SlowInferenceChain

Scalafix semantic rule that flags Scala 3 call sites which can trigger slow type inference when:

• a method omits a type argument,
• that type argument is higher-kinded (for example F[_]),
• that type argument affects the result type, but not as the top-level result constructor itself (for example Resource[F, Unit] is checked, F[Int] is not),
• it only shows up through implicit / context evidence,
• and the result is immediately chained on, including for generators.

This rule is a workaround for the Scala 3 compiler issue scala/scala3#18763.

Setup

Add the rule to your build:

ThisBuild / scalafixDependencies +=
  "org.polyvariant" %% "SlowInferenceChain" % version

Then run Scalafix on your sources:

sbt "scalafix SlowInferenceChain"

or add it to your .scalafix.conf:

rules = [
  DisableSyntax,
  LeakingImplicitClassVal,
  NoAutoTupling,
  NoValInForComprehension,
  RedundantSyntax,
+ SlowInferenceChain,
]

Project layout

• rules/ — the rule implementation
• input/ — test fixtures consumed by Scalafix testkit
• output/ — unused for this linter, kept for the standard testkit layout
• tests/ — the Scalafix test suite

Commands

sbt rules/compile
sbt tests/test

Current fixture coverage

Positive cases:

• chained method call on the inferred result
• for-comprehension generator

Negative cases:

• explicit type argument already present
• plain type parameter rather than a higher-kinded one
• no chaining after the call
• type parameter appears in a non-implicit parameter
• no implicit / given evidence for the type parameter
• matching higher-kinded type parameter and corresponding implicit evidence already exist in an enclosing scope
• return type does not depend on the inferred type parameter
• return type is directly F[...]