feat: detect unconstructed structs

compiler/noirc_frontend/src/elaborator/expressions.rs

@@ -10,6 +10,7 @@ use crate::{
     },
     hir::{
         comptime::{self, InterpreterError},
+        def_map::ModuleId,
         resolution::errors::ResolverError,
         type_check::{generics::TraitGenerics, TypeCheckError},
     },
@@ -532,6 +533,8 @@ impl<'context> Elaborator<'context> {
             }
         };
 
+        self.mark_struct_as_constructed(r#type.clone(), &last_segment.ident);
+
         let turbofish_span = last_segment.turbofish_span();
 
         let struct_generics = self.resolve_struct_turbofish_generics(
@@ -561,6 +564,15 @@ impl<'context> Elaborator<'context> {
         (expr, Type::Struct(struct_type, generics))
     }
 
+    fn mark_struct_as_constructed(&mut self, struct_type: Shared<StructType>, name: &Ident) {
+        let struct_module_id = struct_type.borrow().id.module_id();
+        let module_data = self.get_module(struct_module_id);
+        let parent_local_id = module_data.parent.expect("Expected struct module parent to exist");
+        let parent_module_id =
+            ModuleId { krate: struct_module_id.krate, local_id: parent_local_id };
+        self.interner.usage_tracker.mark_as_used(parent_module_id, name);
+    }
+
     /// Resolve all the fields of a struct constructor expression.
     /// Ensures all fields are present, none are repeated, and all
     /// are part of the struct.

compiler/noirc_frontend/src/hir/def_collector/dc_crate.rs

@@ -508,7 +508,7 @@ impl DefCollector {
                 let ident = ident.clone();
                 let error = CompilationError::ResolverError(ResolverError::UnusedItem {
                     ident,
-                    item_type: unused_item.item_type(),
+                    item: *unused_item,
                 });
                 (error, module.location.file)
             })

compiler/noirc_frontend/src/hir/resolution/errors.rs

@@ -2,7 +2,10 @@ pub use noirc_errors::Span;
 use noirc_errors::{CustomDiagnostic as Diagnostic, FileDiagnostic};
 use thiserror::Error;
 
-use crate::{ast::Ident, hir::comptime::InterpreterError, parser::ParserError, Type};
+use crate::{
+    ast::Ident, hir::comptime::InterpreterError, parser::ParserError, usage_tracker::UnusedItem,
+    Type,
+};
 
 use super::import::PathResolutionError;
 
@@ -20,8 +23,8 @@ pub enum ResolverError {
     DuplicateDefinition { name: String, first_span: Span, second_span: Span },
     #[error("Unused variable")]
     UnusedVariable { ident: Ident },
-    #[error("Unused {item_type}")]
-    UnusedItem { ident: Ident, item_type: &'static str },
+    #[error("Unused {}", item.item_type())]
+    UnusedItem { ident: Ident, item: UnusedItem },
     #[error("Could not find variable in this scope")]
     VariableNotDeclared { name: String, span: Span },
     #[error("path is not an identifier")]
@@ -166,14 +169,24 @@ impl<'a> From<&'a ResolverError> for Diagnostic {
                 diagnostic.unnecessary = true;
                 diagnostic
             }
-            ResolverError::UnusedItem { ident, item_type } => {
+            ResolverError::UnusedItem { ident, item} => {
                 let name = &ident.0.contents;
+                let item_type = item.item_type();
 
-                let mut diagnostic = Diagnostic::simple_warning(
-                    format!("unused {item_type} {name}"),
-                    format!("unused {item_type}"),
-                    ident.span(),
-                );
+                let mut diagnostic =
+                    if let UnusedItem::Struct(..) = item {
+                        Diagnostic::simple_warning(
+                            format!("{item_type} `{name}` is never constructed"),
+                            format!("{item_type} is never constructed"),
+                            ident.span(),
+                        )
+                    } else {
+                        Diagnostic::simple_warning(
+                            format!("unused {item_type} {name}"),
+                            format!("unused {item_type}"),
+                            ident.span(),
+                        )
+                    };
                 diagnostic.unnecessary = true;
                 diagnostic
             }

compiler/noirc_frontend/src/hir/resolution/import.rs

@@ -289,7 +289,7 @@ fn resolve_name_in_module(
         return Err(PathResolutionError::Unresolved(first_segment.clone()));
     }
 
-    usage_tracker.mark_as_used(current_mod_id, first_segment);
+    usage_tracker.mark_as_referenced(current_mod_id, first_segment);
 
     let mut warning: Option<PathResolutionError> = None;
     for (index, (last_segment, current_segment)) in
@@ -356,7 +356,7 @@ fn resolve_name_in_module(
             return Err(PathResolutionError::Unresolved(current_segment.clone()));
         }
 
-        usage_tracker.mark_as_used(current_mod_id, current_segment);
+        usage_tracker.mark_as_referenced(current_mod_id, current_segment);
 
         current_ns = found_ns;
     }

compiler/noirc_frontend/src/tests.rs

@@ -3241,14 +3241,13 @@ fn errors_on_unused_private_import() {
     let errors = get_program_errors(src);
     assert_eq!(errors.len(), 1);
 
-    let CompilationError::ResolverError(ResolverError::UnusedItem { ident, item_type }) =
-        &errors[0].0
+    let CompilationError::ResolverError(ResolverError::UnusedItem { ident, item }) = &errors[0].0
     else {
         panic!("Expected an unused item error");
     };
 
     assert_eq!(ident.to_string(), "bar");
-    assert_eq!(*item_type, "import");
+    assert_eq!(item.item_type(), "import");
 }
 
 #[test]
@@ -3277,14 +3276,13 @@ fn errors_on_unused_pub_crate_import() {
     let errors = get_program_errors(src);
     assert_eq!(errors.len(), 1);
 
-    let CompilationError::ResolverError(ResolverError::UnusedItem { ident, item_type }) =
-        &errors[0].0
+    let CompilationError::ResolverError(ResolverError::UnusedItem { ident, item }) = &errors[0].0
     else {
         panic!("Expected an unused item error");
     };
 
     assert_eq!(ident.to_string(), "bar");
-    assert_eq!(*item_type, "import");
+    assert_eq!(item.item_type(), "import");
 }
 
 #[test]
@@ -3413,14 +3411,13 @@ fn errors_on_unused_function() {
     let errors = get_program_errors(src);
     assert_eq!(errors.len(), 1);
 
-    let CompilationError::ResolverError(ResolverError::UnusedItem { ident, item_type }) =
-        &errors[0].0
+    let CompilationError::ResolverError(ResolverError::UnusedItem { ident, item }) = &errors[0].0
     else {
         panic!("Expected an unused item error");
     };
 
     assert_eq!(ident.to_string(), "foo");
-    assert_eq!(*item_type, "function");
+    assert_eq!(item.item_type(), "function");
 }
 
 #[test]
@@ -3429,6 +3426,8 @@ fn errors_on_unused_struct() {
     struct Foo {}
     struct Bar {}
 
+    pub fn foo(_: Foo) {}
+
     fn main() {
         let _ = Bar {};
     }
@@ -3437,14 +3436,13 @@ fn errors_on_unused_struct() {
     let errors = get_program_errors(src);
     assert_eq!(errors.len(), 1);
 
-    let CompilationError::ResolverError(ResolverError::UnusedItem { ident, item_type }) =
-        &errors[0].0
+    let CompilationError::ResolverError(ResolverError::UnusedItem { ident, item }) = &errors[0].0
     else {
         panic!("Expected an unused item error");
     };
 
     assert_eq!(ident.to_string(), "Foo");
-    assert_eq!(*item_type, "struct");
+    assert_eq!(item.item_type(), "struct");
 }
 
 #[test]
@@ -3465,14 +3463,13 @@ fn errors_on_unused_trait() {
     let errors = get_program_errors(src);
     assert_eq!(errors.len(), 1);
 
-    let CompilationError::ResolverError(ResolverError::UnusedItem { ident, item_type }) =
-        &errors[0].0
+    let CompilationError::ResolverError(ResolverError::UnusedItem { ident, item }) = &errors[0].0
     else {
         panic!("Expected an unused item error");
     };
 
     assert_eq!(ident.to_string(), "Foo");
-    assert_eq!(*item_type, "trait");
+    assert_eq!(item.item_type(), "trait");
 }
 
 #[test]
@@ -3489,14 +3486,13 @@ fn errors_on_unused_type_alias() {
     let errors = get_program_errors(src);
     assert_eq!(errors.len(), 1);
 
-    let CompilationError::ResolverError(ResolverError::UnusedItem { ident, item_type }) =
-        &errors[0].0
+    let CompilationError::ResolverError(ResolverError::UnusedItem { ident, item }) = &errors[0].0
     else {
         panic!("Expected an unused item error");
     };
 
     assert_eq!(ident.to_string(), "Foo");
-    assert_eq!(*item_type, "type alias");
+    assert_eq!(item.item_type(), "type alias");
 }
 
 #[test]

compiler/noirc_frontend/src/usage_tracker.rs

@@ -7,7 +7,7 @@ use crate::{
     node_interner::{FuncId, TraitId, TypeAliasId},
 };
 
-#[derive(Debug)]
+#[derive(Debug, Copy, Clone, PartialEq, Eq)]
 pub enum UnusedItem {
     Import,
     Function(FuncId),
@@ -51,8 +51,29 @@ impl UsageTracker {
         }
     }
 
+    /// Marks an item as being referenced. This doesn't always makes the item as used. For example
+    /// if a struct is referenced it will still be considered unused unless it's constructed somewhere.
+    pub(crate) fn mark_as_referenced(&mut self, current_mod_id: ModuleId, name: &Ident) {
+        let Some(items) = self.unused_items.get_mut(&current_mod_id) else {
+            return;
+        };
+
+        let Some(unused_item) = items.get(name) else {
+            return;
+        };
+
+        if let UnusedItem::Struct(_) = unused_item {
+            return;
+        }
+
+        items.remove(name);
+    }
+
+    /// Marks an item as being used.
     pub(crate) fn mark_as_used(&mut self, current_mod_id: ModuleId, name: &Ident) {
-        self.unused_items.entry(current_mod_id).or_default().remove(name);
+        if let Some(items) = self.unused_items.get_mut(&current_mod_id) {
+            items.remove(name);
+        };
     }
 
     pub(crate) fn unused_items(&self) -> &HashMap<ModuleId, HashMap<Ident, UnusedItem>> {

noir_stdlib/src/meta/mod.nr

@@ -238,11 +238,15 @@ mod tests {
     }
     // docs:end:big-derive-usage-example
 
+    impl DoNothing for Bar {
+        fn do_nothing(_: Self) {}
+    }
+
     // This function is just to remove unused warnings
     fn remove_unused_warnings() {
-        let _: Bar = crate::panic::panic(f"");
-        let _: MyStruct = crate::panic::panic(f"");
-        let _: MyOtherStruct = crate::panic::panic(f"");
+        let _: Bar = Bar { x: 1, y: [2, 3] };
+        let _: MyStruct = MyStruct { my_field: 1 };
+        let _: MyOtherStruct = MyOtherStruct { my_other_field: 2 };
         let _ = derive_do_nothing(crate::panic::panic(f""));
         let _ = derive_do_nothing_alt(crate::panic::panic(f""));
         remove_unused_warnings();