Make extract_requirement_version()'s parsing more robust (#545)

Previously it didn't check the empty string case, which in practice
made no difference (since this function is only used internally during
compilation), but fixing this also improves test coverage, so brings us
one step closer to being able to enforce it at 100% in CI.

Author: edmorley

src/packaging_tool_versions.rs

@@ -19,14 +19,14 @@ pub(crate) const UV_VERSION: &str =
 // Note: Since this is a `const fn` we cannot use iterators and most methods on `str` / `Result`.
 const fn extract_requirement_version(requirement: &'static str) -> Option<&'static str> {
     let mut bytes = requirement.as_bytes();
-    while let [_, rest @ ..] = bytes {
-        if let [b'=', b'=', version @ ..] = rest {
-            if let Ok(version) = str::from_utf8(version.trim_ascii()) {
-                return Some(version);
-            }
-            break;
+    while let [_, tail @ ..] = bytes {
+        if let [b'=', b'=', version @ ..] = tail {
+            return match str::from_utf8(version.trim_ascii()) {
+                Ok(version) if !version.is_empty() => Some(version),
+                _ => None,
+            };
         }
-        bytes = rest;
+        bytes = tail;
     }
     None
 }
@@ -47,6 +47,7 @@ mod tests {
     #[test]
     fn extract_requirement_version_invalid() {
         assert_eq!(extract_requirement_version(""), None);
+        assert_eq!(extract_requirement_version(" == "), None);
         assert_eq!(extract_requirement_version("package"), None);
         assert_eq!(extract_requirement_version("package=<1.2.3"), None);
     }