--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/rust/hg-core/src/config/plain_info.rs	Thu Sep 22 17:16:54 2022 -0400
@@ -0,0 +1,79 @@
+use crate::utils::files::get_bytes_from_os_string;
+use std::env;
+
+/// Keeps information on whether plain mode is active.
+///
+/// Plain mode means that all configuration variables which affect
+/// the behavior and output of Mercurial should be
+/// ignored. Additionally, the output should be stable,
+/// reproducible and suitable for use in scripts or applications.
+///
+/// The only way to trigger plain mode is by setting either the
+/// `HGPLAIN' or `HGPLAINEXCEPT' environment variables.
+///
+/// The return value can either be
+/// - False if HGPLAIN is not set, or feature is in HGPLAINEXCEPT
+/// - False if feature is disabled by default and not included in HGPLAIN
+/// - True otherwise
+#[derive(Clone)]
+pub struct PlainInfo {
+    is_plain: bool,
+    except: Vec<Vec<u8>>,
+}
+
+impl PlainInfo {
+    fn plain_except(except: Vec<Vec<u8>>) -> Self {
+        PlainInfo {
+            is_plain: true,
+            except,
+        }
+    }
+
+    pub fn empty() -> PlainInfo {
+        PlainInfo {
+            is_plain: false,
+            except: vec![],
+        }
+    }
+
+    pub fn from_env() -> PlainInfo {
+        if let Some(except) = env::var_os("HGPLAINEXCEPT") {
+            PlainInfo::plain_except(
+                get_bytes_from_os_string(except)
+                    .split(|&byte| byte == b',')
+                    .map(|x| x.to_vec())
+                    .collect(),
+            )
+        } else {
+            PlainInfo {
+                is_plain: env::var_os("HGPLAIN").is_some(),
+                except: vec![],
+            }
+        }
+    }
+
+    pub fn is_feature_plain(&self, feature: &str) -> bool {
+        return self.is_plain
+            && !self
+                .except
+                .iter()
+                .any(|exception| exception.as_slice() == feature.as_bytes());
+    }
+
+    pub fn is_plain(&self) -> bool {
+        self.is_plain
+    }
+
+    pub fn plainalias(&self) -> bool {
+        self.is_feature_plain("alias")
+    }
+    pub fn plainrevsetalias(&self) -> bool {
+        self.is_feature_plain("revsetalias")
+    }
+    pub fn plaintemplatealias(&self) -> bool {
+        self.is_feature_plain("templatealias")
+    }
+    pub fn plaintweakdefaults(&self) -> bool {
+        self.is_feature_plain("tweakdefaults")
+    }
+}