Skip to content

allow constants to refer to mutable/external memory, but reject such constants as patterns #1859

New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

Sign up for GitHub

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Merged
merged 3 commits into from
Jul 1, 2025
Merged

allow constants to refer to mutable/external memory, but reject such constants as patterns #1859

Show file tree
Hide file tree
Changes from 1 commit
Commits
Show all changes
3 commits
Select commit Hold shift + click to select a range
24a4f32
allow constants to refer to mutable/external memory, but reject such …
RalfJung Jun 14, 2025
dd6e807
Add clarifications
traviscross Jun 24, 2025
5910551
Revise editorially
traviscross Jun 24, 2025
File filter

Filter by extension

Filter by extension
Conversations
Failed to load comments.
Loading
Jump to
Jump to file
Failed to load files.
Loading
Diff view
Diff view
3 changes: 0 additions & 3 deletions src/items/constant-items.md
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -48,9 +48,6 @@ const BITS_N_STRINGS: BitsNStrings<'static> = BitsNStrings {
};
```

r[items.const.final-value-immutable]
The final value of a `const` item cannot contain references to anything mutable.

Copy link
Contributor

@ehuss ehuss Jun 19, 2025

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

We were wondering if it would make sense to change the wording here instead of removing it. Perhaps something like:

"The final value of a const item cannot contain a mutable reference."

Most of us seemed to think this is correct. Perhaps there is some other rule forbidding this already (implicitly or explicitly), but it seems like it wouldn't hurt to be clear about it here?

r[items.const.expr-omission]
The constant expression may only be omitted in a [trait definition].

Expand Down
3 changes: 3 additions & 0 deletions src/patterns.md
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -975,6 +975,9 @@ r[patterns.const.generic]
In particular, the value of `C` must be known at pattern-building time (which is pre-monomorphization).
This means that associated consts that involve generic parameters cannot be used as patterns.

r[patterns.const.immutable]
Finally, the value of `C` must not contain any references to mutable statics (`static mut` or interior mutable) or `extern` statics.

r[patterns.const.translation]
After ensuring all conditions are met, the constant value is translated into a pattern, and now behaves exactly as-if that pattern had been written directly.
In particular, it fully participates in exhaustiveness checking.
Expand Down