LTO is enabled at opt-level=2 even with lto=false

[Rua]

My project has the following profile settings in the Cargo.toml:

[profile.dev]
opt-level = 2
incremental = true
lto = false

[profile.release]
incremental = true
lto = true

I profiled the building of my project using cargo +nightly rustc -- -Zself-profile. There's no --release so I assume it's going to do a dev build. Then, I run summarize summarize on the profiling file that was produced, and it lists as the very first item, therefore taking the most time:

+--------------------------------------------------+-----------+-----------------+----------+------------+
| Item                                             | Self time | % of total time | Time     | Item count |
+--------------------------------------------------+-----------+-----------------+----------+------------+
| LLVM_lto_optimize                                | 172.31s   | 23.797          | 172.31s  | 256        |
+--------------------------------------------------+-----------+-----------------+----------+------------+

This is unexpected, because LTO is explicitly disabled in the dev profile. I'm assuming that opt-level = 2 simply includes LTO by default, but then why am I not able to override it?

rustc --version --verbose:

rustc 1.53.0-nightly (bb491ed23 2021-04-23)
binary: rustc
commit-hash: bb491ed23937aef876622e4beb68ae95938b3bf9
commit-date: 2021-04-23
host: x86_64-unknown-linux-gnu
release: 1.53.0-nightly
LLVM version: 12.0.0

[ehuss]

Hi! The profile setting lto = false still does "thin local LTO" when the opt-level is not zero, which is LTO on the local crate across codegen units. To fully disable LTO, the setting needs to be lto = "off". There's more info in the docs. I realize that might be a little confusing (there is rust-lang/cargo#7491 filed for this), but that is how things evolved over time.

[Rua]

false and "off" are different things? Yes, that is rather confusing indeed! But thanks for the explanation.

[weihanglo]

Close in favor of rust-lang/cargo#7491.