auto merge of #12869 : thestinger/rust/cmp, r=brson

The `Float` trait provides correct `min` and `max` methods on floating
point types, providing a consistent result regardless of the order the
parameters are passed.

These generic functions do not take the necessary performance hit to
correctly support a partial order, so the true requirement should be
given as a type bound.

Closes #12712

diff --cc src/doc/guide-tasks.md
Simple merge

diff --cc src/libtest/lib.rs
Simple merge