​​​​​​

warning: unused manifest key: build
help: build is a valid .cargo/config.toml key

(~/.cargo/config.toml)

[build]
warnings = "deny"

(Cargo.toml)

[package]
name = "temp"
version = "0.1.0"
edition = "2024"

[dependencies]

[build]
warnings = "deny"

$ export | grep -i cargo
declare -x CARGO_BUILD_WARNINGS="deny"

...and it still compiles even w/ warnings ​​