feat: add used_vars in TemplateFix

part of #1070

crates/core/src/replacer.rs

@@ -64,6 +64,16 @@ enum MetaVarExtract {
   Transformed(MetaVariableID),
 }
 
+impl MetaVarExtract {
+  fn used_var(&self) -> &str {
+    match self {
+      MetaVarExtract::Single(s) => s,
+      MetaVarExtract::Multiple(s) => s,
+      MetaVarExtract::Transformed(s) => s,
+    }
+  }
+}
+
 fn split_first_meta_var(
   src: &str,
   meta_char: char,

crates/core/src/replacer/template.rs

@@ -5,9 +5,11 @@ use crate::matcher::NodeMatch;
 use crate::meta_var::MetaVarEnv;
 use crate::source::{Content, Doc};
 
-use std::borrow::Cow;
 use thiserror::Error;
 
+use std::borrow::Cow;
+use std::collections::HashSet;
+
 pub enum TemplateFix {
   // no meta_var, pure text
   Textual(String),
@@ -25,6 +27,14 @@ impl TemplateFix {
   pub fn with_transform<L: Language>(tpl: &str, lang: &L, trans: &[String]) -> Self {
     create_template(tpl, lang.meta_var_char(), trans)
   }
+
+  pub fn used_vars(&self) -> HashSet<&str> {
+    let template = match self {
+      TemplateFix::WithMetaVar(t) => t,
+      TemplateFix::Textual(_) => return HashSet::new(),
+    };
+    template.vars.iter().map(|v| v.0.used_var()).collect()
+  }
 }
 
 impl<D: Doc> Replacer<D> for TemplateFix {
@@ -313,6 +323,16 @@ if (true) {
     test_template_replace("$B $A", &[("A", "World"), ("B", "Hello")], "Hello World");
   }
 
+  #[test]
+  fn test_template_vars() {
+    let tf = TemplateFix::try_new("$A $B $C", &Tsx).expect("ok");
+    assert_eq!(tf.used_vars(), ["A", "B", "C"].into_iter().collect());
+    let tf = TemplateFix::try_new("$a$B$C", &Tsx).expect("ok");
+    assert_eq!(tf.used_vars(), ["B", "C"].into_iter().collect());
+    let tf = TemplateFix::try_new("$a$B$C", &Tsx).expect("ok");
+    assert_eq!(tf.used_vars(), ["B", "C"].into_iter().collect());
+  }
+
   // GH #641
   #[test]
   fn test_multi_row_replace() {