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@@ -231,6 +231,41 @@ if words.len() != 2 {
231} 231}
232``` 232```
233 233
234# Avoid Monomorphization
235
236Rust uses monomorphization to compile generic code, meaning that for each instantiation of a generic functions with concrete types, the function is compiled afresh, *per crate*.
237This allows for exceptionally good performance, but leads to increased compile times.
238Runtime performance obeys 80%/20% rule -- only a small fraction of code is hot.
239Compile time **does not** obey this rule -- all code has to be compiled.
240For this reason, avoid making a lot of code type parametric, *especially* on the boundaries between crates.
241
242```rust
243// Good
244fn frbonicate(f: impl FnMut()) {
245 frobnicate_impl(&mut f)
246}
247fn frobnicate_impl(f: &mut dyn FnMut()) {
248 // lots of code
249}
250
251// Not as good
252fn frbonicate(f: impl FnMut()) {
253 // lots of code
254}
255```
256
257Avoid `AsRef` polymorphism, it pays back only for widely used libraries:
258
259```rust
260// Good
261fn frbonicate(f: &Path) {
262}
263
264// Not as good
265fn frbonicate(f: impl AsRef<Path>) {
266}
267```
268
234# Documentation 269# Documentation
235 270
236For `.md` and `.adoc` files, prefer a sentence-per-line format, don't wrap lines. 271For `.md` and `.adoc` files, prefer a sentence-per-line format, don't wrap lines.