480a72d601
Stabilize `num_midpoint_signed` feature This PR proposes that we stabilize the signed variants of [`iN::midpoint`](https://github.com/rust-lang/rust/issues/110840#issue-1684506201), the operation is equivalent to doing `(a + b) / 2` in a sufficiently large number. The stabilized API surface would be: ```rust /// Calculates the middle point of `self` and `rhs`. /// /// `midpoint(a, b)` is `(a + b) / 2` as if it were performed in a /// sufficiently-large signed integer type. This implies that the result is /// always rounded towards zero and that no overflow will ever occur. impl i{8,16,32,64,128,size} { pub const fn midpoint(self, rhs: Self) -> Self; } ``` T-libs-api previously stabilized the unsigned (and float) variants in #131784, the signed variants were left out because of the rounding that should be used in case of negative midpoint. This stabilization proposal proposes that we round towards zero because: - it makes the obvious `(a + b) / 2` in a sufficiently-large number always true - using another rounding for the positive result would be inconsistent with the unsigned variants - it makes `midpoint(-a, -b)` == `-midpoint(a, b)` always true - it is consistent with `midpoint(a as f64, b as f64) as i64` - it makes it possible to always suggest `midpoint` as a replacement for `(a + b) / 2` expressions *(which we may want to do as a future work given the 21.2k hits on [GitHub Search](https://github.com/search?q=lang%3Arust+%2F%5C%28%5Ba-zA-Z_%5D*+%5C%2B+%5Ba-zA-Z_%5D*%5C%29+%5C%2F+2%2F&type=code&p=1))* `@scottmcm` mentioned a drawback in https://github.com/rust-lang/rust/pull/132191#issuecomment-2439891200: > I'm torn, because rounding towards zero makes it "wider" than other values, which `>> 1` avoids -- `(a + b) >> 1` has the nice behaviour that `midpoint(a, b) + 2 == midpoint(a + 2, b + 2)`. > > But I guess overall sticking with `(a + b) / 2` makes sense as well, and I do like the negation property 🤷 Which I think is outweigh by the advantages cited above. Closes #110840 cc `@rust-lang/libs-api` cc `@scottmcm` r? `@dtolnay`
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