Fix: Constructors no longer fail silently

[BenFordTytherington]

• Closes Missing impl Constructor<()> for ... {} in the bridge silently fails #1233
• A shim trait is used to check whether the user declared a constructor
• This is implemented by the bridge if you add the constructor trait in bridge
• This will then allow you to implemented constructor for your type
• This prevents constructors not being used silently

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (f88253b) to head (bc1bc82).

Additional details and impacted files

@@            Coverage Diff            @@
##              main     #1295   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           75        75           
  Lines        13038     13064   +26     
=========================================
+ Hits         13038     13064   +26     

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[LeonMatthesKDAB]

This works well, but needs some better user-facing documentation.

Please also update the Changelog to mention this new feature :)

[LeonMatthesKDAB]

I just tried the PR, it works well, but the discoverability of what caused the error still needs improving!

If I e.g. remove the constructor declaration from qml_features/properties.rs, what I get is:

Long error message

error[E0277]: the trait bound `RustProperties: ConstructorDeclared<(bool, &'a cxx_qt_lib::QUrl, &'a cxx_qt_lib::QUrl, &'a cxx_qt_lib::QString)>` is not satisfied
   --> examples/qml_features/rust/src/properties.rs:139:5
    |
139 | /     fn route_arguments(
140 | |         arguments: (bool, &'a QUrl, &'a QUrl, &'a QString),
141 | |     ) -> (
142 | |         Self::NewArguments,
143 | |         Self::BaseArguments,
144 | |         Self::InitializeArguments,
145 | |     ) {
    | |_____^ the trait `ConstructorDeclared<(bool, &'a cxx_qt_lib::QUrl, &'a cxx_qt_lib::QUrl, &'a cxx_qt_lib::QString)>` is not implemented for `RustProperties`
    |
    = help: the trait `ConstructorDeclared<(bool, &'a cxx_qt_lib::QUrl, &'a cxx_qt_lib::QUrl, &'a cxx_qt_lib::QString)>` is not implemented for `RustProperties`
            but trait `ConstructorDeclared<()>` is implemented for it
    = help: for that trait implementation, expected `()`, found `(bool, &'a cxx_qt_lib::QUrl, &'a cxx_qt_lib::QUrl, &'a cxx_qt_lib::QString)`
note: required by a bound in `Constructor`
   --> /home/kdab/Documents/projects/3371-Rust-RnD/cxx-qt/crates/cxx-qt/src/lib.rs:313:47
    |
313 | pub trait Constructor<Arguments>: CxxQtType + ConstructorDeclared<Arguments> {
    |                                               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `Constructor`

error[E0277]: the trait bound `RustProperties: ConstructorDeclared<(bool, &'a cxx_qt_lib::QUrl, &'a cxx_qt_lib::QUrl, &'a cxx_qt_lib::QString)>` is not satisfied
   --> examples/qml_features/rust/src/properties.rs:150:5
    |
150 | /     fn new(
151 | |         (connected, connected_url, previous_connected_url, status_message): Self::NewArguments,
152 | |     ) -> Self::Rust {
    | |___________________^ the trait `ConstructorDeclared<(bool, &'a cxx_qt_lib::QUrl, &'a cxx_qt_lib::QUrl, &'a cxx_qt_lib::QString)>` is not implemented for `RustProperties`
    |
    = help: the trait `ConstructorDeclared<(bool, &'a cxx_qt_lib::QUrl, &'a cxx_qt_lib::QUrl, &'a cxx_qt_lib::QString)>` is not implemented for `RustProperties`
            but trait `ConstructorDeclared<()>` is implemented for it
    = help: for that trait implementation, expected `()`, found `(bool, &'a cxx_qt_lib::QUrl, &'a cxx_qt_lib::QUrl, &'a cxx_qt_lib::QString)`
note: required by a bound in `Constructor`
   --> /home/kdab/Documents/projects/3371-Rust-RnD/cxx-qt/crates/cxx-qt/src/lib.rs:313:47
    |
313 | pub trait Constructor<Arguments>: CxxQtType + ConstructorDeclared<Arguments> {
    |                                               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `Constructor`

error[E0277]: the trait bound `RustProperties: ConstructorDeclared<(bool, &'a cxx_qt_lib::QUrl, &'a cxx_qt_lib::QUrl, &'a cxx_qt_lib::QString)>` is not satisfied
   --> examples/qml_features/rust/src/properties.rs:134:75
    |
134 | impl<'a> cxx_qt::Constructor<(bool, &'a QUrl, &'a QUrl, &'a QString)> for qobject::RustProperties {
    |                                                                           ^^^^^^^^^^^^^^^^^^^^^^^ the trait `ConstructorDeclared<(bool, &'a cxx_qt_lib::QUrl, &'a cxx_qt_lib::QUrl, &'a cxx_qt_lib::QString)>` is not implemented for `RustProperties`
    |
    = help: the trait `ConstructorDeclared<(bool, &'a cxx_qt_lib::QUrl, &'a cxx_qt_lib::QUrl, &'a cxx_qt_lib::QString)>` is not implemented for `RustProperties`
            but trait `ConstructorDeclared<()>` is implemented for it
    = help: for that trait implementation, expected `()`, found `(bool, &'a cxx_qt_lib::QUrl, &'a cxx_qt_lib::QUrl, &'a cxx_qt_lib::QString)`
note: required by a bound in `Constructor`
   --> /home/kdab/Documents/projects/3371-Rust-RnD/cxx-qt/crates/cxx-qt/src/lib.rs:313:47
    |
313 | pub trait Constructor<Arguments>: CxxQtType + ConstructorDeclared<Arguments> {
    |                                               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `Constructor`

So now I know I did something wrong, but what?

The compiler helpfully points me me to the Constructor trait. But if I look at the docs for the constructor trait, I cannot click on ConstructorDeclared, and I cannot find the trait mentioned anywhere in the docs.

So either we:

• Make this not #[doc(hidden)] and add documentation that says that you should not implement the trait yourself, but that adding the declaration to the bridge generates an implementation.
• Describe this in the doc comment of the Constructor trait.