Created attachment 700080 [details]
Seems that rust is hardcoding old CC/CXX somewhere, and not respecting the environment at librsvg build time.
error: linker `x86_64-pc-linux-gnu-gcc-10.2.0` not found
= note: No such file or directory (os error 2)
error: aborting due to previous error
error: could not compile `libm`
process didn't exit successfully: `rustc --crate-name build_script_build --edition=2018 /tmp/portage/gnome-base/librsvg-2.50.4/work/librsvg-2.50.4/vendor/libm/build.rs --error-format=json --json=diagnostic-rendered-ansi --crate-type bin --emit=dep-info,link -C embed-bitcode=no -C debuginfo=2 -C debug-assertions=off --cfg 'feature="default"' -C metadata=5cf14f69d9d81a55 -C extra-filename=-5cf14f69d9d81a55 --out-dir /tmp/portage/gnome-base/librsvg-2.50.4/work/librsvg-2.50.4-abi_x86_32.x86/target/release/build/libm-5cf14f69d9d81a55 -L dependency=/tmp/portage/gnome-base/librsvg-2.50.4/work/librsvg-2.50.4-abi_x86_32.x86/target/release/deps --cap-lints allow` (exit code: 1)
warning: build failed, waiting for other jobs to finish...
Created attachment 700083 [details]
It seems that you have set up the old gcc-10.2.0 somewhere, for instance via /etc/portage/env ; and maybe forgotten about it? Because you don't have that version on your system anymore, and thus the newer gcc-10.3.0 is not recognized.
No, I don't have 'set up the old gcc-10.2.0 somewhere'. It's Rust or some stupid Rust package hardcoding old value of CC.