Suppress unused type parameter error when type has error field

[sanxiyn]

Fix #35075.

[rust-highfive]

r? @nrc

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[nrc]

Hmm, this seems to be the wrong fix, but maybe it is the best we can do. Ideally we would see that T is used in the field type with unknown type, rather than always assuming a type parameter is used by an error type. E.g., struct Foo<T> { bar: Bar } would still give an unused T error. I assume that we don't keep the type param info around for error types and therefore this is really hard to do? Is that right? In that case then I think we should take this PR, since I think overall it gives a better experience.

[sanxiyn]

I assume that we don't keep the type param info around for error types and therefore this is really hard to do? Is that right?

Yes, this is the case.

[nrc]

@bors: r+

[bors]

📌 Commit 0365215 has been approved by nrc

[bors]

⌛ Testing commit 0365215 with merge d648a16...

[bors]

☀️ Test successful - auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-64-cargotest, auto-linux-64-cross-freebsd, auto-linux-64-debug-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-opt-mir, auto-linux-64-opt-rustbuild, auto-linux-64-x-android-t, auto-linux-cross-opt, auto-linux-musl-64-opt, auto-mac-32-opt, auto-mac-64-opt, auto-mac-64-opt-rustbuild, auto-mac-cross-ios-opt, auto-win-gnu-32-opt, auto-win-gnu-32-opt-rustbuild, auto-win-gnu-64-opt, auto-win-msvc-32-opt, auto-win-msvc-64-cargotest, auto-win-msvc-64-opt, auto-win-msvc-64-opt-mir, auto-win-msvc-64-opt-rustbuild
Approved by: nrc
Pushing d648a16 to master...