Interfaces errors emit better notes

[0x6e]

When an interface is not satisfied by the implementing component we emit notes that attempt to highlight the particular syntax that is causing the problem.

error: Cannot implement 'ValidInterface'.
       - 'value' must be a 'int' property (found a 'float' property)
   --> internal/compiler/tests/syntax/interfaces/implement_self.slint:92:15
    |
 92 |     implement ValidInterface <=> self;
    |               ^^^^^^^^^^^^^^^
note: 'ValidInterface' declares 'value' as 'in-out property <int> value;'
   --> internal/compiler/tests/syntax/interfaces/implement_self.slint:95:22
    |
 95 |     in-out property <float> value;
    |                      ^^^^^

In some cases, it is not possible to highlight the particular syntax causing the issue (e.g. missing pure keyword) so we fall back to emitting a note on the name identifier of the declarations:

error: Cannot implement 'Inverter'.
       - 'invert' must be 'pure'
   --> internal/compiler/tests/syntax/interfaces/implement_self.slint:125:15
    |
125 |     implement Inverter <=> self;
    |               ^^^^^^^^^
note: 'Inverter' declares 'invert' as 'pure callback invert(bool) -> bool;'
   --> internal/compiler/tests/syntax/interfaces/implement_self.slint:129:14
    |
129 |     callback invert(bool) -> bool;
    |              ^^^^^^

In some cases we may emit multiple notes for the same error. We aggregate errors for each implement statement into one error string, and emit notes for each part of the syntax that is incorrect:

error: Cannot implement 'OutOnlyInterface'.
       - 'count' must be a 'int' property (found a 'float' property)
       - 'count' must be 'out' (found 'in-out')
   --> internal/compiler/tests/syntax/interfaces/implement_self.slint:286:15
    |
286 |     implement OutOnlyInterface <=> self;
    |               ^^^^^^^^^^^^^^^^^
note: 'OutOnlyInterface' declares 'count' as 'out property <int> count;'
   --> internal/compiler/tests/syntax/interfaces/implement_self.slint:289:22
    |
289 |     in-out property <float> count;
    |                      ^^^^^
note: 'OutOnlyInterface' declares 'count' as 'out property <int> count;'
   --> internal/compiler/tests/syntax/interfaces/implement_self.slint:289:5
    |
289 |     in-out property <float> count;
    |     ^^^^^^

This might appear a bit repetitive, but the error text must contain the information needed to identify the error - the note does not appear in the IDE (otherwise we might emit one generic error for the implement statement, with specific notes for the problematic declarations).

[LeonMatthes]

The more detailed spans are great, but I find the new note diagnostics somewhat misleading.
But we can probably fix that with a bit of rewording.

[LeonMatthes]

Hm, I find this phrasing somewhat confusing.

This sounds like the diagnostic is pointing at the declaration within ValidInterface, but it is instead pointing to the conflicting implementation that we are checking against.
That is the right thing to point at, but the message should not suggest that it is showing the ValidInterface declaration.

Maybe a rewording could be:

Suggested change

	// > <note{'ValidInterface' declares 'value' as 'in-out property <int> value;'}
	// > <note{'value' declared here conflicts with 'ValidInterface' (expected 'in-out property <int> value;')}

I'm still not entirely happy with it, so suggestions appreciated :)

[0x6e]

I wasn't super happy with this either, and agree that the wording probably could be improved. In particular I wondered if the note should also say what the problem is. However, the error diagnostic is what is visible to the user in the IDE, and must contain the error. The note is only visible in the compiler output (AFAIK) and always relates to the error, so we get:

• error: there is an error here, caused by the implement keyword (normally) and these are the problems;
• notes: this is where the declaration that conflicts with the interface is, it is probably this particular part of the declaration, and this is what the interface declares/actually expects.

Maybe the note should say something more like: 'Foo' declares 'bar' here. 'Interface' expects 'in-out' property (found 'in'). Previously you said you wanted the code that the interface expects, but I'm not sure how best to fit that in as well. It could be part of the error.