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Plus a tiny bit of reformatting.
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Stabilize `unsafe_attributes`
# Stabilization report
## Summary
This is a tracking issue for the RFC 3325: unsafe attributes
We are stabilizing `#![feature(unsafe_attributes)]`, which makes certain attributes considered 'unsafe', meaning that they must be surrounded by an `unsafe(...)`, as in `#[unsafe(no_mangle)]`.
RFC: rust-lang/rfcs#3325
Tracking issue: #123757
## What is stabilized
### Summary of stabilization
Certain attributes will now be designated as unsafe attributes, namely, `no_mangle`, `export_name`, and `link_section` (stable only), and these attributes will need to be called by surrounding them in `unsafe(...)` syntax. On editions prior to 2024, this is simply an edition lint, but it will become a hard error in 2024. This also works in `cfg_attr`, but `unsafe` is not allowed for any other attributes, including proc-macros ones.
```rust
#[unsafe(no_mangle)]
fn a() {}
#[cfg_attr(any(), unsafe(export_name = "c"))]
fn b() {}
```
For a table showing the attributes that were considered to be included in the list to require unsafe, and subsequent reasoning about why each such attribute was or was not included, see [this comment here](https://github.com/rust-lang/rust/pull/124214#issuecomment-2124753464)
## Tests
The relevant tests are in `tests/ui/rust-2024/unsafe-attributes` and `tests/ui/attributes/unsafe`.
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This lets us compare a `Token` with a `TokenKind`. It's used a lot, but
can be used even more, avoiding the need for some `.kind` uses.
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Get rid of some `#[allow(rustc::untranslatable_diagnostic)]`
`@rustbot` label +A-translation
cc https://github.com/rust-lang/rust/issues/100717
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Use more slice patterns inside the compiler
Nothing super noteworthy. Just replacing the common 'fragile' pattern of "length check followed by indexing or unwrap" with slice patterns for legibility and 'robustness'.
r? ghost
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This makes it possible for the `unsafe(...)` syntax to only be
valid at the top level, and the `NestedMetaItem`s will automatically
reject `unsafe(...)`.
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Tell users not to file a bug when using internal library features
Actually fixes #97501. I don't think we should suppress the suggestion to add `#![feature(..)]`, though I guess I could be convinced otherwise.
r? `@Nilstrieb` cc `@RalfJung`
Didn't add a test b/c I don't think we test this for lang features either, but I can confirm it does work.
```
warning: the feature `core_intrinsics` is internal to the compiler or standard library
--> /home/michael/test.rs:1:12
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1 | #![feature(core_intrinsics)]
| ^^^^^^^^^^^^^^^
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= note: using it is strongly discouraged
= note: `#[warn(internal_features)]` on by default
thread 'rustc' panicked at compiler/rustc_mir_transform/src/validate.rs:94:25:
broken MIR in Item(DefId(0:6 ~ test[42db]::{impl#0}::add)) (after phase change to runtime-optimized) at bb0[0]:
Cannot perform arithmetic Add on type WrapInt8
stack backtrace:
0: begin_panic_handler
at ./library/std/src/panicking.rs:665:5
1: panic_fmt
at ./library/core/src/panicking.rs:74:14
2: fail<alloc::string::String>
at ./compiler/rustc_mir_transform/src/validate.rs:146:9
3: run_pass
at ./compiler/rustc_mir_transform/src/validate.rs:94:13
4: validate_body
at ./compiler/rustc_mir_transform/src/pass_manager.rs:193:5
5: run_passes_inner
at ./compiler/rustc_mir_transform/src/pass_manager.rs:176:13
6: rustc_mir_transform::pass_manager::run_passes
at ./compiler/rustc_mir_transform/src/pass_manager.rs:87:5
7: run_optimization_passes
at ./compiler/rustc_mir_transform/src/lib.rs:561:5
8: inner_optimized_mir
at ./compiler/rustc_mir_transform/src/lib.rs:667:5
9: optimized_mir
at ./compiler/rustc_mir_transform/src/lib.rs:630:21
10: {closure#0}
at ./compiler/rustc_query_impl/src/plumbing.rs:285:13
[... omitted 22 frames ...]
11: query_get_at<rustc_query_system::query::caches::DefIdCache<rustc_middle::query::erase::Erased<[u8; 8]>>>
at ./compiler/rustc_middle/src/query/plumbing.rs:145:17
12: instance_mir
13: collect_items_of_instance
at ./compiler/rustc_monomorphize/src/collector.rs:1203:16
14: {closure#0}
at ./compiler/rustc_monomorphize/src/collector.rs:447:17
15: maybe_grow<(), rustc_monomorphize::collector::collect_items_rec::{closure_env#0}>
at /home/michael/.cargo/registry/src/index.crates.io-6f17d22bba15001f/stacker-0.1.15/src/lib.rs:55:9
16: ensure_sufficient_stack<(), rustc_monomorphize::collector::collect_items_rec::{closure_env#0}>
at ./compiler/rustc_data_structures/src/stack.rs:17:5
17: collect_items_rec
at ./compiler/rustc_monomorphize/src/collector.rs:446:13
18: collect_items_rec
at ./compiler/rustc_monomorphize/src/collector.rs:526:13
19: {closure#0}
at ./compiler/rustc_monomorphize/src/collector.rs:1597:17
20: {closure#0}<rustc_middle::mir::mono::MonoItem, alloc::vec::Vec<rustc_middle::mir::mono::MonoItem, alloc::alloc::Global>, rustc_monomorphize::collector::collect_crate_mono_items::{closure#1}::{closure_env#0}>
at ./compiler/rustc_data_structures/src/sync/parallel.rs:182:34
21: call_once<(), rustc_data_structures::sync::parallel::enabled::par_for_each_in::{closure#0}::{closure#0}::{closure_env#0}<rustc_middle::mir::mono::MonoItem, alloc::vec::Vec<rustc_middle::mir::mono::MonoItem, alloc::alloc::Global>, rustc_monomorphize::collector::collect_crate_mono_items::{closure#1}::{closure_env#0}>>
at ./library/core/src/panic/unwind_safe.rs:272:9
22: do_call<core::panic::unwind_safe::AssertUnwindSafe<rustc_data_structures::sync::parallel::enabled::par_for_each_in::{closure#0}::{closure#0}::{closure_env#0}<rustc_middle::mir::mono::MonoItem, alloc::vec::Vec<rustc_middle::mir::mono::MonoItem, alloc::alloc::Global>, rustc_monomorphize::collector::collect_crate_mono_items::{closure#1}::{closure_env#0}>>, ()>
at ./library/std/src/panicking.rs:557:40
23: try<(), core::panic::unwind_safe::AssertUnwindSafe<rustc_data_structures::sync::parallel::enabled::par_for_each_in::{closure#0}::{closure#0}::{closure_env#0}<rustc_middle::mir::mono::MonoItem, alloc::vec::Vec<rustc_middle::mir::mono::MonoItem, alloc::alloc::Global>, rustc_monomorphize::collector::collect_crate_mono_items::{closure#1}::{closure_env#0}>>>
at ./library/std/src/panicking.rs:521:19
24: run<(), rustc_data_structures::sync::parallel::enabled::par_for_each_in::{closure#0}::{closure#1}::{closure_env#0}<rustc_middle::mir::mono::MonoItem, alloc::vec::Vec<rustc_middle::mir::mono::MonoItem, alloc::alloc::Global>, rustc_monomorphize::collector::collect_crate_mono_items::{closure#1}::{closure_env#0}>>
at ./compiler/rustc_data_structures/src/sync/parallel.rs:28:9
25: {closure#1}<rustc_middle::mir::mono::MonoItem, alloc::vec::Vec<rustc_middle::mir::mono::MonoItem, alloc::alloc::Global>, rustc_monomorphize::collector::collect_crate_mono_items::{closure#1}::{closure_env#0}>
at ./compiler/rustc_data_structures/src/sync/parallel.rs:186:21
26: {closure#0}<rustc_middle::mir::mono::MonoItem, rustc_data_structures::sync::parallel::enabled::par_for_each_in::{closure#0}::{closure_env#1}<rustc_middle::mir::mono::MonoItem, alloc::vec::Vec<rustc_middle::mir::mono::MonoItem, alloc::alloc::Global>, rustc_monomorphize::collector::collect_crate_mono_items::{closure#1}::{closure_env#0}>>
at ./library/core/src/iter/traits/iterator.rs:815:29
27: fold<rustc_middle::mir::mono::MonoItem, alloc::alloc::Global, (), core::iter::traits::iterator::Iterator::for_each::call::{closure_env#0}<rustc_middle::mir::mono::MonoItem, rustc_data_structures::sync::parallel::enabled::par_for_each_in::{closure#0}::{closure_env#1}<rustc_middle::mir::mono::MonoItem, alloc::vec::Vec<rustc_middle::mir::mono::MonoItem, alloc::alloc::Global>, rustc_monomorphize::collector::collect_crate_mono_items::{closure#1}::{closure_env#0}>>>
at ./library/alloc/src/vec/into_iter.rs:317:25
28: for_each<alloc::vec::into_iter::IntoIter<rustc_middle::mir::mono::MonoItem, alloc::alloc::Global>, rustc_data_structures::sync::parallel::enabled::par_for_each_in::{closure#0}::{closure_env#1}<rustc_middle::mir::mono::MonoItem, alloc::vec::Vec<rustc_middle::mir::mono::MonoItem, alloc::alloc::Global>, rustc_monomorphize::collector::collect_crate_mono_items::{closure#1}::{closure_env#0}>>
at ./library/core/src/iter/traits/iterator.rs:818:9
29: {closure#0}<rustc_middle::mir::mono::MonoItem, alloc::vec::Vec<rustc_middle::mir::mono::MonoItem, alloc::alloc::Global>, rustc_monomorphize::collector::collect_crate_mono_items::{closure#1}::{closure_env#0}>
at ./compiler/rustc_data_structures/src/sync/parallel.rs:185:17
30: parallel_guard<(), rustc_data_structures::sync::parallel::enabled::par_for_each_in::{closure_env#0}<rustc_middle::mir::mono::MonoItem, alloc::vec::Vec<rustc_middle::mir::mono::MonoItem, alloc::alloc::Global>, rustc_monomorphize::collector::collect_crate_mono_items::{closure#1}::{closure_env#0}>>
at ./compiler/rustc_data_structures/src/sync/parallel.rs:44:15
31: par_for_each_in<rustc_middle::mir::mono::MonoItem, alloc::vec::Vec<rustc_middle::mir::mono::MonoItem, alloc::alloc::Global>, rustc_monomorphize::collector::collect_crate_mono_items::{closure#1}::{closure_env#0}>
at ./compiler/rustc_data_structures/src/sync/parallel.rs:178:9
32: {closure#1}
at ./compiler/rustc_monomorphize/src/collector.rs:1595:13
33: run<(), rustc_monomorphize::collector::collect_crate_mono_items::{closure_env#1}>
at ./compiler/rustc_data_structures/src/profiling.rs:754:9
34: time<(), rustc_monomorphize::collector::collect_crate_mono_items::{closure_env#1}>
at ./compiler/rustc_session/src/utils.rs:16:9
35: collect_crate_mono_items
at ./compiler/rustc_monomorphize/src/collector.rs:1594:9
36: collect_and_partition_mono_items
at ./compiler/rustc_monomorphize/src/partitioning.rs:1124:30
37: {closure#0}
at ./compiler/rustc_query_impl/src/plumbing.rs:281:9
[... omitted 22 frames ...]
38: query_get_at<rustc_query_system::query::caches::SingleCache<rustc_middle::query::erase::Erased<[u8; 24]>>>
at ./compiler/rustc_middle/src/query/plumbing.rs:145:17
39: collect_and_partition_mono_items
at ./compiler/rustc_middle/src/query/plumbing.rs:423:31
40: collect_and_partition_mono_items
at ./compiler/rustc_middle/src/query/plumbing.rs:414:17
41: codegen_crate<rustc_codegen_llvm::LlvmCodegenBackend>
at ./compiler/rustc_codegen_ssa/src/base.rs:596:25
42: codegen_crate
at ./compiler/rustc_codegen_llvm/src/lib.rs:361:18
43: {closure#0}
at ./compiler/rustc_interface/src/passes.rs:1027:9
44: run<alloc::boxed::Box<dyn core::any::Any, alloc::alloc::Global>, rustc_interface::passes::start_codegen::{closure_env#0}>
at ./compiler/rustc_data_structures/src/profiling.rs:754:9
45: time<alloc::boxed::Box<dyn core::any::Any, alloc::alloc::Global>, rustc_interface::passes::start_codegen::{closure_env#0}>
at ./compiler/rustc_session/src/utils.rs:16:9
46: start_codegen
at ./compiler/rustc_interface/src/passes.rs:1026:19
47: codegen_and_build_linker
at ./compiler/rustc_interface/src/queries.rs:128:31
48: {closure#6}
at ./compiler/rustc_driver_impl/src/lib.rs:451:25
49: {closure#1}<rustc_driver_impl::run_compiler::{closure#0}::{closure#1}::{closure_env#6}, core::result::Result<core::option::Option<rustc_interface::queries::Linker>, rustc_span::ErrorGuaranteed>>
at ./compiler/rustc_middle/src/ty/context.rs:1336:37
50: {closure#0}<rustc_middle::ty::context::{impl#19}::enter::{closure_env#1}<rustc_driver_impl::run_compiler::{closure#0}::{closure#1}::{closure_env#6}, core::result::Result<core::option::Option<rustc_interface::queries::Linker>, rustc_span::ErrorGuaranteed>>, core::result::Result<core::option::Option<rustc_interface::queries::Linker>, rustc_span::ErrorGuaranteed>>
at ./compiler/rustc_middle/src/ty/context/tls.rs:82:9
51: try_with<core::cell::Cell<*const ()>, rustc_middle::ty::context::tls::enter_context::{closure_env#0}<rustc_middle::ty::context::{impl#19}::enter::{closure_env#1}<rustc_driver_impl::run_compiler::{closure#0}::{closure#1}::{closure_env#6}, core::result::Result<core::option::Option<rustc_interface::queries::Linker>, rustc_span::ErrorGuaranteed>>, core::result::Result<core::option::Option<rustc_interface::queries::Linker>, rustc_span::ErrorGuaranteed>>, core::result::Result<core::option::Option<rustc_interface::queries::Linker>, rustc_span::ErrorGuaranteed>>
at ./library/std/src/thread/local.rs:283:12
52: with<core::cell::Cell<*const ()>, rustc_middle::ty::context::tls::enter_context::{closure_env#0}<rustc_middle::ty::context::{impl#19}::enter::{closure_env#1}<rustc_driver_impl::run_compiler::{closure#0}::{closure#1}::{closure_env#6}, core::result::Result<core::option::Option<rustc_interface::queries::Linker>, rustc_span::ErrorGuaranteed>>, core::result::Result<core::option::Option<rustc_interface::queries::Linker>, rustc_span::ErrorGuaranteed>>, core::result::Result<core::option::Option<rustc_interface::queries::Linker>, rustc_span::ErrorGuaranteed>>
at ./library/std/src/thread/local.rs:260:9
53: enter_context<rustc_middle::ty::context::{impl#19}::enter::{closure_env#1}<rustc_driver_impl::run_compiler::{closure#0}::{closure#1}::{closure_env#6}, core::result::Result<core::option::Option<rustc_interface::queries::Linker>, rustc_span::ErrorGuaranteed>>, core::result::Result<core::option::Option<rustc_interface::queries::Linker>, rustc_span::ErrorGuaranteed>>
at ./compiler/rustc_middle/src/ty/context/tls.rs:79:5
54: enter<rustc_driver_impl::run_compiler::{closure#0}::{closure#1}::{closure_env#6}, core::result::Result<core::option::Option<rustc_interface::queries::Linker>, rustc_span::ErrorGuaranteed>>
at ./compiler/rustc_middle/src/ty/context.rs:1336:9
55: <rustc_interface::queries::QueryResult<&rustc_middle::ty::context::GlobalCtxt>>::enter::<core::result::Result<core::option::Option<rustc_interface::queries::Linker>, rustc_span::ErrorGuaranteed>, rustc_driver_impl::run_compiler::{closure#0}::{closure#1}::{closure#6}>
at ./compiler/rustc_interface/src/queries.rs:64:9
56: {closure#1}
at ./compiler/rustc_driver_impl/src/lib.rs:450:13
57: enter<rustc_driver_impl::run_compiler::{closure#0}::{closure_env#1}, core::result::Result<core::option::Option<rustc_interface::queries::Linker>, rustc_span::ErrorGuaranteed>>
at ./compiler/rustc_interface/src/queries.rs:209:19
58: {closure#0}
at ./compiler/rustc_driver_impl/src/lib.rs:388:22
59: {closure#1}<core::result::Result<(), rustc_span::ErrorGuaranteed>, rustc_driver_impl::run_compiler::{closure_env#0}>
at ./compiler/rustc_interface/src/interface.rs:502:27
60: {closure#0}<rustc_interface::interface::run_compiler::{closure_env#1}<core::result::Result<(), rustc_span::ErrorGuaranteed>, rustc_driver_impl::run_compiler::{closure_env#0}>, core::result::Result<(), rustc_span::ErrorGuaranteed>>
at ./compiler/rustc_interface/src/util.rs:154:13
61: {closure#0}<rustc_interface::util::run_in_thread_pool_with_globals::{closure_env#0}<rustc_interface::interface::run_compiler::{closure_env#1}<core::result::Result<(), rustc_span::ErrorGuaranteed>, rustc_driver_impl::run_compiler::{closure_env#0}>, core::result::Result<(), rustc_span::ErrorGuaranteed>>, core::result::Result<(), rustc_span::ErrorGuaranteed>>
at ./compiler/rustc_interface/src/util.rs:106:21
62: set<rustc_span::SessionGlobals, rustc_interface::util::run_in_thread_with_globals::{closure#0}::{closure#0}::{closure_env#0}<rustc_interface::util::run_in_thread_pool_with_globals::{closure_env#0}<rustc_interface::interface::run_compiler::{closure_env#1}<core::result::Result<(), rustc_span::ErrorGuaranteed>, rustc_driver_impl::run_compiler::{closure_env#0}>, core::result::Result<(), rustc_span::ErrorGuaranteed>>, core::result::Result<(), rustc_span::ErrorGuaranteed>>, core::result::Result<(), rustc_span::ErrorGuaranteed>>
at /home/michael/.cargo/registry/src/index.crates.io-6f17d22bba15001f/scoped-tls-1.0.1/src/lib.rs:137:9
63: create_session_globals_then<core::result::Result<(), rustc_span::ErrorGuaranteed>, rustc_interface::util::run_in_thread_with_globals::{closure#0}::{closure#0}::{closure_env#0}<rustc_interface::util::run_in_thread_pool_with_globals::{closure_env#0}<rustc_interface::interface::run_compiler::{closure_env#1}<core::result::Result<(), rustc_span::ErrorGuaranteed>, rustc_driver_impl::run_compiler::{closure_env#0}>, core::result::Result<(), rustc_span::ErrorGuaranteed>>, core::result::Result<(), rustc_span::ErrorGuaranteed>>>
at ./compiler/rustc_span/src/lib.rs:134:5
64: {closure#0}<rustc_interface::util::run_in_thread_pool_with_globals::{closure_env#0}<rustc_interface::interface::run_compiler::{closure_env#1}<core::result::Result<(), rustc_span::ErrorGuaranteed>, rustc_driver_impl::run_compiler::{closure_env#0}>, core::result::Result<(), rustc_span::ErrorGuaranteed>>, core::result::Result<(), rustc_span::ErrorGuaranteed>>
at ./compiler/rustc_interface/src/util.rs:105:17
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.
error: the compiler unexpectedly panicked. this is a bug.
note: using internal features is not supported and expected to cause internal compiler errors when used incorrectly
note: rustc 1.82.0-dev running on x86_64-unknown-linux-gnu
query stack during panic:
#0 [optimized_mir] optimizing MIR for `<impl at /home/michael/test.rs:9:1: 9:32>::add`
#1 [collect_and_partition_mono_items] collect_and_partition_mono_items
end of query stack
```
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The previous commit updated `rustfmt.toml` appropriately. This commit is
the outcome of running `x fmt --all` with the new formatting options.
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`missing_fragment_specifier` has been a future compatibility warning
since 2017. Uplifting it to an unconditional hard error was attempted in
2020, but eventually reverted due to fallout.
Make it an error only in edition >= 2024, leaving the lint for older
editions. This change will make it easier to support more macro syntax
that relies on usage of `$`.
Fixes <https://github.com/rust-lang/rust/issues/40107>
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Improve spans on evaluated `cfg_attr`s.
When converting something like `#![cfg_attr(cond, attr)]` into `#![attr]`, we currently duplicate the `#` token and the `!` token. But weirdly, there is also this comment:
// We don't really have a good span to use for the synthesized `[]`
// in `#[attr]`, so just use the span of the `#` token.
Maybe that comment used to be true? But now it is false: we can duplicate the existing delimiters (and their spans and spacing), much like we do for the `#` and `!`.
This commit does that, thus removing the incorrect comment, and improving the spans on `Group`s in a few proc-macro tests.
`@petrochenkov`
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When converting something like `#![cfg_attr(cond, attr)]` into
`#![attr]`, we currently duplicate the `#` token and the `!` token. But
weirdly, there is also this comment:
// We don't really have a good span to use for the synthesized `[]`
// in `#[attr]`, so just use the span of the `#` token.
Maybe that comment used to be true? But now it is false: we can
duplicate the existing delimiters (and their spans and spacing), much
like we do for the `#` and `!`.
This commit does that, thus removing the incorrect comment, and
improving the spans on `Group`s in a few proc-macro tests.
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function, just like with immutable visitors
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[`macro_metavar_expr_concat`] Add support for literals
Adds support for literals in macro parameters.
```rust
macro_rules! with_literal {
($literal:literal) => {
const ${concat(FOO, $literal)}: i32 = 1;
}
}
fn main() {
with_literal!("_BAR");
assert_eq!(FOO_BAR, 1);
}
```
cc #124225
r? ``@petrochenkov``
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More attribute cleanups
A follow-up to #127308.
r? ```@petrochenkov```
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Explaining things that took me some time to work out.
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This commit does the following.
- Pulls the code out of `AttrTokenStream::to_token_trees` into a new
function `attrs_and_tokens_to_token_trees`.
- Simplifies `TokenStream::from_ast` by calling the new function. This
is nicer than the old way, which created a temporary
`AttrTokenStream` containing a single `AttrsTarget` (which required
some cloning) just to call `to_token_trees` on it. (It is good to
remove this use of `AttrsTarget` which isn't related to `cfg_attr`
expansion.)
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In various functions where the attribute being processed is known to be
a `#[cfg_attr(...)]` attribute. I find this a helpful reminder.
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Returning `Vec<TokenTree>` works better for the call sites than
returning `TokenStream`.
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[`macro_metavar_expr_concat`] Add support for literals
Adds support for things like `${concat($variable, 123)}` or `${concat("hello", "_world")}` .
cc #124225
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All the branches produce either zero or one elements.
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- `AttributesData` -> `AttrsTarget`
- `AttrTokenTree::Attributes` -> `AttrTokenTree::AttrsTarget`
- `FlatToken::AttrTarget` -> `FlatToken::AttrsTarget`
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To distinguish it from the `HasTokens` method.
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The number of source code bytes can't exceed a `u32`'s range, so a token
position also can't. This reduces the size of `Parser` and
`LazyAttrTokenStreamImpl` by eight bytes each.
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Some `Nonterminal` removal precursors
Small things to prepare for #124141, more or less.
r? ```@oli-obk```
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Just some extra sanity checking, making explicit some values not
possible in code working with token trees -- we shouldn't be seeing
explicit delimiter tokens, because they should be represented as
`TokenTree::Delimited`.
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Add hard error and migration lint for unsafe attrs
More implementation work for https://github.com/rust-lang/rust/issues/123757
This adds the migration lint for unsafe attributes, as well as making it a hard error in Rust 2024.
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Rework pattern and expression nonterminal kinds.
Some tweaks to `NonterminalKind` that will assist with #124141. Details in the individual commits.
r? compiler-errors
cc ```@eholk```
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Merge `PatParam`/`PatWithOr`, and `Expr`/`Expr2021`, for a few reasons.
- It's conceptually nice, because the two pattern kinds and the two
expression kinds are very similar.
- With expressions in particular, there are several places where both
expression kinds get the same treatment.
- It removes one unreachable match arm.
- Most importantly, for #124141 I will need to introduce a new type
`MetaVarKind` that is very similar to `NonterminalKind`, but records a
couple of extra fields for expression metavars. It's nicer to have a
single `MetaVarKind::Expr` expression variant to hold those extra
fields instead of duplicating them across two variants
`MetaVarKind::{Expr,Expr2021}`. And then it makes sense for patterns
to be treated the same way, and for `NonterminalKind` to also be
treated the same way.
I also clarified the comments, because I have long found them a little
hard to understand.
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So we can omit the `token::` qualifier, which gives more space to some
cramped code.
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Make edition dependent `:expr` macro fragment act like the edition-dependent `:pat` fragment does
Parse the `:expr` fragment as `:expr_2021` in editions <=2021, and as `:expr` in edition 2024. This is similar to how we parse `:pat` as `:pat_param` in edition <=2018 and `:pat_with_or` in >=2021, and means we can get rid of a span dependency from `nonterminal_may_begin_with`.
Specifically, this fixes a theoretical regression since the `expr_2021` macro fragment previously would allow `const {}` if the *caller* is edition 2024. This is inconsistent with the way that the `pat` macro fragment was upgraded, and also leads to surprising behavior when a macro *caller* crate upgrades to edtion 2024, since they may have parsing changes that they never asked for (with no way of opting out of it).
This PR also allows using `expr_2021` in all editions. Why was this was disallowed in the first place? It's purely additive, and also it's still feature gated?
r? ```@fmease``` ```@eholk``` cc ```@vincenzopalazzo```
cc #123865
Tracking:
- https://github.com/rust-lang/rust/issues/123742
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