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thread_local! doc improvements + tests for operational pedantics #163387

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10 changes: 7 additions & 3 deletions library/std/src/sys/thread_local/native/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -81,9 +81,10 @@ pub macro thread_local_inner {

// used to generate the `LocalKey` value for `thread_local!`
(@key $t:ty, $(#[$align_attr:meta])*, $init:expr) => {{
// We intentionally have an argument-position `'static` lifetime so that elided lifetimes in `$t`
// become `'static` like they do for `const`s and `static`s, including in the other two
// `thread_local!` implementations.
// NOTE: The `PhantomData` here is a nice trick to ensure that *all* usages of `$t` can be
// inferred to have static lifetimes. This function definition would normally *not* be such
// a definition, but because lifetime elision tends to prefer lifetimes matching arguments
// to a function, the `'static` lifetime inside the `PhantomData` qualifies here.
Comment on lines +84 to +87

@RalfJung RalfJung Sep 27, 2026 •
edited by rustbot
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Suggested change
// NOTE: The `PhantomData` here is a nice trick to ensure that *all* usages of `$t` can be
// inferred to have static lifetimes. This function definition would normally *not* be such
// a definition, but because lifetime elision tends to prefer lifetimes matching arguments
// to a function, the `'static` lifetime inside the `PhantomData` qualifies here.
// NOTE: The `PhantomData` here is a nice trick to ensure that *all* usages of `$t` can be
// inferred to have static lifetimes: if `$t` has missing lifetimes, like `&str`,
// then lifetime elision will pick up the only lifetime in the argument,
// `'static`. The we type-check `$init` against that expanded type.

View changes since the review

#[allow(mismatched_lifetime_syntaxes)]
#[inline]
fn __rust_std_internal_init_fn(_lifetime_elision: $crate::marker::PhantomData<&'static ()>) -> $t {
Expand All @@ -92,10 +93,12 @@ pub macro thread_local_inner {

unsafe {
$crate::thread::LocalKey::new(const {
// NOTE: `$t`'s lifetimes can always be inferred in turbofish position
if $crate::mem::needs_drop::<$t>() {
|__rust_std_internal_init| {
#[thread_local]
$(#[$align_attr])*
// NOTE: `$t`'s lifetimes can always be inferred `'static` for `static`s
static __RUST_STD_INTERNAL_VAL: $crate::thread::local_impl::LazyStorage<$t, ()>
= $crate::thread::local_impl::LazyStorage::new();
__RUST_STD_INTERNAL_VAL.get_or_init(__rust_std_internal_init, || __rust_std_internal_init_fn($crate::marker::PhantomData))
Expand All @@ -104,6 +107,7 @@ pub macro thread_local_inner {
|__rust_std_internal_init| {
#[thread_local]
$(#[$align_attr])*
// NOTE: `$t`'s lifetimes can always be inferred `'static` for `static`s
static __RUST_STD_INTERNAL_VAL: $crate::thread::local_impl::LazyStorage<$t, !>
= $crate::thread::local_impl::LazyStorage::new();
__RUST_STD_INTERNAL_VAL.get_or_init(__rust_std_internal_init, || __rust_std_internal_init_fn($crate::marker::PhantomData))
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9 changes: 6 additions & 3 deletions library/std/src/sys/thread_local/os.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -20,9 +20,10 @@ pub macro thread_local_inner {

// used to generate the `LocalKey` value for `thread_local!`.
(@key $t:ty, $($(#[$($align_attr:tt)*])+)?, $init:expr) => {{
// We intentionally have an argument-position `'static` lifetime so that elided lifetimes in `$t`
// become `'static` like they do for `const`s and `static`s, including in the other two
// `thread_local!` implementations.
// NOTE: The `PhantomData` here is a nice trick to ensure that *all* usages of `$t` can be
// inferred to have static lifetimes. This function definition would normally *not* be such
// a definition, but because lifetime elision tends to prefer lifetimes matching arguments
// to a function, the `'static` lifetime inside the `PhantomData` qualifies here.
#[allow(mismatched_lifetime_syntaxes)]
#[inline]
fn __rust_std_internal_init_fn(_lifetime_elision: $crate::marker::PhantomData<&'static ()>) -> $t { $init }
Expand All @@ -32,6 +33,7 @@ pub macro thread_local_inner {
// in `tests/thread.rs` if these types are renamed.
unsafe {
$crate::thread::LocalKey::new(|__rust_std_internal_init| {
// NOTE: `$t`'s lifetimes can always be inferred `'static` for `static`s
static __RUST_STD_INTERNAL_VAL: $crate::thread::local_impl::Storage<$t, {
$({
// Ensure that attributes have valid syntax
Expand All @@ -42,6 +44,7 @@ pub macro thread_local_inner {
})?

#[allow(unused_mut)]
// NOTE: `$t`'s lifetimes can always be inferred in turbofish position
let mut final_align = $crate::thread::local_impl::value_align::<$t>();
$($($crate::thread::local_impl::thread_local_inner!(@align final_align, $($align_attr)*);)+)?
final_align
Expand Down
18 changes: 18 additions & 0 deletions tests/ui/thread-local/non-static-lifetimes.rs
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@clarfonthey clarfonthey Sep 26, 2026 •
edited by rustbot
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cc @RalfJung since you mentioned having tests for this. Admittedly, I kind of struggled to come up with anything for this that might be even remotely sensible (and these aren't that), but, please feel free to comment whether this is satisfactory.

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@RalfJung RalfJung Sep 27, 2026

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Yeah that's fair, I wasn't sure either. It just felt like something we should test for.

Original file line number Diff line number Diff line change
@@ -0,0 +1,18 @@
use std::borrow::Cow;

fn main() {
let x = "";
std::thread_local!(static A: &str = x);
//~^ ERROR attempt to use a non-constant value in a constant [E0435]
}

fn test<'a>(x: &'a str) {
std::thread_local!(static A: Cow<'a, str> = Cow::Borrowed(""));
//~^ ERROR can't use generic parameters from outer item [E0401]
//~| ERROR can't use generic parameters from outer item [E0401]
//~| ERROR can't use generic parameters from outer item [E0401]
//~| ERROR can't use generic parameters from outer item [E0401]
//~| ERROR can't use generic parameters from outer item [E0401]

// wow, aren't macros great?
}
61 changes: 61 additions & 0 deletions tests/ui/thread-local/non-static-lifetimes.stderr
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Original file line number Diff line number Diff line change
@@ -0,0 +1,61 @@
error[E0435]: attempt to use a non-constant value in a constant
--> $DIR/non-static-lifetimes.rs:5:41
|
LL | std::thread_local!(static A: &str = x);
| - ^
| |
| this would need to be a `let`

error[E0401]: can't use generic parameters from outer item
--> $DIR/non-static-lifetimes.rs:10:38
|
LL | fn test<'a>(x: &'a str) {
| -- lifetime parameter from outer item
LL | std::thread_local!(static A: Cow<'a, str> = Cow::Borrowed(""));
| ^^ use of generic parameter from outer item

error[E0401]: can't use generic parameters from outer item
--> $DIR/non-static-lifetimes.rs:10:38
|
LL | fn test<'a>(x: &'a str) {
| -- lifetime parameter from outer item
LL | std::thread_local!(static A: Cow<'a, str> = Cow::Borrowed(""));
| ---------------------------------^^---------------------------
| | |
| | use of generic parameter from outer item
| lifetime `'a` is missing in item created through this procedural macro

error[E0401]: can't use generic parameters from outer item
--> $DIR/non-static-lifetimes.rs:10:38
|
LL | fn test<'a>(x: &'a str) {
| -- lifetime parameter from outer item
LL | std::thread_local!(static A: Cow<'a, str> = Cow::Borrowed(""));
| ^^ use of generic parameter from outer item
|
= note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`

error[E0401]: can't use generic parameters from outer item
--> $DIR/non-static-lifetimes.rs:10:38
|
LL | fn test<'a>(x: &'a str) {
| -- lifetime parameter from outer item
LL | std::thread_local!(static A: Cow<'a, str> = Cow::Borrowed(""));
| ^^ use of generic parameter from outer item
|
= note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`

error[E0401]: can't use generic parameters from outer item
--> $DIR/non-static-lifetimes.rs:10:38
|
LL | fn test<'a>(x: &'a str) {
| -- lifetime parameter from outer item
LL | std::thread_local!(static A: Cow<'a, str> = Cow::Borrowed(""));
| ^^ use of generic parameter from outer item
|
= note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`

error: aborting due to 6 previous errors

Some errors have detailed explanations: E0401, E0435.
For more information about an error, try `rustc --explain E0401`.
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