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document Rust 1.90 changes #580

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6cecf63
document Rust 1.90 changes
tshepang Oct 10, 2025
f669cc9
bump version
tshepang Oct 10, 2025
b874b5d
the restriction was not documented
tshepang Oct 13, 2025
a9fdd2d
remove an obsolete restriction
tshepang Oct 16, 2025
af19a30
restore an adjusted restriction
tshepang Oct 18, 2025
47ce8fd
that equivalent paragraph was actually moved to a new place
tshepang Oct 18, 2025
00c1bfd
add restrictions on constant expressions
tshepang Oct 18, 2025
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21 changes: 21 additions & 0 deletions src/changelog.rst
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Expand Up @@ -18,6 +18,23 @@ with the change that has been applied due to it.
just the language changes that had an impact to the FLS. See the `release
notes`_ for a full list of changes.

Language changes in Rust 1.90.0
-------------------------------

- `Split up the unknown_or_malformed_diagnostic_attributes lint <https://github.com/rust-lang/rust/pull/140717>`_

- No change: lints are not part of the FLS

- `Allow constants whose final value has references to mutable/external memory, but reject such constants as patterns <https://github.com/rust-lang/rust/pull/140942>`_

- This lifted restriction was not specified in the FLS
- The restriction on patterns is documented in :p:`fls_wJ9f906BlBvg`
- New paragraph: :p:`fls_zyuxqty09SDO`

- `Allow volatile access to non-Rust memory, including address 0 <https://github.com/rust-lang/rust/pull/141260>`_

- No change: lints are not part of the FLS

Language changes in Rust 1.89.0
-------------------------------

Expand Down Expand Up @@ -317,6 +334,10 @@ Language changes in Rust 1.83.0

* Changed paragraphs: :p:`fls_to4e7imq2c0w`, :p:`fls_6g7c1kjrmfnr`, :p:`fls_hkbwa8xx2fwx`

* New paragraph: :p:`fls_ooOYxhVh8hZo`

* Removed paragraph: :p:`fls_ox6sgl9n43g2`

* `Allow creating references to statics in \`const\` initializers. <https://github.com/rust-lang/rust/pull/129759>`_

* No change: This previous restriction is not specified in the FLS
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8 changes: 5 additions & 3 deletions src/expressions.rst
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Expand Up @@ -312,9 +312,11 @@ A :t:`constant context` is a :t:`construct` that requires a
* :dp:`fls_ucFupTeCyylb`
The :t:`block expression` of a :t:`const block expression`.

:dp:`fls_ox6sgl9n43g2`
It is a static error to create a :t:`mutable reference` in a
:t:`constant context`.
* :dp:`fls_zyuxqty09SDO`
All forms of :t:`[borrow]s` except those of expressions that would be subject to
:t:`drop scope extension`,
and which are either :t:`[mutable borrow]s`
or borrows of expressions that result in values with :t:`interior mutability`.
Comment on lines +315 to +319
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@traviscross traviscross Oct 18, 2025 •
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Two things. One, it's OK for the expression to be subject to drop scope extension. It's not OK for that extension to extend the scope outside of the initializer to the end of the program.

const _: () = { let x = { &mut 0u8 }; x; }; //~ OK
//                             ^^^
// The drop scope of this temporary is extended.

Two, the comma in "..., and which are either..." causes me to read it as a clause independent from "except...". I.e., this says (applying commutativity) "all forms of borrows 1) that are either mutable or result in values with interior mutability and 2) where the expression would not be subject to drop scope extension".


:dp:`fls_od0h3v40kjp6`
An invocation of the :std:`core::ptr::addr_of` :t:`macro` expands to a
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6 changes: 6 additions & 0 deletions src/patterns.rst
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Expand Up @@ -370,6 +370,12 @@ When a :t:`path pattern` refers to an :t:`associated constant` or a
:t:`constant`, the :t:`type` of the :t:`associated constant` or :t:`constant`
shall be :t:`structurally equal`.

:dp:`fls_wJ9f906BlBvg`
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Adding some traceability here to the Reference PR portion which means the FLS need be updated.

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what is meant by "need be updated"

When a :t:`path pattern` refers to an :t:`associated constant` or a
:t:`constant`, the :t:`constant` must not contain any references to
:t:`[mutable static]s`, :t:`[static]s` with :t:`interior mutability`,
or :t:`[external static]s`.

:dp:`fls_hF19K8sWU8FF`
When the type of the :t:`path pattern` is of an :t:`enum type` or
:t:`struct type`, then the :t:`enum type` or :t:`struct type` shall be subject
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3 changes: 3 additions & 0 deletions src/values.rst
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Expand Up @@ -95,6 +95,9 @@ The :t:`type specification` of a :t:`constant` shall have ``'static``
The :t:`type` of a :t:`constant` shall implement the :std:`core::marker::Sized`
:t:`trait`.

:dp:`fls_ooOYxhVh8hZo`
The type of a :t:`constant` cannot be a :t:`mutable reference type`.

Comment on lines +98 to +100
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Two things. One, even with the other rule, this needs to talk about containing a mutable reference rather than being one.

Two, it's not enough to talk about types here. We actually do this reasoning by value, not by type.

trait Tr {}
impl<T: ?Sized> Tr for T {}
static mut X: u8 = 0;
const _: &dyn Tr = unsafe { &&mut X }; //~ ERROR
//       ^^^^^^^
// This type does not contain any mutable references.

:dp:`fls_ndmfqxjpvsqy`
A :t:`constant initializer` is a :t:`construct` that provides the :t:`value` of
its related :t:`constant`.
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2 changes: 1 addition & 1 deletion version.rst
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@@ -1,4 +1,4 @@
.. SPDX-License-Identifier: MIT OR Apache-2.0
SPDX-FileCopyrightText: The Ferrocene Developers
SPDX-FileCopyrightText: The Rust Project Developers
.. |spec_version| replace:: 1.88.0
.. |spec_version| replace:: 1.90.0