first step towards efficiently obtaining git config information (#450)

Author: Byron

git-repository/src/env.rs

@@ -19,9 +19,53 @@ pub fn args_os() -> impl Iterator<Item = OsString> {
     })
 }
 
-/// Convert the given `input` into a `BString`, useful as `parse(try_from_os_str = <me>)` function.
+/// Convert the given `input` into a `BString`, useful as `#[clap(parse(try_from_os_str = git::env::os_str_to_bstring))]` function.
 pub fn os_str_to_bstring(input: &OsStr) -> Result<BString, String> {
     Vec::from_os_string(input.into())
         .map(Into::into)
         .map_err(|_| input.to_string_lossy().into_owned())
 }
+
+/// Environment information involving the `git` program itself.
+pub mod git {
+    use crate::bstr::{BStr, ByteSlice};
+
+    fn first_file_from_config_with_origin(source: &BStr) -> Option<&BStr> {
+        let file = source.strip_prefix(b"file:")?;
+        let end_pos = file.find_byte(b'\t')?;
+        file[..end_pos].as_bstr().into()
+    }
+
+    #[cfg(test)]
+    mod tests {
+        use crate::env::git;
+
+        #[test]
+        fn first_file_from_config_with_origin() {
+            let macos = "file:/Applications/Xcode.app/Contents/Developer/usr/share/git-core/gitconfig	credential.helper=osxkeychain\nfile:/Users/byron/.gitconfig	push.default=simple\n";
+            let win_msys =
+                "file:C:/git-sdk-64/etc/gitconfig	core.symlinks=false\r\nfile:C:/git-sdk-64/etc/gitconfig	core.autocrlf=true";
+            let win_cmd = "file:C:/Program Files/Git/etc/gitconfig	diff.astextplain.textconv=astextplain\r\nfile:C:/Program Files/Git/etc/gitconfig	filter.lfs.clean=git-lfs clean -- %f\r\n";
+            let linux = "file:/home/parallels/.gitconfig	core.excludesfile=~/.gitignore\n";
+            let bogus = "something unexpected";
+            let empty = "";
+
+            for (source, expected) in [
+                (
+                    macos,
+                    Some("/Applications/Xcode.app/Contents/Developer/usr/share/git-core/gitconfig"),
+                ),
+                (win_msys, Some("C:/git-sdk-64/etc/gitconfig")),
+                (win_cmd, Some("C:/Program Files/Git/etc/gitconfig")),
+                (linux, Some("/home/parallels/.gitconfig")),
+                (bogus, None),
+                (empty, None),
+            ] {
+                assert_eq!(
+                    git::first_file_from_config_with_origin(source.into()),
+                    expected.map(Into::into)
+                );
+            }
+        }
+    }
+}