That's actually not the case:
This option lets you set the default threshold for inlining a function. It takes an unsigned integer as a value. Inlining is based on a cost model, where a higher threshold will allow more inlining.
The default depends on the opt-level:
opt-level Threshold 0 N/A, only inlines always-inline functions 1 N/A, only inlines always-inline functions and LLVM lifetime intrinsics 2 225 3 275 s 75 z 25
Interestingly, setting a custom
-C inline-threshold=0 still enables some inlining. I've seen some reasonable improvements in debug runtime speed in personal projects just by setting an inline threshold of 25.