tl;dr This worked for me (iTerm2 on Mac)

$ cargo --color always build | less -R 2>&1

Problem

When I use lv (or bat) to read the output of cargo build, the output is not colored, even with --color always (i.e. $ cargo --color always build |& lv).

https://twitter.com/yhara/status/1360518450012016643

The Cause

By saving the output to file, I found that cargo and rustc are working correctly -- both emits ANSI escape sequence. However, it seems that rustc uses codes like ESC[38;5;12m, which (I heard) is an extension of ANSI and not supported by all softwares.

https://twitter.com/yhara/status/1360532292079292418

Solution

I found that less -R can pass through the extended code. So the answer is

$ cargo --color always build |& less -R

And you can also use bat by

$ cargo --color always build |& bat --pager='less -R'

(I see no reason to do that though.)