Docs are missing possible values for LTO

[0xpr03]

Problem

Docs are missing the possible values for lto.
I just got to know via #rust irc about

[profile.release]
lto = "fat"

This value isn't listed or documented in the cargo docs.

Steps

1. Go to https://doc.rust-lang.org/cargo/reference/config.html#configuration-keys it's not documented

Possible Solution(s)

Document all possible values.

Notes
I was told that lto is enabled by default for release builds, going by the docs that isn't true, is it ?
At least for me it made a difference of ~3 MiB to set lto = "fat"

[ehuss]

The LTO settings and defaults are documented at https://doc.rust-lang.org/cargo/reference/manifest.html#the-profile-sections
true is the same as "fat LTO". Debug/release default to LTO off (false).

[magicmaaaaan]

One thing that's not clear is if enabling lto affects codegen-units.

There's a note in the code, but that condition seems to set codegen-units regardless.

[0xpr03]

I'd also like to have some more clarification about "fat" etc being the same. And a note that you can trim down the size even more by using this.
As I've said @ehuss it's not about the option itself, it's about the value and it's effect.

[ehuss]

Yea, I think it could definitely mention that true is the same as "fat". As for documenting exactly what LTO does, and what the different modes are, I would think that should live in the rustc book and the cargo docs could just point there. It currently doesn't say too much, deferring to the LLVM docs. There are a variety of codegen options (like s/z optimizations) that are underdocumented, but it's not always clear what should be said. These kinds of things change over time, and the documentation will almost certainly fall out of date.

[ehuss]

There's also a rustc issue rust-lang/rust#48518 about the codegen units/lto interaction.