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Return error in ProviderCollection[] on undeclared provider

Currently, ctx.attrs.foo[UnknownInfo] returns None if foo is a provider collection.

This RFC proposes these changes:

  • ctx.attrs.foo[UnknownInfo] is an error
  • UnknownInfo in ctx.attrs.foo is False
  • ctx.attrs.foo.get(UnknownInfo) returns None

Why​

Better diagnostics when accessing unknown provider. E. g. when writing:

ctx.attrs.foo[UnknownInfo].bar

Currently, the error is:

Object of type `NoneType` has no attribute `bar`

Instead, the error will be something like:

provider collection does not contain `UnknownInfo`,
    defined providers are `FooInfo`, `BarInfo`.

Bazel​

In bazel, [] on unknown provider is an error, like this:

Error: <target //optional_provider:n2> (rule '_sum')
    doesn't contain declared provider 'UnknownInfo'