Should `lto = false` be treated as LTO "off"?

Currently, a Cargo profile of lto = false is actually "thin local" LTO where LTO is performed across the local crate's codegen units. I'm wondering if that should actually be treated as -C lto=off where LTO is completely disabled?

This would change the behavior for manifests that set lto = false, which shows up occasionally since people copy/paste all the defaults.

Unfortunately there would be no way to explicitly express the default if this was changed, which I think would be a problem (or annoyance). We could add an explicit string ('thin-local'), and translate that to no -C lto flag, or add that to rustc itself.

Another option is to keep the current behavior, and just document that the default of false actually still performs some form of LTO, and that the string "off" should be used to completely turn it off. cc https://github.com/rust-lang/rust/pull/65136 where I am proposing to document these flags on the rustc side.

I kinda lean towards the documentation side, since otherwise it would change behavior for some projects, and probably isn't too important to be absolutely correct here.

Start by reviewing Cargo's profile handling alongside the linked rustc PR about documenting LTO flags. Resolve whether lto = false should retain its current behavior, map to -C lto=off, or gain an explicit thin-local form; document or test the chosen semantics before considering the issue done.