gonna show this

src/main.rs

@@ -26,6 +26,7 @@ pub fn main() {
     }
 }
 
+#[must_use]
 pub fn do_it() -> Option<String> {
     let buf = if let Some(buf) = env::args().nth(1) {
         buf
@@ -48,6 +49,7 @@ pub enum Sexp {
 }
 
 impl Sexp {
+    #[must_use]
     pub fn nil() -> Self {
         Self::List(Vec::new())
     }
@@ -80,7 +82,7 @@ impl Sexp {
     pub fn list(&self) -> Option<&[Sexp]> {
         match self {
             Sexp::Atom(_) => None,
-            Sexp::List(xs) => Some(&xs),
+            Sexp::List(xs) => Some(xs),
         }
     }
 
@@ -204,6 +206,7 @@ pub enum Token<'src> {
     Atom(&'src str),
 }
 
+#[must_use]
 pub fn lex(src: &str) -> Vec<Token> {
     // maybe a bad idea?
     let mut out = Vec::with_capacity(src.len());
@@ -323,14 +326,17 @@ pub enum Rule {
 }
 
 impl Rule {
+    #[must_use]
     pub fn forall(vars: Vec<Symbol>, lhs: Sexp, rhs: Sexp) -> Rule {
         Rule::Forall { vars, lhs, rhs }
     }
 
+    #[must_use]
     pub fn concrete(lhs: Sexp, rhs: Sexp) -> Rule {
         Rule::Concrete { lhs, rhs }
     }
 
+    #[must_use]
     pub fn concrify(&self, sexp: &Sexp) -> Rule {
         match self {
             Rule::Forall { vars, lhs, rhs } => {
@@ -359,6 +365,7 @@ impl Rule {
         }
     }
 
+    #[must_use]
     pub fn concretions(&self, targets: &HashSet<&Sexp>) -> HashSet<Rule> {
         if self.is_concrete() {
             let mut out = HashSet::with_capacity(1);
@@ -438,6 +445,7 @@ impl Rule {
         matches!(self, Self::Concrete { .. })
     }
 
+    #[must_use]
     pub fn var_replace(from: Symbol, to: &Sexp) -> Rule {
         Rule::Concrete {
             lhs: Sexp::Atom(from),
@@ -480,6 +488,7 @@ impl fmt::Display for Rule {
     }
 }
 
+#[must_use]
 pub fn could_match(vars: Option<&[Symbol]>, lhs: &Sexp, expr: &Sexp) -> bool {
     let is_var = |var: &Symbol| -> bool {
         match vars {