feat: add is_absolute() for git-style absolute checks (#450)

This essentially means that starting slashes are always absolute, even
on windows.

Author: Byron

git-path/src/lib.rs

@@ -58,6 +58,9 @@ pub struct Spec(bstr::BString);
 mod convert;
 pub use convert::*;
 
+mod util;
+pub use util::is_absolute;
+
 mod spec;
 
 ///

git-path/src/util.rs

@@ -0,0 +1,8 @@
+use std::path::Path;
+
+/// return true if `path` is absolute, which depends on the platform but is always true if it starts with a `slash`, hence looks like
+/// a linux path.
+pub fn is_absolute(path: impl AsRef<Path>) -> bool {
+    let path = path.as_ref();
+    path.is_absolute() || path.to_str().and_then(|s| s.chars().next()) == Some('/')
+}

git-path/tests/path.rs

@@ -2,3 +2,4 @@ pub type Result<T = ()> = std::result::Result<T, Box<dyn std::error::Error>>;
 
 mod convert;
 mod realpath;
+mod util;

git-path/tests/util/mod.rs

@@ -0,0 +1,42 @@
+mod is_absolute {
+    #[test]
+    fn absolute_linux_path_is_true() {
+        assert!(git_path::is_absolute("/"));
+        assert!(git_path::is_absolute("/abs/path"));
+    }
+
+    #[test]
+    fn relative_linux_path_is_false() {
+        assert!(!git_path::is_absolute("./relative/path"));
+        assert!(!git_path::is_absolute("relative/path"));
+    }
+
+    #[cfg(not(windows))]
+    mod not_on_windows {
+        #[test]
+        fn drive_prefixes_are_false() {
+            assert!(!git_path::is_absolute("c:\\abs/path"));
+            assert!(!git_path::is_absolute("c:\\abs\\path"));
+        }
+    }
+
+    #[cfg(windows)]
+    mod on_windows {
+        #[test]
+        fn drive_prefixes_are_true() {
+            assert!(git_path::is_absolute("c:\\abs/path"));
+            assert!(git_path::is_absolute("c:\\abs\\path"));
+        }
+
+        #[test]
+        fn relative_paths_with_backslashes_are_false() {
+            assert!(!git_path::is_absolute(".\\rel/path"));
+            assert!(!git_path::is_absolute("rel\\path"));
+        }
+
+        #[test]
+        fn path_starting_with_backslash_is_false() {
+            assert!(!git_path::is_absolute("\\rel\\path"));
+        }
+    }
+}