Ok, you're right, but we're playing with words here :) I understand the file containing the source code of the generic function is read only once, but I guess the AST of the generic function is processed many times, at least once for each instantiation of the function? And I'd venture in guessing that the biggest cost is in processing the AST and compiling it, not in reading/parsing the source?
Generating specialized code from a generic AST is in no way analogous to the exponential-time explosion that is the C++ header system, which is what Go is referencing when it says that it has been designed to read each source file only once. All languages with proper module systems have this property (which is to say, basically all languages that aren't C or C++).
Yes, designing a proper module system and getting rid of the header system is the best way to improve compilation time. But it was not my point. There are other factors impacting compilation time. Look at Scala for example: it has a proper module system, but compilation is still relatively slow, even if better than C++. This is the reason why I'm interested in learning how Rust compiles generic functions, and what is the impact on the compilation time (because the compiler has to generate a version of the function for each possible T), the binary size, and the pressure on the CPU cache (because having many versions of the function makes harder to keep all of them in the cache at the same time).
The Rust compiler actually has built-in diagnostics to let you profile the time that each phase of compilation takes. On mobile but I believe that it's `rustc -Z time-passes`, and whenever I use it on my own code 95% of the time consists of LLVM performing codegen. Nearly every other compiler pass takes approximately 0% of the compilation time (coherence checking being the notable exception, for reasons I have the discern).
It's not playing with words: the parser only runs once, and the processs/reprocessing AST is much faster than interpreting the original source multiple times (e.g. it can be indexed cheaply).
