Make `into` a preview feature #24090

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odersky merged 3 commits into scala:main from dotty-staging:add-into-preview

Oct 2, 2025

Merged

Make `into` a preview feature #24090

odersky merged 3 commits into scala:main from dotty-staging:add-into-preview

Oct 2, 2025

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odersky

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@odersky odersky commented Sep 27, 2025

Since SIP 71 is now accepted, we can ship it as -preview in 3.8 and standard in 3.9.

Note to tooling implementers

There are two relevant pieces that need to be implemented in IDEs:

into is a new soft modifier for classes, traits, and enums. Types with into are legal conversion target types. See isConversionTargetType in Types.scala.
The Conversion.into type is a regular opaque type, so should not need any support by itself. However, there's a rule that no-flip covariant occurrences of into[T] in method parameters are mapped toT @$into, where $into is an annotation in annotation.internal. The annotation is "silent" which means it should not be shown in inferred types. The purpose of that mapping is that into[...] does not have to be explicitly stripped in method bodies. See suppressInto and revealInto in TypeOps.

/cc @unkarjedy @tgodzik

@odersky


 Make into a preview feature

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@odersky odersky requested a review from a team as a code owner

September 27, 2025 10:32

@odersky


 Update TastyFormat to include INTO

0e80aae

@odersky odersky requested a review from hamzaremmal

September 27, 2025 17:02

@odersky odersky assigned hamzaremmal

Sep 27, 2025

@Gedochao Gedochao requested a review from WojciechMazur

September 29, 2025 08:17

WojciechMazur

WojciechMazur reviewed

Sep 29, 2025

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library/src/scala/annotation/internal/$into.scala

* `T`. Hence, we don't need to use `.underlying` to go from an into type to its

* underlying type in the types of local parameters.

@experimental

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@WojciechMazur WojciechMazur Sep 29, 2025

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Shouldn't it now use @scala.annotation.internal.preview annotation instead of making it stable?

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@hamzaremmal hamzaremmal Sep 29, 2025

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This is not really needed. It is an internal annotation that people will not use manually.

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@sjrd sjrd Sep 29, 2025

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Well, even then. In theory it's part of a preview feature, and could be altered. (Very very unlikely in this case, but let's follow the procedure, so that the next person who comes to this PR in search for a reference of how a feature becomes preview doesn't get misled.)

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@hamzaremmal hamzaremmal Sep 30, 2025

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I agree, if we were to be pedantic, we should put it. I just claimed that from an implementation point of view, it is not really needed.

library/src/scala/runtime/stdLibPatches/language.scala Outdated

* @see [[https://dotty.epfl.ch/docs/reference/experimental/into-modifier]]

@compileTimeOnly("`into` can only be used at compile time in import statements")

@deprecated("The into language import is no longer needed since the feature is now standard", since = "3.9")

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@WojciechMazur WojciechMazur Sep 29, 2025

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Not sure about this one, and how it should be handled. From the perspective of user it would be better to now get hint about preview, and standard when it's actually standardised in 3.9 or later

Suggested change

@deprecated("The into language import is no longer needed since the feature is now standard", since = "3.9")

@deprecated("The into language import is no longer needed since the feature is now in preview", since = "3.9")

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@Gedochao Gedochao Sep 29, 2025

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perhaps hint about preview since 3.8...
and standard whenever we stabilise

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@hamzaremmal hamzaremmal Sep 29, 2025

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We should not say since 3.9.0 in 3.8.0. It makes no sense. I think we should say (preview, 3.8.0) and change it to (stable, 3.9.0) later or say directly (stable, 3.8.0).

@tgodzik tgodzik added the needs-minor-release label

Sep 29, 2025

hamzaremmal

hamzaremmal requested changes

Sep 29, 2025

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library/src/scala/annotation/internal/$into.scala

* `T`. Hence, we don't need to use `.underlying` to go from an into type to its

* underlying type in the types of local parameters.

@experimental

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@hamzaremmal hamzaremmal Sep 29, 2025

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This is not really needed. It is an internal annotation that people will not use manually.

library/src/scala/runtime/stdLibPatches/language.scala Outdated

* @see [[https://dotty.epfl.ch/docs/reference/experimental/into-modifier]]

@compileTimeOnly("`into` can only be used at compile time in import statements")

@deprecated("The into language import is no longer needed since the feature is now standard", since = "3.9")

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@hamzaremmal hamzaremmal Sep 29, 2025

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Suggested change

@deprecated("The into language import is no longer needed since the feature is now standard", since = "3.9")

@deprecated("The into language import is no longer needed since the feature is now standard", since = "3.8.0")

library/src/scala/language.scala Outdated

* @see [[https://dotty.epfl.ch/docs/reference/experimental/into-modifier]]

@compileTimeOnly("`into` can only be used at compile time in import statements")

@deprecated("The into language import is no longer needed since the feature is now standard", since = "3.9")

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@hamzaremmal hamzaremmal Sep 29, 2025

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Suggested change

@deprecated("The into language import is no longer needed since the feature is now standard", since = "3.9")

@deprecated("The into language import is no longer needed since the feature is now standard", since = "3.8.0")

tasty/src/dotty/tools/tasty/TastyFormat.scala Outdated

Comment on lines 231 to 232

TRACKED

INTO -- a tracked class parameter / a dependent class

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@hamzaremmal hamzaremmal Sep 29, 2025

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Suggested change

TRACKED

INTO-- a tracked classparameter/ a dependent class

TRACKED-- a tracked classparameter/ a dependent class

INTO

library/src/scala/Conversion.scala

* conversions are tried from the type of `t` to `T`. `into[T]` types are erased to `T`

* in all covariant positions of the types of parameter symbols.

@experimental

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@hamzaremmal hamzaremmal Sep 29, 2025

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This one is important, if we don't mark this alias with @preview, we will not be able to enforce it.

library/src/scala/Conversion.scala

/** Unwrap an `into` */

extension [T](x: into[T])

@experimental def underlying: T = x

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@hamzaremmal hamzaremmal Sep 29, 2025

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This one should also be marked as @preview, otherwise we will be compiling the library with -preview

@odersky


 Address review comments

dd0c26d

 - Make all aspects of into a -preview feature
 - Drop experimental import in tests

hamzaremmal

hamzaremmal approved these changes

Oct 1, 2025

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@odersky odersky merged commit e95afce into scala:main

Oct 2, 2025

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@odersky odersky deleted the add-into-preview branch

October 2, 2025 16:09

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@odersky @sjrd @Gedochao @WojciechMazur @hamzaremmal @tgodzik

Make into a preview feature #24090

Make into a preview feature #24090

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@odersky odersky commented Sep 27, 2025

Note to tooling implementers

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Make `into` a preview feature #24090

Make `into` a preview feature #24090