std::io: impl IsTerminal for {Empty, Repeat, Sink, Take}

[GrigorenkoPV]

Closes #161941

Yeah, these are instastable

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[clarfonthey]

@rfcbot fcp merge libs

[rust-rfcbot]

@clarfonthey has proposed to merge this. The next step is review by the rest of the tagged team members:

• @Amanieu
• @BurntSushi
• @cuviper
• @dtolnay
• @joshtriplett
• @the8472
• @thomcc

Concerns:

• always-false (std::io: impl IsTerminal for {Empty, Repeat, Sink, Take} #161947 (comment))

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[joshtriplett]

We might also want to implement it for BufReader<R> and BufWriter<W> if the underlying types implement IsTerminal, and pass it through.

[the8472]

@rfcbot concern always-false

Depending on which semantics we agree to document in #162012 it might not want this for Sink if it always returns false.

[GrigorenkoPV]

We might also want to implement it for BufReader<R> and BufWriter<W> if the underlying types implement IsTerminal, and pass it through.

Those two are in alloc with inner being a private field, while IsTerminal is a sealed trait in std. So this will require some non-obvious rearranging.

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Some peculiar trivia: BufWriter<T> has T: ?Sized + Write, BufReader<T> has T: ?Sized (no + Read), Take<T> has T (no : ?Sized or : Read).