Interface error improvements by 0x6e · Pull Request #12594 · slint-ui/slint

0x6e · 2026-07-22T15:50:17Z

Reorganise the interface syntax tests (17 -> 9 files) around the compiler's validation stages so every diagnostic is asserted exactly once: declaration-body rules, interface/target resolution, self-implementation conformance, child delegation, cross-statement conflicts and parser-level grammar errors, keeping the two regression guards untouched. Drop the runtime test that duplicated implements_defaults.

We then iterated through the various errors to:

group multiple errors for the same statement into one error;
provide better descriptions of what the expected interface API should be;
consolidate the error creation code into functions that are re-used.

This resulted in us being able to remove interfaces::validate_function_implementations and rename interfaces::validate_properties_and_callbacks to interfaces::validate_self_implement_statements which no longer filters out function declarations.

Relates to #1870.

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LeonMatthes

Much better.

It would be great if we can show the actual code that needs to be added.
And I think we should keep the check errors on the implement statement instead of the implementation candidate (with potentially a note diagnostic on the candidate).

LeonMatthes

This is definitely an improvement.

I left a few more ideas for further refinement. As discussed, these can be done as another follow-up.

Reorganise the interface syntax tests (17 -> 9 files) around the compiler's validation stages so every diagnostic is asserted exactly once: declaration-body rules, interface/target resolution, self-implementation conformance, child delegation, cross-statement conflicts and parser-level grammar errors, keeping the two regression guards untouched. Drop the runtime test that duplicated implements_defaults. The child-forward "Cannot override binding" branch was unreachable - a pre-existing binding for an interface member is always intercepted by the earlier base-conflict or declaration-conflict checks - so replace it with a debug_assert. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

The `uses` keyword has been replaced with the `implement` statement. Remove the old language from the syntax errors.

Rewrite the diagnostics emitted by `property_matches_interface` so that a mismatch between a component's API and an implemented interface is described precisely — the member's kind (property / callback / function), its visibility, and its purity. Multiple mismatches on a single member are presented as a multi-line list.

…gnostic Rather than emitting a error for every issue with a child-implement statement, consolidate them into one error with a list of issues.

… diagnostic for the `implement` statement Rather than emitting an error per conflict consolidate them all into a single error with a list of conflicts.

…a single diagnostic for the `implement` statement Rather than emitting an error per conflict consolidate them all into a single error with a list of conflicts. This function may still emit other diagnostics where the error source is not the `implement` statement.

Re-use the `missing_type_description` lambda to describe a missing property or callback in `validate_interface_member_implementation`.

Rather than saying "Cannot implement 'Interface'" we shorten it to "'Interface' error." followed by the list of conflicts. This is less verbose, doesn't mention the `implement` keyword where it is not the cause of the error and is just a bit nicer to read.

Remove `interfaces::validate_function_implementations` and rename `interfaces::validate_properties_and_callbacks` to `validate_self_implement_statements`. The new function no longer filters our function declarations, allowing us to use the same checking code and errors for all interface API.

…rface function implementation The langtype::Function implementation of PartialEq means that the compiler would emit an error if the implementation of an interface did not have exactly matching parameter names. We do not require this to satisfy interface API, so introduce our own comparison function for langtype::Type.

…don't match For example, `public` or `protected` function visibility doesn't make much sense in a property context (expects `in`, `out`, `in-out`). In this case, we emit the full type that the interface expects so that the user can get it right in one iteration.

Improve the parsing and compiler error to make it clear that interface inheritance is not supported (yet).

Add a syntax test to verify that `@rust-attr` does not support annotating interfaces.

Interface errors that occur due to a missing member, or a member being fully the wrong type (e.g. property instead of callback) now print the required Slint code.

Add a generic note to the unsatisfactory statement. In the future we can make this more specific.

Before we were emitting notes before the relevant errors which was not very useful when looking at `slint-compiler` output. Now we emit the notes after the error, giving us more understandable compiler output: ``` error: Cannot implement 'ValidInterface'. - 'reset' must be a 'public function() -> void' (found a 'int' property) - 'speak' must be a 'callback() -> void' (found a 'int' property) - 'value' must be an in-out 'int' property (found a 'callback() -> void') --> internal/compiler/tests/syntax/interfaces/implement_self.slint:255:15 | 255 | implement ValidInterface <=> self; | ^^^^^^^^^^^^^^^ note: 'reset' does not satisfy 'ValidInterface' --> internal/compiler/tests/syntax/interfaces/implement_self.slint:262:5 | 262 | in-out property <int> reset; | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ note: 'speak' does not satisfy 'ValidInterface' --> internal/compiler/tests/syntax/interfaces/implement_self.slint:260:5 | 260 | in-out property <int> speak; | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ note: 'value' does not satisfy 'ValidInterface' --> internal/compiler/tests/syntax/interfaces/implement_self.slint:258:5 | 258 | callback value(); | ^^^^^^^^^^^^^^^^^ ```

…ing uses Verify that the new `implement` keyword doesn't break existing uses of the name "implement" for properties/callback/functions/etc.

0x6e force-pushed the nathan/interface-error-improvements branch from 6d2dab7 to 290ba4a Compare July 22, 2026 16:00

0x6e marked this pull request as ready for review July 22, 2026 17:00

0x6e requested a review from LeonMatthes July 22, 2026 17:01

github-actions Bot deployed to pr-preview-12594 July 22, 2026 17:19 View deployment

LeonMatthes reviewed Jul 27, 2026

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0x6e mentioned this pull request Jul 27, 2026

Interfaces: implement validates, it does not copy implementations #12597

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0x6e force-pushed the nathan/interface-error-improvements branch from 290ba4a to b1d9c5a Compare July 27, 2026 10:52

github-actions Bot deployed to pr-preview-12594 July 27, 2026 11:43 View deployment

0x6e force-pushed the nathan/interface-error-improvements branch 2 times, most recently from 468adca to 2c5e839 Compare July 27, 2026 18:17

0x6e force-pushed the nathan/interface-error-improvements branch from 2c5e839 to 796eb63 Compare July 27, 2026 20:10

github-actions Bot deployed to pr-preview-12594 July 27, 2026 20:49 View deployment

Base automatically changed from nathan/function-callback-display to master July 28, 2026 07:56

0x6e force-pushed the nathan/interface-error-improvements branch from 796eb63 to ccc102e Compare July 28, 2026 09:10

0x6e requested a review from LeonMatthes July 28, 2026 09:11

github-actions Bot deployed to pr-preview-12594 July 28, 2026 09:41 View deployment

LeonMatthes approved these changes Jul 28, 2026

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Comment thread internal/compiler/tests/syntax/interfaces/interface_resolution.slint

Comment thread internal/compiler/object_tree/interfaces.rs Outdated

Comment thread internal/compiler/object_tree/interfaces.rs

Comment thread internal/compiler/tests/syntax/interfaces/implement_self.slint

0x6e force-pushed the nathan/interface-error-improvements branch 2 times, most recently from 2031621 to 097664a Compare July 29, 2026 14:56

0x6e and others added 7 commits July 29, 2026 18:01

compiler: interface errors no longer reference "using" an interface

1f7c562

The `uses` keyword has been replaced with the `implement` statement. Remove the old language from the syntax errors.

compiler: interfaces::element_implements_interface emits a single dia…

724eb0f

…gnostic Rather than emitting a error for every issue with a child-implement statement, consolidate them into one error with a list of issues.

compiler: interfaces::apply_child_implement_statements emits a single…

a3ca0b0

… diagnostic for the `implement` statement Rather than emitting an error per conflict consolidate them all into a single error with a list of conflicts.

compiler: consolidate the missing interface property/callback error

5152c01

Re-use the `missing_type_description` lambda to describe a missing property or callback in `validate_interface_member_implementation`.

0x6e added 10 commits July 29, 2026 18:01

compiler: interface inheritance is not supported

ab35dcf

Improve the parsing and compiler error to make it clear that interface inheritance is not supported (yet).

compiler: @rust-attr does not support interfaces

2f74145

Add a syntax test to verify that `@rust-attr` does not support annotating interfaces.

compiler: expected code provided for missing interface API

e84d3bb

Interface errors that occur due to a missing member, or a member being fully the wrong type (e.g. property instead of callback) now print the required Slint code.

compiler: interface errors occur on the implement statement

85db20d

Add a generic note to the unsatisfactory statement. In the future we can make this more specific.

compiler: verify that the new implement keyword doesn't break exist…

b7ae663

…ing uses Verify that the new `implement` keyword doesn't break existing uses of the name "implement" for properties/callback/functions/etc.

0x6e force-pushed the nathan/interface-error-improvements branch from 097664a to b7ae663 Compare July 29, 2026 17:02

github-actions Bot deployed to pr-preview-12594 July 29, 2026 18:04 View deployment

0x6e merged commit 52c7894 into master Jul 29, 2026
95 of 97 checks passed

0x6e deleted the nathan/interface-error-improvements branch July 29, 2026 18:11

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Interface error improvements - #12594

Interface error improvements#12594
0x6e merged 17 commits into
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0x6e commented Jul 22, 2026 •

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0x6e commented Jul 22, 2026 •

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