clippy: replace flat_map with filter_map

BasePattern::as_identifier returns an Option; when iterating we can
filter out the Nones with `filter_map`. We were instead using
`flat_map`, which is much more general: it turns the Options into
iterators (which will yield 0 or 1 items) and then yields everything
from the iterators.

This is potentially slower and more complicated to read.

Author: apoelstra

Cargo.toml

@@ -73,7 +73,7 @@ explicit_deref_methods = "warn"
 explicit_into_iter_loop = "warn"
 explicit_iter_loop = "warn"
 filter_map_next = "warn"
-flat_map_option = "allow"
+flat_map_option = "warn"
 float_cmp = "warn"
 fn_params_excessive_bools = "warn"
 from_iter_instead_of_collect = "warn"

src/pattern.rs

@@ -259,7 +259,7 @@ impl BasePattern {
 
     /// Get an iterator over all identifiers inside the pattern.
     pub fn identifiers(&self) -> impl Iterator<Item = &Identifier> {
-        self.pre_order_iter().flat_map(BasePattern::as_identifier)
+        self.pre_order_iter().filter_map(BasePattern::as_identifier)
     }
 
     /// Check if all `identifiers` are contained inside the pattern.