Hi Ludo,

running gdb in an i686 transparent qemu emulation gives me:

$ gdb --args /gnu/store/gpc2m2f79rsgy15m65x0s5av7a41rp8b-mrustc-0.8.0/bin/mrustc src/tools/cargo/src/crates-io/lib.rs --crate-name crates_io --crate-type rlib --crate-tag 0_9_0 -g --cfg debug_assertions -O -o output/cargo-build/libcrates_io-0_9_0.hir -L output/cargo-build --extern curl=output/cargo-build/libcurl-0_4_6.hir --extern error_chain=output/cargo-build/liberror_chain-0_10_0.hir --extern serde=output/cargo-build/libserde-1_0_6.hir --extern serde_derive=output/cargo-build/libserde_derive-1_0_6.hir --extern serde_json=output/cargo-build/libserde_json-1_0_2.hir --extern url=output/cargo-build/liburl-1_4_0.hir -L output -L /gnu/store/gpc2m2f79rsgy15m65x0s5av7a41rp8b-mrustc-0.8.0/lib/mrust
(gdb) set follow-fork-mode child
(gdb) r
Starting program: /gnu/store/gpc2m2f79rsgy15m65x0s5av7a41rp8b-mrustc-0.8.0/bin/mrustc src/tools/cargo/src/crates-io/lib.rs --crate-name crates_io --crate-type rlib --crate-tag 0_9_0 -g --cfg debug_assertions -O -o output/cargo-build/libcrates_io-0_9_0.hir -L output/cargo-build --extern curl=output/cargo-build/libcurl-0_4_6.hir --extern error_chain=output/cargo-build/liberror_chain-0_10_0.hir --extern serde=output/cargo-build/libserde-1_0_6.hir --extern serde_derive=output/cargo-build/libserde_derive-1_0_6.hir --extern serde_json=output/cargo-build/libserde_json-1_0_2.hir --extern url=output/cargo-build/liburl-1_4_0.hir -L output -L /gnu/store/gpc2m2f79rsgy15m65x0s5av7a41rp8b-mrustc-0.8.0/lib/mrust
Parse: V V V
(0.01 s) Parse: DONE
LoadCrates: V V V
(2.33 s) LoadCrates: DONE
Expand: V V V
[Attaching after process 7067 vfork to child process 7071]
[New inferior 2 (process 7071)]
[Detaching vfork parent process 7067 after child exec]
[Inferior 1 (process 7067) detached]
process 7071 is executing new program: /tmp/guix-build-rust-1.19.0.drv-6/rustc-1.19.0-src/output/cargo-build/libserde_derive-1_0_6.hir-plugin
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/gnu/store/qk8irm5yp2khq4dj7zcpcaa1g87pfyvw-glibc-2.28/lib/libthread_db.so.1".
munmap_chunk(): invalid pointer

Thread 2.1 "libserde_derive" received signal SIGABRT, Aborted.
[Switching to Thread 0xf7c9de80 (LWP 7071)]
0xf7fd4b59 in ?? ()
(gdb) bt
#0  0xf7fd4b59 in ?? ()
#1  0x00000000 in ?? ()

I've reported it upstream as https://github.com/thepowersgang/mrustc/issues/108 .