fix: use a distinct warning for discarded let assignments to avoid confusion closes #763

crates/aiken-lang/src/tests/check.rs

@@ -1191,7 +1191,7 @@ fn discarded_let_bindings() {
     let (warnings, ast) = check(parse(source_code)).unwrap();
 
     assert!(matches!(warnings[0], Warning::UnusedVariable { ref name, .. } if name == "unused"));
-    assert!(matches!(warnings[1], Warning::UnusedVariable { ref name, .. } if name == "_"));
+    assert!(matches!(warnings[1], Warning::DiscardedLetAssignment { ref name, .. } if name == "_"));
 
     // Controls that unused let-bindings have been erased from the transformed AST.
     match ast.definitions.first() {

crates/aiken-lang/src/tipo/error.rs

@@ -1535,6 +1535,28 @@ pub enum Warning {
         name: String,
     },
 
+    #[error(
+        "I came across a discarded variable in a let assignment: {}.\n",
+        name.if_supports_color(Stderr, |s| s.purple())
+    )]
+    #[diagnostic(help("{}", formatdoc! {
+        r#"If you do want to enforce some side-effects, use {keyword_expect} with {name} instead of {keyword_let}.
+
+           You should also know that, unlike in typical imperative languages, unused let-bindings are {fully_ignored} in Aiken.
+           They will not produce any side-effect (such as error calls). Programs with or without unused variables are semantically equivalent.
+        "#,
+        fully_ignored = "fully_ignored".if_supports_color(Stderr, |s| s.bold()),
+        keyword_expect = "expect".if_supports_color(Stderr, |s| s.yellow()),
+        keyword_let = "let".if_supports_color(Stderr, |s| s.yellow()),
+        name = name.if_supports_color(Stderr, |s| s.yellow())
+    }))]
+    #[diagnostic(code("unused::discarded_let_assignment"))]
+    DiscardedLetAssignment {
+        #[label("discarded")]
+        location: Span,
+        name: String,
+    },
+
     #[error(
         "I came across a validator in a {} module which means\nI'm going to ignore it.\n",
         "lib/".if_supports_color(Stderr, |s| s.purple())
@@ -1595,6 +1617,7 @@ impl ExtraData for Warning {
             | Warning::UnusedPrivateModuleConstant { .. }
             | Warning::UnusedType { .. }
             | Warning::UnusedVariable { .. }
+            | Warning::DiscardedLetAssignment { .. }
             | Warning::Utf8ByteArrayIsValidHexString { .. }
             | Warning::ValidatorInLibraryModule { .. } => None,
             Warning::UnusedImportedModule { location, .. } => {

crates/aiken-lang/src/tipo/expr.rs

@@ -1707,7 +1707,8 @@ impl<'a, 'b> ExprTyper<'a, 'b> {
             .warnings
             .iter()
             .filter_map(|w| match w {
-                Warning::UnusedVariable { location, .. } => Some(*location),
+                Warning::UnusedVariable { location, .. }
+                | Warning::DiscardedLetAssignment { location, .. } => Some(*location),
                 _ => None,
             })
             .collect::<Vec<_>>();

crates/aiken-lang/src/tipo/pattern.rs

@@ -10,7 +10,7 @@ use itertools::Itertools;
 
 use super::{
     environment::{assert_no_labeled_arguments, collapse_links, EntityKind, Environment},
-    error::Error,
+    error::{Error, Warning},
     hydrator::Hydrator,
     PatternConstructor, Type, ValueConstructorVariant,
 };
@@ -150,7 +150,11 @@ impl<'a, 'b> PatternTyper<'a, 'b> {
                 if is_assignment {
                     // Register declaration for the unused variable detection
                     self.environment
-                        .init_usage(name.to_string(), EntityKind::Variable, location);
+                        .warnings
+                        .push(Warning::DiscardedLetAssignment {
+                            name: name.clone(),
+                            location,
+                        });
                 };
                 Ok(Pattern::Discard { name, location })
             }