internal: document that ascription is preferred to a turbo fish

crates/ide_assists/src/handlers/sort_items.rs

@@ -131,7 +131,7 @@ impl AddRewrite for Assists {
         target: TextRange,
     ) -> Option<()> {
         self.add(AssistId("sort_items", AssistKind::RefactorRewrite), label, target, |builder| {
-            let mutable: Vec<_> = old.into_iter().map(|it| builder.make_mut(it)).collect();
+            let mutable: Vec<T> = old.into_iter().map(|it| builder.make_mut(it)).collect();
             mutable
                 .into_iter()
                 .zip(new)

crates/vfs/src/loader.rs

@@ -161,7 +161,7 @@ impl Directories {
     ///   - This path is longer than any element in `self.exclude` that is a prefix
     ///     of `path`. In case of equality, exclusion wins.
     fn includes_path(&self, path: &AbsPath) -> bool {
-        let mut include = None::<&AbsPathBuf>;
+        let mut include: Option<&AbsPathBuf> = None;
         for incl in &self.include {
             if path.starts_with(incl) {
                 include = Some(match include {

docs/dev/style.md

@@ -950,6 +950,26 @@ At the same time, it is more crowded -- it takes more time to visually scan it.
 **Rationale:** consistency, playing to language's strengths.
 Rust has first-class support for imperative control flow constructs like `for` and `if`, while functions are less first-class due to lack of universal function type, currying, and non-first-class effects (`?`, `.await`).
 
+## Turbofish
+
+Prefer type ascription over the turbofish.
+When ascribing types, avoid `_`
+
+```rust
+// GOOD
+let mutable: Vec<T> = old.into_iter().map(|it| builder.make_mut(it)).collect();
+
+// BAD
+let mutable: Vec<_> = old.into_iter().map(|it| builder.make_mut(it)).collect();
+
+// BAD
+let mutable = old.into_iter().map(|it| builder.make_mut(it)).collect::<Vec<_>>();
+```
+
+**Rationale:** consistency, readability.
+If compiler struggles to infer the type, the human would as well.
+Having the result type specified up-front helps with understanding what the chain of iterator methods is doing.
+
 ## Helper Functions
 
 Avoid creating singe-use helper functions: