Optimize flattening in apache airflow workspace (#11313)

## Motivation

No-op `uv lock` in apache airflow
(891c67f210ab7c877d1f00ea6ea3d3cdbb0e96ef) is slow, which makes `uv run`
slow, too.

Reference project:

```
$ hyperfine "uv run python -c \"print('hi')\""
Benchmark 1: uv run python -c "print('hi')"
Time (mean ± σ):      16.3 ms ±   1.5 ms    [User: 9.8 ms, System: 6.4 ms]
Range (min … max):    13.0 ms …  20.0 ms    186 runs
```

Apache airflow before:

```
$ hyperfine "uv run python -c \"print('hi')\""
Benchmark 1: uv run python -c "print('hi')"
Time (mean ± σ):     161.0 ms ±   5.2 ms    [User: 135.3 ms, System: 24.1 ms]
Range (min … max):   155.0 ms … 176.3 ms    18 runs
```

## Optimization

`FlatRequiresDist::from_requirements` is taking 50% of main thread
runtime.

Before:


![image](https://github.com/user-attachments/assets/10ea76eb-d1e9-477c-b400-39e653eb8f3a)

After both commits:


![image](https://github.com/user-attachments/assets/5c578ff6-f80b-46bb-9b5f-8be8435c3d85)

Apache airflow after the first commit:

```
$ hyperfine "uv-profiling run python -c \"print('hi')\""
Benchmark 1: uv-profiling run python -c "print('hi')"
  Time (mean ± σ):     122.3 ms ±   5.4 ms    [User: 96.1 ms, System: 24.7 ms]
  Range (min … max):   114.0 ms … 133.2 ms    23 runs
```

Apache airflow after the second commit:

```
$ hyperfine "uv-profiling run python -c \"print('hi')\""
Benchmark 1: uv-profiling run python -c "print('hi')"
  Time (mean ± σ):     108.5 ms ±   3.4 ms    [User: 83.2 ms, System: 24.2 ms]
  Range (min … max):   103.6 ms … 119.9 ms    28 runs
```

crates/uv-distribution/src/metadata/build_requires.rs

@@ -106,7 +106,7 @@ impl BuildRequires {
                         project_workspace.project_root(),
                         project_sources,
                         project_indexes,
-                        extra.as_ref(),
+                        extra.as_deref(),
                         group,
                         locations,
                         project_workspace.workspace(),
@@ -181,7 +181,7 @@ impl BuildRequires {
                         workspace.install_path(),
                         project_sources,
                         project_indexes,
-                        extra.as_ref(),
+                        extra.as_deref(),
                         group,
                         locations,
                         workspace,

crates/uv-distribution/src/metadata/lowering.rs

@@ -362,7 +362,7 @@ impl LoweredRequirement {
                     requirement
                         .marker
                         .top_level_extra_name()
-                        .is_some_and(|extra| extra == *target)
+                        .is_some_and(|extra| &*extra == target)
                 })
             })
             .cloned()

crates/uv-distribution/src/metadata/requires_dist.rs

@@ -209,7 +209,7 @@ impl RequiresDist {
                         project_workspace.project_root(),
                         project_sources,
                         project_indexes,
-                        extra.as_ref(),
+                        extra.as_deref(),
                         group,
                         locations,
                         project_workspace.workspace(),
@@ -262,7 +262,7 @@ impl RequiresDist {
                     // If there is no such requirement with the extra, error.
                     if !metadata.requires_dist.iter().any(|requirement| {
                         requirement.name == *name
-                            && requirement.marker.top_level_extra_name().as_ref() == Some(extra)
+                            && requirement.marker.top_level_extra_name().as_deref() == Some(extra)
                     }) {
                         return Err(MetadataError::IncompleteSourceExtra(
                             name.clone(),
@@ -370,6 +370,12 @@ impl FlatRequiresDist {
             return Self(requirements);
         }
 
+        // Memoize the top level extras, in the same order as `requirements`
+        let top_level_extras: Vec<_> = requirements
+            .iter()
+            .map(|req| req.marker.top_level_extra_name())
+            .collect();
+
         // Transitively process all extras that are recursively included.
         let mut flattened = requirements.clone();
         let mut seen = FxHashSet::<(ExtraName, MarkerTree)>::default();
@@ -384,8 +390,10 @@ impl FlatRequiresDist {
             }
 
             // Find the requirements for the extra.
-            for requirement in &requirements {
+            for (requirement, top_level_extra) in requirements.iter().zip(top_level_extras.iter()) {
-                if requirement.marker.top_level_extra_name().as_ref() == Some(&extra) {
+                if top_level_extra.as_deref() != Some(&extra) {
+                    continue;
+                }
                 let requirement = {
                     let mut marker = marker;
                     marker.and(requirement.marker);
@@ -413,7 +421,6 @@ impl FlatRequiresDist {
                 }
             }
         }
-        }
 
         // Drop all the self-references now that we've flattened them out.
         flattened.retain(|req| req.name != *name);

crates/uv-pep508/src/marker/tree.rs

@@ -1,3 +1,4 @@
+use std::borrow::Cow;
 use std::cmp::Ordering;
 use std::fmt::{self, Display, Formatter};
 use std::ops::{Bound, Deref};
@@ -772,7 +773,7 @@ impl MarkerTree {
     }
 
     /// Returns the underlying [`MarkerTreeKind`] of the root node.
-    pub fn kind(&self) -> MarkerTreeKind<'_> {
+    pub fn kind(self) -> MarkerTreeKind<'static> {
         if self.is_true() {
             return MarkerTreeKind::True;
         }
@@ -1002,11 +1003,19 @@ impl MarkerTree {
     ///
     /// ASSUMPTION: There is one `extra = "..."`, and it's either the only marker or part of the
     /// main conjunction.
-    pub fn top_level_extra_name(self) -> Option<ExtraName> {
+    pub fn top_level_extra_name(self) -> Option<Cow<'static, ExtraName>> {
+        // Fast path: The marker is only a `extra == "..."`.
+        if let MarkerTreeKind::Extra(marker) = self.kind() {
+            if marker.edge(true).is_true() {
+                let CanonicalMarkerValueExtra::Extra(extra) = marker.name;
+                return Some(Cow::Borrowed(extra));
+            }
+        }
+
         let extra_expression = self.top_level_extra()?;
 
         match extra_expression {
-            MarkerExpression::Extra { name, .. } => name.into_extra(),
+            MarkerExpression::Extra { name, .. } => name.into_extra().map(Cow::Owned),
             _ => unreachable!(),
         }
     }