Add fallible closure to idioms

SUMMARY.md

@@ -19,6 +19,7 @@
   - [Privacy For Extensibility](./idioms/priv-extend.md)
   - [Easy doc initialization](./idioms/rustdoc-init.md)
   - [Temporary mutability](./idioms/temporary-mutability.md)
+  - [Fallible closure](./idioms/fallible-closure.md)
 
 - [Design Patterns](./patterns/index.md)
   - [Behavioural](./patterns/behavioural/intro.md)

idioms/fallible-closure.md

@@ -0,0 +1,35 @@
+# Fallible closure inside infallible function
+
+## Description
+
+Build a fallible closure inside of an apparently infallible function.
+
+## Example
+
+```rust
+use std::path::Path;
+
+// this function hides its failure states
+fn file_stem(path: &Path) -> &str {
+    // this closure is fallible, so it can make use of `?`
+    // fn() -> Option<&str>
+    let inner = || Some(path.file_stem()?.to_str()?);
+    inner().unwrap_or("untitled")
+}
+```
+
+## Motivation
+
+You may sometimes want to use the [`?`] operator inside an infallible function,
+in those cases a closure inside your function could be the simple solution.
+
+## Advantages
+
+This allows using the terser syntax permitted by the [`?`] operator inside an
+infallible context.
+
+## Disadvantages
+
+This is hiding failure states from the consumer of the API.
+
+[`?`]: https://doc.rust-lang.org/reference/expressions/operator-expr.html#the-question-mark-operator