mark std::string::String::new() and std::vec::Vec::new() as #[must_use].

[matthiaskrgr]

No description provided.

[rust-highfive]

r? @TimNN

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[hanna-kruppe]

Could you say a bit about the motivation? With many other methods where must_use is being proposed, I can see an argument about not using the result hiding a specific, subtle bug (e.g., calling String::trim and expecting it to work in-place) or seriously wasting resources (e.g., anything that allocates). But neither of those reasons applies to {Vec,String}::new().

[leoyvens]

I suppose that the motivation is that if the return value is dead code then the constructing call is always dead code.

[scottmcm]

In #48926 (comment), @est31 proposed a new lint that would obviate the need for changes like this (since both methods are const) which I think is a better approach than manually annotating all these.

[TimNN]

I think we'll need some more consensus on general guidelines which std function should be marked #[must_use], so I'm marking this PR as blocked on #48926 for the time being.

[TimNN]

Thank you for your PR! It doesn't look like there will be a decision about how to handle #[must_use] functions in the standard library any time soon, so I'm going to close this PR for now.