Add support for path completion (#2608)

Co-authored-by: Michael Davis <mcarsondavis@gmail.com>
Co-authored-by: Pascal Kuthe <pascalkuthe@pm.me>

helix-stdx/src/env.rs

@@ -1,9 +1,12 @@
 use std::{
-    ffi::OsStr,
+    borrow::Cow,
+    ffi::{OsStr, OsString},
     path::{Path, PathBuf},
     sync::RwLock,
 };
 
+use once_cell::sync::Lazy;
+
 static CWD: RwLock<Option<PathBuf>> = RwLock::new(None);
 
 // Get the current working directory.
@@ -59,6 +62,93 @@ pub fn which<T: AsRef<OsStr>>(
     })
 }
 
+fn find_brace_end(src: &[u8]) -> Option<usize> {
+    use regex_automata::meta::Regex;
+
+    static REGEX: Lazy<Regex> = Lazy::new(|| Regex::builder().build("[{}]").unwrap());
+    let mut depth = 0;
+    for mat in REGEX.find_iter(src) {
+        let pos = mat.start();
+        match src[pos] {
+            b'{' => depth += 1,
+            b'}' if depth == 0 => return Some(pos),
+            b'}' => depth -= 1,
+            _ => unreachable!(),
+        }
+    }
+    None
+}
+
+fn expand_impl(src: &OsStr, mut resolve: impl FnMut(&OsStr) -> Option<OsString>) -> Cow<OsStr> {
+    use regex_automata::meta::Regex;
+
+    static REGEX: Lazy<Regex> = Lazy::new(|| {
+        Regex::builder()
+            .build_many(&[
+                r"\$\{([^\}:]+):-",
+                r"\$\{([^\}:]+):=",
+                r"\$\{([^\}-]+)-",
+                r"\$\{([^\}=]+)=",
+                r"\$\{([^\}]+)",
+                r"\$(\w+)",
+            ])
+            .unwrap()
+    });
+
+    let bytes = src.as_encoded_bytes();
+    let mut res = Vec::with_capacity(bytes.len());
+    let mut pos = 0;
+    for captures in REGEX.captures_iter(bytes) {
+        let mat = captures.get_match().unwrap();
+        let pattern_id = mat.pattern().as_usize();
+        let mut range = mat.range();
+        let var = &bytes[captures.get_group(1).unwrap().range()];
+        let default = if pattern_id != 5 {
+            let Some(bracket_pos) = find_brace_end(&bytes[range.end..]) else {
+                break;
+            };
+            let default = &bytes[range.end..range.end + bracket_pos];
+            range.end += bracket_pos + 1;
+            default
+        } else {
+            &[]
+        };
+        // safety: this is a codepoint aligned substring of an osstr (always valid)
+        let var = unsafe { OsStr::from_encoded_bytes_unchecked(var) };
+        let expansion = resolve(var);
+        let expansion = match &expansion {
+            Some(val) => {
+                if val.is_empty() && pattern_id < 2 {
+                    default
+                } else {
+                    val.as_encoded_bytes()
+                }
+            }
+            None => default,
+        };
+        res.extend_from_slice(&bytes[pos..range.start]);
+        pos = range.end;
+        res.extend_from_slice(expansion);
+    }
+    if pos == 0 {
+        src.into()
+    } else {
+        res.extend_from_slice(&bytes[pos..]);
+        // safety: this is a composition of valid osstr (and codepoint aligned slices which are also valid)
+        unsafe { OsString::from_encoded_bytes_unchecked(res) }.into()
+    }
+}
+
+/// performs substitution of enviorment variables. Supports the following (POSIX) syntax:
+///
+/// * `$<var>`, `${<var>}`
+/// * `${<var>:-<default>}`, `${<var>-<default>}`
+/// * `${<var>:=<default>}`, `${<var>=default}`
+///
+pub fn expand<S: AsRef<OsStr> + ?Sized>(src: &S) -> Cow<OsStr> {
+    expand_impl(src.as_ref(), |var| std::env::var_os(var))
+}
+
 #[derive(Debug)]
 pub struct ExecutableNotFoundError {
     command: String,
@@ -75,7 +165,9 @@ impl std::error::Error for ExecutableNotFoundError {}
 
 #[cfg(test)]
 mod tests {
-    use super::{current_working_dir, set_current_working_dir};
+    use std::ffi::{OsStr, OsString};
+
+    use super::{current_working_dir, expand_impl, set_current_working_dir};
 
     #[test]
     fn current_dir_is_set() {
@@ -88,4 +180,34 @@ mod tests {
         let cwd = current_working_dir();
         assert_eq!(cwd, new_path);
     }
+
+    macro_rules! assert_env_expand {
+        ($env: expr, $lhs: expr, $rhs: expr) => {
+            assert_eq!(&*expand_impl($lhs.as_ref(), $env), OsStr::new($rhs));
+        };
+    }
+
+    /// paths that should work on all platforms
+    #[test]
+    fn test_env_expand() {
+        let env = |var: &OsStr| -> Option<OsString> {
+            match var.to_str().unwrap() {
+                "FOO" => Some("foo".into()),
+                "EMPTY" => Some("".into()),
+                _ => None,
+            }
+        };
+        assert_env_expand!(env, "pass_trough", "pass_trough");
+        assert_env_expand!(env, "$FOO", "foo");
+        assert_env_expand!(env, "bar/$FOO/baz", "bar/foo/baz");
+        assert_env_expand!(env, "bar/${FOO}/baz", "bar/foo/baz");
+        assert_env_expand!(env, "baz/${BAR:-bar}/foo", "baz/bar/foo");
+        assert_env_expand!(env, "baz/${BAR:=bar}/foo", "baz/bar/foo");
+        assert_env_expand!(env, "baz/${BAR-bar}/foo", "baz/bar/foo");
+        assert_env_expand!(env, "baz/${BAR=bar}/foo", "baz/bar/foo");
+        assert_env_expand!(env, "baz/${EMPTY:-bar}/foo", "baz/bar/foo");
+        assert_env_expand!(env, "baz/${EMPTY:=bar}/foo", "baz/bar/foo");
+        assert_env_expand!(env, "baz/${EMPTY-bar}/foo", "baz//foo");
+        assert_env_expand!(env, "baz/${EMPTY=bar}/foo", "baz//foo");
+    }
 }