The failure seems to occur in the check phase, see the build logs
referenced from:

https://data.guix-patches.cbaines.net/gnu/store/c3f7d3ziwjfkwg3j7xz47dj44sb2l5av-rust-1.26.2.drv

This looks like a relevant error:

---- [compile-fail] compile-fail/issue-15919.rs stdout ----
	

executing "/tmp/guix-build-rust-1.26.2.drv-0/rustc-1.26.2-src/build/aarch64-unknown-linux-gnu/stage2/bin/rustc" "/tmp/guix-build-rust-1.26.2.drv-0/rustc-1.26.2-src/src/test/compile-fail/issue-15919.rs" "-L" "/tmp/guix-build-rust-1.26.2.drv-0/rustc-1.26.2-src/build/aarch64-unknown-linux-gnu/test/compile-fail" "--target=aarch64-unknown-linux-gnu" "-Zui-testing" "-C" "prefer-dynamic" "-o" "/tmp/guix-build-rust-1.26.2.drv-0/rustc-1.26.2-src/build/aarch64-unknown-linux-gnu/test/compile-fail/issue-15919.stage2-aarch64-unknown-linux-gnu" "-Crpath" "-O" "-Zmiri" "-Zunstable-options" "-Lnative=/tmp/guix-build-rust-1.26.2.drv-0/rustc-1.26.2-src/build/aarch64-unknown-linux-gnu/native/rust-test-helpers" "-L" "/tmp/guix-build-rust-1.26.2.drv-0/rustc-1.26.2-src/build/aarch64-unknown-linux-gnu/test/compile-fail/issue-15919.stage2-aarch64-unknown-linux-gnu.aux" "-A" "unused"
------stdout------------------------------

------stderr------------------------------
error: the type `[usize; 18446744073709551615]` is too big for the current architecture