Can’t use LTO

[shivjm]

With (thin or fat) LTO enabled in the release profile, I immediately get this error:

rust_maplit> unpacking sources
rust_maplit> unpacking source archive /nix/store/g5pwgk95hnam0dw6x5kbz3yzsdh54f6g-crate-maplit-1.0.2.tar.gz
rust_maplit> source root is maplit-1.0.2
rust_maplit> setting SOURCE_DATE_EPOCH to timestamp 1566667343 of file maplit-1.0.2/Cargo.toml.orig
rust_maplit> patching sources
rust_maplit> configuring
rust_maplit> building
rust_maplit> Building lib: RUSTC 'src/lib.rs' '--out-dir=/nix/store/mdndwnmzpcxq6x3s2kryf1z1rf5852kw-rust_maplit-1.0.2/lib' '--crate-name=maplit' '--crate-type=lib' '--emit=metadata,link' '-Cembed-bitcode=no' '-Cextra-filename=-295a311dcdfdc3f7' '--color=always' '-Copt-level=3' '-Cdebuginfo=0' '-Cdebug-assertions=no' '-Coverflow-checks=no' '-Clto=fat' '-Cpanic=unwind' '-Ccodegen-units=1' '--cap-lints=allow' '-Cmetadata=295a311dcdfdc3f7' '-Ldependency=/nix/store/lwwq906vh2hg0p8g7hqbfb7rr50h6r0c-rust_maplit-1.0.2-dev/rust-support/deps-closure'
rust_maplit> error: options `-C embed-bitcode=no` and `-C lto` are incompatible

I can see -Cembed-bitcode=no is hard-coded:

https://github.com/oxalica/nocargo/blob/e3cd8e3396e85966e71f96f93586d64c797f56b9/build-rust-crate/builder-build-script.sh#L44-L51

I’m completely new to Nix; is this required for nocargo to function? Would it be possible to disable that flag or make it optional? Does LTO not make sense in this context?

[shivjm]

I’ve been experimenting to see if I could implement this, but I don’t think I know enough about linkers or about Nix to proceed.

[oxalica]

Fixed in 69dc641ff910f2350480db76d619d2efad318d92

[shivjm]

That’s great news, thank you! Is there any way this can work even with proc macro crates? This is the output I get, for example:

rust_async-trait> unpacking sources
rust_async-trait> unpacking source archive /nix/store/gagh6m6s2nrish3rnyvdal4xfd6y7j0r-crate-async-trait-0.1.57.tar.gz
rust_async-trait> source root is async-trait-0.1.57
rust_async-trait> setting SOURCE_DATE_EPOCH to timestamp 1153704088 of file async-trait-0.1.57/tests/ui/unsupported-self.stderr
rust_async-trait> patching sources
rust_async-trait> configuring
rust_async-trait> export OUT_DIR=/nix/store/30ryrbm27b00m0hszsd9l03nj1sih664-rust_async-trait-0.1.57-build/rust-support/out-dir
rust_async-trait> building
rust_async-trait> Building lib: RUSTC 'src/lib.rs' '--out-dir=/nix/store/l5cdqnw8d0fi054g3q571gnsdpagl9q2-rust_async-trait-0.1.57/lib' '--crate-name=async_trait' '--crate-type=proc-macro' '--emit=metadata,link' '-Cextra-filename=-e32805d186f61ce1' '-Copt-level=3' '-Cdebug-assertions=no' '-Coverflow-checks=no' '-Clto=thin' '-Ccodegen-units=1' '--cap-lints=allow' '--color=always' '-Cmetadata=e32805d186f61ce1' '--edition=2018' '--extern=proc_macro' '--extern=proc_macro2=/nix/store/ww3jdgaa70f5ly33853339js0masmz4r-rust_proc-macro2-1.0.43/lib/libproc_macro2-184e1004c4ac269c.rlib' '--extern=quote=/nix/store/q8ilfbznqfgy6dnza70fd8rrr9zvcj4b-rust_quote-1.0.21/lib/libquote-5598a87da415ddfe.rlib' '--extern=syn=/nix/store/4vgjxg6j41g0qw8kqg7w8fdrfwb8cf7f-rust_syn-1.0.99/lib/libsyn-ef320449b0c06c65.rlib' '-Ldependency=/nix/store/8jaycq86y6i17rppd7r4k30papvllijl-rust_async-trait-0.1.57-dev/rust-support/deps-closure'
rust_async-trait> error: lto can only be run for executables, cdylibs and static library outputs
rust_async-trait> error: aborting due to previous error
error: builder for '/nix/store/hg18yyha0lrzjbc4mcfsngqhfyn6da3p-rust_async-trait-0.1.57.drv' failed with exit code 1

[oxalica]

Is there any way this can work even with proc macro crates?

Should be fixed in a6681f1aca683a6d190768438fb298e912214e5d.

Also if you run into more problems, please open new issues.

[shivjm]

That works. Thank you very much for fixing both things so quickly. And of course, if I have any further issues, I’ll open new… er, issues.