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The team behind Swift shared a new blog post yesterday detailing their new diagnostic architecture which is being implemented as a part of the upcoming Swift 5.2 release (expected early November). This new architecture aims to improve the diagnostics from the compiler. It will make it easier to improve/port existing diagnostics to be used by new feature implementors.

The Swift compiler uses type checker to ensure the correctness of a program. Type checker enforces rules about how types are used in source code and show when those rules are violated. However, it guesses the exact location of an error which is not helpful in certain scenarios because it is not specific or actionable.

With the new diagnostic infrastructure, the team is trying to implement a new type checker that attempts to “fix” problems right at the point where they occur while remembering the fixes it has applied. This way, type checker can pinpoint errors in more kinds of programs.

New constraint fix resolves inconsistent situations

The type checker converts the source code into a constraint system, which represents relationships among the types in the code. The Constraint System first generates the constraint and then solves them. The process of constraint generation produces a constraint system that relates the types of various subexpressions within an expression. The constraint solver takes a given set of constraints and determines the most specific type binding for each of the type variables in the constraint system.

To improve the constraint solving mechanism, the new diagnostic infrastructure employs a constraint fix that tries to resolve inconsistent situations where the solver gets stuck with no other types to attempt. This fix captures all useful information about the error location from the solver and uses that later for diagnostics. While the former approach guesses where the error is located, the new approach has a symbiotic relationship with the solver which provides all of the error locations to it.

How constraint fix works

A constraint fix is created whenever a constraint failure is detected. The constraint fix captures three crucial pieces of information about a failure: kind of failure that occurred, location in the source code where the failure came from, and types and declarations involved in the failure. The constraint solver then accumulates these fixes. Once it arrives at a solution, it looks at the fixes that were part of a solution and produces actionable errors or warnings.

The Swift team has shared examples of improved diagnostics and SwiftUI examples to better demonstrate the workings of this new diagnostic infrastructure.

People are quite excited about these error improvements in Swift and shared their views on the associated thread in Swift Forums.

“Omg better error messages! That’s so awesome! Right now error messages are the worst part of swift, I’m excited. I hope it will be the end of types called _ in error messages”

“Awesome work @xedin! The blog post is highly informative and I thoroughly enjoyed reading it. I very much look forward to seeing the new diagnostic architecture in action.”

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