codegen-units=1 not honored

[matthiaskrgr]

cargo new foo
cd foo
RUSTFLAGS="-Ztime-passes -Ccodegen-units=1" cargo build  -j 1

When checking the output, it looks like there are several codegen units used when calling llvm, or is this something else?

    time: 0.010; rss: 116MB	monomorphization collection
    time: 0.000; rss: 116MB	codegen unit partitioning
    time: 0.000; rss: 118MB	write allocator module
    time: 0.000; rss: 119MB	llvm function passes [1p3g4pb8qsux2ebh]
    time: 0.000; rss: 119MB	llvm module passes [1p3g4pb8qsux2ebh]
    time: 0.002; rss: 121MB	codegen passes [1p3g4pb8qsux2ebh]
    time: 0.000; rss: 122MB	llvm function passes [2h4slianzkuyn4qa]
    time: 0.000; rss: 122MB	llvm module passes [2h4slianzkuyn4qa]
    time: 0.002; rss: 123MB	codegen passes [2h4slianzkuyn4qa]
    time: 0.000; rss: 123MB	llvm function passes [1uwbbgiy3kdescgz]
    time: 0.000; rss: 123MB	llvm module passes [1uwbbgiy3kdescgz]
    time: 0.001; rss: 123MB	codegen passes [1uwbbgiy3kdescgz]
    time: 0.000; rss: 123MB	llvm function passes [5e3yhkplw0iikeko]
    time: 0.000; rss: 123MB	llvm module passes [5e3yhkplw0iikeko]
    time: 0.002; rss: 123MB	codegen passes [5e3yhkplw0iikeko]
    time: 0.005; rss: 125MB	codegen to LLVM IR
    time: 0.000; rss: 125MB	assert dep graph
    time: 0.000; rss: 125MB	llvm function passes [3wor41lao02720f1]
    time: 0.000; rss: 125MB	llvm module passes [3wor41lao02720f1]

rustc 1.40.0-nightly (4a8c5b20c 2019-10-23)

[andjo403]

codegen-units is only honored when incremental is off.
is is controled from here https://github.com/rust-lang/rust/blob/master/src/librustc_mir/monomorphize/partitioning.rs#L896
e.g. this will only give one codegen-unit CARGO_INCREMENTAL=0 RUSTFLAGS="-Ztime-passes -Ccodegen-units=1" cargo build

[ehuss]

I added a note to my documentation PR (#65136) to mention that it is disabled: 3538ffb

[matthiaskrgr]

If this is a pebkac, could there be a warning that one specified flags basically "unused"?

[jonas-schievink]

Fixed by #70156