Fix compilation with serde 1.0.122
Fix compilation with serde 1.0.122

This may or may not be a bug in serde, but let's commit a fix here first
in any case.

Closes #9101

nipunn1313/cargo,nipunn1313/cargo,rust-lang/cargo,rust-lang/cargo,eminence/cargo,eminence/cargo,eminence/cargo,rust-lang/cargo,rustyhorde/cargo,nipunn1313/cargo,rust-lang/cargo,eminence/cargo,rustyhorde/cargo,nipunn1313/cargo,rustyhorde/cargo,rustyhorde/cargo