feat: allow user implemented mappings to be marked as default

[latonz]

Fixes #1008

• Preparation for feat: allow property mappings to use specific conversion #1072
• RMG060 default severity is info for now, since otherwise this would result in lots of warnings for before-/aftermap. This should be changed with the next breaking change (see Proposed breaking changes for Mapperly 4.0 #1131).

[codecov]

Codecov Report

Attention: Patch coverage is 94.31818% with 10 lines in your changes are missing coverage. Please review.

Project coverage is 91.32%. Comparing base (4e4937c) to head (2080a7f).

Files	Patch %	Lines
src/Riok.Mapperly/Descriptors/MappingCollection.cs	92.13%	3 Missing and 4 partials ⚠️
...Riok.Mapperly.Abstractions/UserMappingAttribute.cs	0.00%	1 Missing ⚠️
...Mapperly/Configuration/UserMappingConfiguration.cs	66.66%	1 Missing ⚠️
.../Riok.Mapperly/Diagnostics/DiagnosticCollection.cs	0.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1071      +/-   ##
==========================================
- Coverage   91.35%   91.32%   -0.04%     
==========================================
  Files         222      224       +2     
  Lines        7312     7447     +135     
  Branches      940      952      +12     
==========================================
+ Hits         6680     6801     +121     
- Misses        410      420      +10     
- Partials      222      226       +4     

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[latonz]

I'm not sure whether or not we should consider the first user implemented mapping as default if AutoUserMappings = false, wdyt @CommonGuy?

[CommonGuy]

Do we really need this? Let's take this as an example:

[UserMapping]
public static int Method1(int s) => s;

[UserMapping(Default = true)]
public static int Method2(int s) => s;

// some partial mapping method would follow here

Is there any scenario where Method1 will ever be used as a user mapping by Mapperly? As far as I can see, Method2 will always be picked. We could use [UserMapping(Ignore = true)] on Method1 here, which would have the same effect.

But maybe there are some scenarios (generic mapping method?) where multiple user methods could apply? If that is the case, then this new feature makes sense, but those scenarios should be used in the tests instead of mapping from int to int.

Instead of using a Default property, maybe an Order or Priority property (default 0, higher priority is picked first) would make more sense? Let's imagine three user implemented methods:

• Method1 with Priority = 1
• Method2 with Priority = 2
• Method3 with Priority = 3

For a partial method "MyMethod1", all three methods are suitable, so Method 3 is picked. For another partial method "MyMethod2", only Method1 and Method2 are suitable, so Method2 is picked. This wouldn't work if only Default = true could be defined on Method3.

---

Regardless of implementation, IMO the behavior should be:

• One matching method -> everything is fine
• Multiple matching methods, one has a default/best priority -> everything is fine
• Multiple matching methods with default=false/priority=0 -> emit a warning, even though this is a slightly breaking change if someone has "Report Warnings as Errors" enabled. Relying on method ordering is dangerous and this is fixed easily
• Multiple matching methods with multiple Default = true/same priority >0 -> emit an error

I don't think there is an advantage to making Default a nullable boolean, could default it to false.

[latonz]

Using default for now instead of priority as we do it in other parts (eg. FormatProvider). For now there is no real reason to use priority. If we want to introduce a priority concept we should do this consistently across the entire Mapperly API surface (for example including FormatProvider and ObjectFactory). This could later be introduced without a breaking change by assigning a default priority value to instances which have Default = true set.

[github-actions]

🎉 This PR is included in version 3.5.0-next.1 🎉

The release is available on:

• GitHub release
• v3.5.0-next.1

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