Avoid allocating for names in the PEP 508 parser (#10476)

## Summary

We can read from the slice directly. I don't think this will affect
performance today, because `from_str` will then allocate, but it
_should_ be a speedup once #10475 merges, since we can then avoid
allocating a `String` and go straight from `str` to `ArcStr`.

Author: charliermarsh

crates/uv-pep508/src/lib.rs

@@ -394,12 +394,9 @@ fn parse_name<T: Pep508Url>(cursor: &mut Cursor) -> Result<PackageName, Pep508Er
     // https://peps.python.org/pep-0508/#names
     // ^([A-Z0-9]|[A-Z0-9][A-Z0-9._-]*[A-Z0-9])$ with re.IGNORECASE
     let start = cursor.pos();
-    let mut name = String::new();
 
     if let Some((index, char)) = cursor.next() {
-        if matches!(char, 'A'..='Z' | 'a'..='z' | '0'..='9') {
-            name.push(char);
-        } else {
+        if !matches!(char, 'A'..='Z' | 'a'..='z' | '0'..='9') {
             // Check if the user added a filesystem path without a package name. pip supports this
             // in `requirements.txt`, but it doesn't adhere to the PEP 508 grammar.
             let mut clone = cursor.clone().at(start);
@@ -431,26 +428,24 @@ fn parse_name<T: Pep508Url>(cursor: &mut Cursor) -> Result<PackageName, Pep508Er
     }
 
     loop {
-        match cursor.peek() {
-            Some((index, char @ ('A'..='Z' | 'a'..='z' | '0'..='9' | '.' | '-' | '_'))) => {
-                name.push(char);
-                cursor.next();
-                // [.-_] can't be the final character
-                if cursor.peek().is_none() && matches!(char, '.' | '-' | '_') {
-                    return Err(Pep508Error {
-                        message: Pep508ErrorSource::String(format!(
-                            "Package name must end with an alphanumeric character, not '{char}'"
-                        )),
-                        start: index,
-                        len: char.len_utf8(),
-                        input: cursor.to_string(),
-                    });
-                }
-            }
-            Some(_) | None => {
-                return Ok(PackageName::new(name)
-                    .expect("`PackageName` validation should match PEP 508 parsing"));
+        if let Some((index, char @ ('A'..='Z' | 'a'..='z' | '0'..='9' | '.' | '-' | '_'))) =
+            cursor.peek()
+        {
+            cursor.next();
+            // [.-_] can't be the final character
+            if cursor.peek().is_none() && matches!(char, '.' | '-' | '_') {
+                return Err(Pep508Error {
+                    message: Pep508ErrorSource::String(format!(
+                        "Package name must end with an alphanumeric character, not `{char}`"
+                    )),
+                    start: index,
+                    len: char.len_utf8(),
+                    input: cursor.to_string(),
+                });
             }
+        } else {
+            let len = cursor.pos() - start;
+            return Ok(PackageName::from_str(cursor.slice(start, len)).unwrap());
         }
     }
 }
@@ -1033,7 +1028,7 @@ mod tests {
         assert_snapshot!(
             parse_pep508_err("name_"),
             @"
-        Package name must end with an alphanumeric character, not '_'
+        Package name must end with an alphanumeric character, not `_`
         name_
             ^"
         );