Expand opening background

Author: joshtriplett

content/inside-rust/call-for-testing-hint-mostly-unused.md

@@ -14,11 +14,16 @@ future.
 
 ## Background
 
-Some crates provide comprehensive APIs with a very large surface area, yet many
-of their users need only a few entry points. In such cases, the compiler
-currently spends time generating code for the entire crate, and then ends up
+When building a Rust library crate, the compiler generates compiled code for as
+much of the crate as it can[^1], which
+gets linked into later crates into the dependency graph. However, some crates
+provide comprehensive APIs with a very large surface area, yet many of their
+users need only a few entry points. In such cases, the compiler currently
+spends time generating code for the entire crate, and the linker then ends up
 throwing most of that code away as unused.
 
+[^1]: Roughly, everything that isn't generic and isn't inlined.
+
 This can waste a substantial amount of compile time. Some large crates can take
 minutes to compile, and when you use these large crates as dependencies, they
 can take a disproportionate amount of the entire compilation time of your