Track global capabilities #24883
Merged
Track global capabilities #24883
+704
−566
Conversation
This file contains hidden or bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
bishabosha
reviewed
Jan 3, 2026
| -- [E007] Type Mismatch Error: tests/neg-custom-args/captures/object-uses.scala:15:18 ---------------------------------- | ||
| 15 | val _: Object = b // error | ||
| | ^ | ||
| | Found: (A.this.b : A.this.B.type) |
Member
There was a problem hiding this comment.
singleton types hiding capture sets, this seems like something that might want to be recovered in error messages
Contributor
Author
There was a problem hiding this comment.
A hint is in the error note below. It should be made clearer, for sure.
odersky
force-pushed
the
global-caps
branch
2 times, most recently
from
cb4c1c1 to
6a79065
Compare
Also: - Add interpolation of types of module vals, so that we can decide whether they are tracked or not - Treat non-capture checked global refs as non-trackable
1. Add capture sets also for self types of global classes 2. Also mark free capabilities in global classes 3. Handle interactions with opaque type refinements on self types
We already have a sentinel value in rootEnv; use that instead.
Require explicit types also for static definitions that have capture sets in their (result-) types.
Compensate by allowing Unscoped fresh capabilities to escape to outer levels.
Also: Don't require a uses clause if an object only refers to external capabilities. Those can't be declared anyway.
bracevac
approved these changes
Jan 8, 2026
Sign up for free
to join this conversation on GitHub.
Already have an account?
Sign in to comment
3 participants
Add this suggestion to a batch that can be applied as a single commit.
This suggestion is invalid because no changes were made to the code.
Suggestions cannot be applied while the pull request is closed.
Suggestions cannot be applied while viewing a subset of changes.
Only one suggestion per line can be applied in a batch.
Add this suggestion to a batch that can be applied as a single commit.
Applying suggestions on deleted lines is not supported.
You must change the existing code in this line in order to create a valid suggestion.
Outdated suggestions cannot be applied.
This suggestion has been applied or marked resolved.
Suggestions cannot be applied from pending reviews.
Suggestions cannot be applied on multi-line comments.
Suggestions cannot be applied while the pull request is queued to merge.
Suggestion cannot be applied right now. Please check back later.
This is a fundamental paradigm shift. One of the tenets of the traditional capability model is that an object holds a capability only if it created the capability itself, or it was given to it as a parameter in a method call. This rules out global capabilities. If capabilities are only runtime checked, this restriction is necessary, since otherwise everyone can access global capabilities without any safety checks.
The downside of this rule is that it makes it somewhat inconvenient to implement programs with a fine-grained capability structure. For instance, traditional software can use a global output stream for printing or a global network socket for communicating. Passing these capabilities into the main function and from there on down to all code that needs to print or communicate is possible, but can become tedious.
With capture checking we have a new tool that allows us to support global capabilities. In that context, every object or function that uses a global capability has to declare that usage in its capture set. Conversely, we can demand objects and functions that don't access global capabilities simply by constraining their types.
So far, the capture checker did not track global capabilities since they were presumed to be impossible according to the traditional capability model. This PR aims to change that by properly handling global capabilities.
Based on #24881