Silence the warning about forgetting the vendoring

[jneem]

When sparse crates.io is used, dependencies from crates.io always come from ReplacedSources, as the non-sparse crates.io gets replaced by the sparse one. As far as diagnostics go, this case shouldn't "count" as a replaced source.

I had trouble adding a test for this change, because the test harness replaces everything with a dummy registry.

Fixes #12802

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[ehuss]

Thanks!

I believe you can use the replace_crates_io() function to replace crates.io without using the typical source replacement. See the test builtin_source_replacement as an example. Can you give that a try and see if it works?

Also, can you update the docstring on Registry::is_replaced to mention that it is false for built-in replacements?

Perhaps @arlosi could give this a second set of eyes to determine if this is the best strategy to fix the issue. I think it should be fine. This method is only used for this one error message. I just have a slight concern that it could cause confusion in the future (returning false for something that is replaced internally).

[jneem]

Thanks for the hint! I managed to get a test working (which fails before this PR, and passes after).

I just have a slight concern that it could cause confusion in the future (returning false for something that is replaced internally).

I was a little worried about this too. I think the usages of ReplacedSource::is_builtin_replacement suggests that it's often necessary to distinguish between builtin and non-builtin replacements. So if it ever becomes necessary to do so on the Source trait, we could replace is_replaced by a method returning a three-value enum (NotReplaced, BuiltinReplaced, OtherReplaced)