep_graph/graph.rs
View File

@ -887,7 +887,7 @@ pub(crate) fn next_virtual_depnode_index(&self) -> DepNodeIndex {
pub struct WorkProduct {
pub cgu_name: String,
/// Saved file associated with this CGU.
pub saved_file: Option<String>,
pub saved_file: String,
}
// Index type for `DepNodeData`'s edges.

2
compiler/rustc_typeck/src/astconv/mod.rs
View File

@ -24,6 +24,7 @@
use rustc_hir::intravisit::{walk_generics, Visitor as _};
use rustc_hir::lang_items::LangItem;
use rustc_hir::{GenericArg, GenericArgs};
use rustc_middle::middle::stability::AllowUnstable;
use rustc_middle::ty::subst::{self, GenericArgKind, InternalSubsts, Subst, SubstsRef};
use rustc_middle::ty::GenericParamDefKind;
use rustc_middle::ty::{self, Const, DefIdTree, EarlyBinder, Ty, TyCtxt, TypeFoldable};
@ -426,6 +427,7 @@ fn provided_kind(
Some(arg.id()),
arg.span(),
None,
AllowUnstable::No,
|_, _| {
// Default generic parameters may not be marked
// with stability attributes, i.e. when the

1
library/std/src/sys/unix/process/process_unix.rs
View File

@ -730,6 +730,7 @@ fn signal_string(signal: i32) -> &'static str {
libc::SIGVTALRM => " (SIGVTALRM)",
libc::SIGPROF => " (SIGPROF)",
libc::SIGWINCH => " (SIGWINCH)",
#[cfg(not(target_os = "haiku"))]
libc::SIGIO => " (SIGIO)",
libc::SIGSYS => " (SIGSYS)",
// For information on Linux signals, run `man 7 signal`

30
src/test/ui/stability-attribute/allow-unstable-reexport.rs Normal file
View File

@ -0,0 +1,30 @@
// Allow an unstable re-export without requiring a feature gate.
// #94972
// aux-build:lint-stability.rs
// aux-build:lint-stability-reexport.rs
#![feature(staged_api)]
#![stable(feature = "lint_stability", since = "1.0.0")]
extern crate lint_stability;
extern crate lint_stability_reexport;
#[unstable(feature = "unstable_test_feature", issue = "none")]
pub use lint_stability::unstable;
// We want to confirm that using a re-export through another crate behaves
// the same way as using an item directly
#[unstable(feature = "unstable_test_feature", issue = "none")]
pub use lint_stability_reexport::unstable_text;
// Ensure items which aren't marked as unstable can't re-export unstable items
#[stable(feature = "lint_stability", since = "1.0.0")]
pub use lint_stability::unstable as unstable2;
//~^ ERROR use of unstable library feature 'unstable_test_feature'
fn main() {
// Since we didn't enable the feature in this crate, we still can't
// use these items, even though they're in scope from the `use`s which are now allowed.
unstable(); //~ ERROR use of unstable library feature 'unstable_test_feature'
unstable_text(); //~ ERROR use of unstable library feature 'unstable_test_feature'
}

27
src/test/ui/stability-attribute/allow-unstable-reexport.stderr Normal file
View File

@ -0,0 +1,27 @@
error[E0658]: use of unstable library feature 'unstable_test_feature'
--> $DIR/allow-unstable-reexport.rs:22:9
|
LL | pub use lint_stability::unstable as unstable2;
| ^^^^^^^^^^^^^^^^^^^^^^^^
|
= help: add `#![feature(unstable_test_feature)]` to the crate attributes to enable
error[E0658]: use of unstable library feature 'unstable_test_feature'
--> $DIR/allow-unstable-reexport.rs:28:5
|
LL | unstable();
| ^^^^^^^^
|
= help: add `#![feature(unstable_test_feature)]` to the crate attributes to enable
error[E0658]: use of unstable library feature 'unstable_test_feature': text
--> $DIR/allow-unstable-reexport.rs:29:5
|
LL | unstable_text();
| ^^^^^^^^^^^^^
|
= help: add `#![feature(unstable_test_feature)]` to the crate attributes to enable
error: aborting due to 3 previous errors
For more information about this error, try `rustc --explain E0658`.

9
src/test/ui/stability-attribute/auxiliary/lint-stability-reexport.rs Normal file
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@ -0,0 +1,9 @@
#![crate_type = "lib"]
#![feature(staged_api)]
#![stable(feature = "lint_stability", since = "1.0.0")]
extern crate lint_stability;
// Re-exporting without enabling the feature "unstable_test_feature" in this crate
#[unstable(feature = "unstable_test_feature", issue = "none")]
pub use lint_stability::unstable_text;

28
src/test/ui/unsized/issue-97732.rs Normal file
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// check-pass
#![feature(coerce_unsized)]
// Ensure that unsizing structs that contain ZSTs at non-zero offsets don't ICE
use std::ops::CoerceUnsized;
#[repr(C)]
pub struct BoxWithZstTail<T: ?Sized>(Box<T>, ());
impl<S: ?Sized, T: ?Sized> CoerceUnsized<BoxWithZstTail<T>> for BoxWithZstTail<S> where
Box<S>: CoerceUnsized<Box<T>>
{
}
pub fn noop_dyn_upcast_with_zst_tail(
b: BoxWithZstTail<dyn ToString + Send>,
) -> BoxWithZstTail<dyn ToString> {
b
}
fn main() {
let original = "foo";
let boxed = BoxWithZstTail(Box::new(original) as Box<dyn ToString + Send>, ());
let noop_upcasted = noop_dyn_upcast_with_zst_tail(boxed);
assert_eq!(original, noop_upcasted.0.to_string());
}