fix: don't crash on AsTraitPath with empty path

compiler/noirc_frontend/src/ast/statement.rs

@@ -454,9 +454,8 @@ impl Path {
         self.last_segment().ident
     }
 
-    pub fn first_name(&self) -> &str {
-        assert!(!self.segments.is_empty());
-        &self.segments.first().unwrap().ident.0.contents
+    pub fn first_name(&self) -> Option<&str> {
+        self.segments.first().map(|segment| segment.ident.0.contents.as_str())
     }
 
     pub fn last_name(&self) -> &str {

compiler/noirc_frontend/src/elaborator/path_resolution.rs

@@ -107,7 +107,7 @@ impl<'context> Elaborator<'context> {
     pub(super) fn resolve_path(&mut self, mut path: Path) -> PathResolutionResult {
         let mut module_id = self.module_id();
 
-        if path.kind == PathKind::Plain && path.first_name() == SELF_TYPE_NAME {
+        if path.kind == PathKind::Plain && path.first_name() == Some(SELF_TYPE_NAME) {
             if let Some(Type::Struct(struct_type, _)) = &self.self_type {
                 let struct_type = struct_type.borrow();
                 if path.segments.len() == 1 {

compiler/noirc_frontend/src/elaborator/types.rs

@@ -192,7 +192,7 @@
 
     // Resolve Self::Foo to an associated type on the current trait or trait impl
     fn lookup_associated_type_on_self(&self, path: &Path) -> Option<Type> {
-        if path.segments.len() == 2 && path.first_name() == SELF_TYPE_NAME {
+        if path.segments.len() == 2 && path.first_name() == Some(SELF_TYPE_NAME) {
             if let Some(trait_id) = self.current_trait {
                 let the_trait = self.interner.get_trait(trait_id);
                 if let Some(typ) = the_trait.get_associated_type(path.last_name()) {
@@ -420,7 +420,7 @@
                 // TODO(https://github.com/noir-lang/noir/issues/6238):
                 // support non-u32 generics here
                 if !kind.unifies(&Kind::u32()) {
                     let error = TypeCheckError::EvaluatedGlobalIsntU32 {
                         expected_kind: Kind::u32().to_string(),
                         expr_kind: kind.to_string(),
                         expr_span: path.span(),

compiler/noirc_frontend/src/tests.rs

@@ -61,6 +61,15 @@
 }
 
 pub(crate) fn get_program(src: &str) -> (ParsedModule, Context, Vec<(CompilationError, FileId)>) {
+    get_program_with_maybe_parser_errors(
+        src, false, // allow parser errors
+    )
+}
+
+pub(crate) fn get_program_with_maybe_parser_errors(
+    src: &str,
+    allow_parser_errors: bool,
+) -> (ParsedModule, Context, Vec<(CompilationError, FileId)>) {
     let root = std::path::Path::new("/");
     let fm = FileManager::new(root);
 
@@ -73,7 +82,7 @@
     let mut errors = vecmap(parser_errors, |e| (e.into(), root_file_id));
     remove_experimental_warnings(&mut errors);
 
-    if !has_parser_error(&errors) {
+    if allow_parser_errors || !has_parser_error(&errors) {
         let inner_attributes: Vec<SecondaryAttribute> = program
             .items
             .iter()
@@ -1988,7 +1997,7 @@
 }
 
 // TODO(https://github.com/noir-lang/noir/issues/6238):
 // The EvaluatedGlobalIsntU32 warning is a stopgap
 // (originally from https://github.com/noir-lang/noir/issues/6125)
 #[test]
 fn numeric_generic_field_larger_than_u32() {
@@ -2005,7 +2014,7 @@
     assert_eq!(errors.len(), 2);
     assert!(matches!(
         errors[0].0,
         CompilationError::TypeError(TypeCheckError::EvaluatedGlobalIsntU32 { .. }),
     ));
     assert!(matches!(
         errors[1].0,
@@ -2014,7 +2023,7 @@
 }
 
 // TODO(https://github.com/noir-lang/noir/issues/6238):
 // The EvaluatedGlobalIsntU32 warning is a stopgap
 // (originally from https://github.com/noir-lang/noir/issues/6126)
 #[test]
 fn numeric_generic_field_arithmetic_larger_than_u32() {
@@ -2043,7 +2052,7 @@
 
     assert!(matches!(
         errors[0].0,
         CompilationError::TypeError(TypeCheckError::EvaluatedGlobalIsntU32 { .. }),
     ));
 
     assert!(matches!(
@@ -2179,7 +2188,7 @@
     assert_eq!(errors.len(), 3);
 
     // TODO(https://github.com/noir-lang/noir/issues/6238):
     // The EvaluatedGlobalIsntU32 warning is a stopgap
     assert!(matches!(
         errors[0].0,
         CompilationError::TypeError(TypeCheckError::EvaluatedGlobalIsntU32 { .. }),
@@ -3202,7 +3211,7 @@
 }
 
 #[test]
 fn arithmetic_generics_canonicalization_deduplication_regression() {
     let source = r#"
         struct ArrData<let N: u32> {
             a: [Field; N],
@@ -3536,7 +3545,7 @@
 
 #[test]
 fn unconditional_recursion_fail() {
     let srcs = vec![
         r#"
         fn main() {
             main()
@@ -3598,7 +3607,7 @@
         "#,
     ];
 
     for src in srcs {
         let errors = get_program_errors(src);
         assert!(
             !errors.is_empty(),
@@ -3617,7 +3626,7 @@
 
 #[test]
 fn unconditional_recursion_pass() {
     let srcs = vec![
         r#"
         fn main() {
             if false { main(); }

compiler/noirc_frontend/src/tests/traits.rs

@@ -1,6 +1,6 @@
 use crate::hir::def_collector::dc_crate::CompilationError;
 use crate::hir::resolution::errors::ResolverError;
-use crate::tests::get_program_errors;
+use crate::tests::{get_program_errors, get_program_with_maybe_parser_errors};
 
 use super::assert_no_errors;
 
@@ -390,3 +390,19 @@ fn trait_bounds_which_are_dependent_on_generic_types_are_resolved_correctly() {
     "#;
     assert_no_errors(src);
 }
+
+#[test]
+fn does_not_crash_on_as_trait_path_with_empty_path() {
+    let src = r#"
+        struct Foo {
+            x: <N>,
+        }
+
+        fn main() {}
+    "#;
+
+    let (_, _, errors) = get_program_with_maybe_parser_errors(
+        src, true, // allow parser errors
+    );
+    assert!(!errors.is_empty());
+}