chore!: Ban nested slices (#4018)

# Description

## Problem\*

Resolves #4017 

## Summary\*

After this PR #3979 and an
internal discussion we have decided to temporarily block nested slices.

This PR adds a check in the frontend and in the SSA against nested
slices. The check in the frontend makes sure any struct fields with a
nested slice (but without generics) are not blocked as well as any type
declarations with nested slices. In order to account for generics in
structs we also have a checked array codegen that makes sure we do not
have a nested slice.

## Additional Context

The actual nested slice code in the ACIR gen is somewhat intertwined so
I felt it would be best for a separate PR which will be a followup to
this one.

## Documentation\*

Check one:
- [] No documentation needed.
- [X] Documentation included in this PR.
- [ ] **[Exceptional Case]** Documentation to be submitted in a separate
PR.

# PR Checklist\*

- [X] I have tested the changes locally.
- [X] I have formatted the changes with [Prettier](https://prettier.io/)
and/or `cargo fmt` on default settings.

Author: vezenovm

compiler/noirc_evaluator/src/errors.rs

@@ -42,6 +42,8 @@ pub enum RuntimeError {
     UnknownLoopBound { call_stack: CallStack },
     #[error("Argument is not constant")]
     AssertConstantFailed { call_stack: CallStack },
+    #[error("Nested slices are not supported")]
+    NestedSlice { call_stack: CallStack },
 }
 
 // We avoid showing the actual lhs and rhs since most of the time they are just 0
@@ -129,7 +131,8 @@ impl RuntimeError {
             | RuntimeError::UnknownLoopBound { call_stack }
             | RuntimeError::AssertConstantFailed { call_stack }
             | RuntimeError::IntegerOutOfBounds { call_stack, .. }
-            | RuntimeError::UnsupportedIntegerSize { call_stack, .. } => call_stack,
+            | RuntimeError::UnsupportedIntegerSize { call_stack, .. }
+            | RuntimeError::NestedSlice { call_stack, .. } => call_stack,
         }
     }
 }

compiler/noirc_evaluator/src/ssa/ssa_gen/mod.rs

@@ -187,12 +187,14 @@ impl<'a> FunctionContext<'a> {
 
                 let typ = Self::convert_type(&array.typ).flatten();
                 Ok(match array.typ {
-                    ast::Type::Array(_, _) => self.codegen_array(elements, typ[0].clone()),
+                    ast::Type::Array(_, _) => {
+                        self.codegen_array_checked(elements, typ[0].clone())?
+                    }
                     ast::Type::Slice(_) => {
                         let slice_length =
                             self.builder.field_constant(array.contents.len() as u128);
-
-                        let slice_contents = self.codegen_array(elements, typ[1].clone());
+                        let slice_contents =
+                            self.codegen_array_checked(elements, typ[1].clone())?;
                         Tree::Branch(vec![slice_length.into(), slice_contents])
                     }
                     _ => unreachable!(
@@ -231,6 +233,18 @@ impl<'a> FunctionContext<'a> {
         self.codegen_array(elements, typ)
     }
 
+    // Codegen an array but make sure that we do not have a nested slice
+    fn codegen_array_checked(
+        &mut self,
+        elements: Vec<Values>,
+        typ: Type,
+    ) -> Result<Values, RuntimeError> {
+        if typ.is_nested_slice() {
+            return Err(RuntimeError::NestedSlice { call_stack: self.builder.get_call_stack() });
+        }
+        Ok(self.codegen_array(elements, typ))
+    }
+
     /// Codegen an array by allocating enough space for each element and inserting separate
     /// store instructions until each element is stored. The store instructions will be separated
     /// by add instructions to calculate the new offset address to store to next.

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

@@ -82,6 +82,8 @@ pub enum ResolverError {
     NonCrateFunctionCalled { name: String, span: Span },
     #[error("Only sized types may be used in the entry point to a program")]
     InvalidTypeForEntryPoint { span: Span },
+    #[error("Nested slices are not supported")]
+    NestedSlices { span: Span },
 }
 
 impl ResolverError {
@@ -304,6 +306,11 @@ impl From<ResolverError> for Diagnostic {
             ResolverError::InvalidTypeForEntryPoint { span } => Diagnostic::simple_error(
                 "Only sized types may be used in the entry point to a program".to_string(),
                 "Slices, references, or any type containing them may not be used in main or a contract function".to_string(), span),
+            ResolverError::NestedSlices { span } => Diagnostic::simple_error(
+                "Nested slices are not supported".into(),
+                "Try to use a constant sized array instead".into(),
+                span,
+            ),
         }
     }
 }

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

@@ -705,7 +705,12 @@ impl<'a> Resolver<'a> {
 
     /// Translates an UnresolvedType to a Type
     pub fn resolve_type(&mut self, typ: UnresolvedType) -> Type {
-        self.resolve_type_inner(typ, &mut vec![])
+        let span = typ.span;
+        let resolved_type = self.resolve_type_inner(typ, &mut vec![]);
+        if resolved_type.is_nested_slice() {
+            self.errors.push(ResolverError::NestedSlices { span: span.unwrap() });
+        }
+        resolved_type
     }
 
     pub fn resolve_type_aliases(

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

@@ -24,6 +24,10 @@ pub(crate) fn resolve_structs(
     crate_id: CrateId,
 ) -> Vec<(CompilationError, FileId)> {
     let mut errors: Vec<(CompilationError, FileId)> = vec![];
+    // This is necessary to avoid cloning the entire struct map
+    // when adding checks after each struct field is resolved.
+    let struct_ids = structs.keys().copied().collect::<Vec<_>>();
+
     // Resolve each field in each struct.
     // Each struct should already be present in the NodeInterner after def collection.
     for (type_id, typ) in structs {
@@ -35,6 +39,28 @@ pub(crate) fn resolve_structs(
             struct_def.generics = generics;
         });
     }
+
+    // Check whether the struct fields have nested slices
+    // We need to check after all structs are resolved to
+    // make sure every struct's fields is accurately set.
+    for id in struct_ids {
+        let struct_type = context.def_interner.get_struct(id);
+        // Only handle structs without generics as any generics args will be checked
+        // after monomorphization when performing SSA codegen
+        if struct_type.borrow().generics.is_empty() {
+            let fields = struct_type.borrow().get_fields(&[]);
+            for field in fields.iter() {
+                if field.1.is_nested_slice() {
+                    errors.push((
+                        ResolverError::NestedSlices { span: struct_type.borrow().location.span }
+                            .into(),
+                        struct_type.borrow().location.file,
+                    ));
+                }
+            }
+        }
+    }
+
     errors
 }
 
@@ -49,5 +75,6 @@ fn resolve_struct_fields(
     let (generics, fields, errors) =
         Resolver::new(&mut context.def_interner, &path_resolver, &context.def_maps, file_id)
             .resolve_struct_fields(unresolved.struct_def);
+
     (generics, fields, errors)
 }

compiler/noirc_frontend/src/hir_def/types.rs

@@ -143,6 +143,43 @@ impl Type {
             | Type::Error => unreachable!("This type cannot exist as a parameter to main"),
         }
     }
+
+    pub(crate) fn is_nested_slice(&self) -> bool {
+        match self {
+            Type::Array(size, elem) => {
+                if let Type::NotConstant = size.as_ref() {
+                    elem.as_ref().contains_slice()
+                } else {
+                    false
+                }
+            }
+            _ => false,
+        }
+    }
+
+    fn contains_slice(&self) -> bool {
+        match self {
+            Type::Array(size, _) => matches!(size.as_ref(), Type::NotConstant),
+            Type::Struct(struct_typ, generics) => {
+                let fields = struct_typ.borrow().get_fields(generics);
+                for field in fields.iter() {
+                    if field.1.contains_slice() {
+                        return true;
+                    }
+                }
+                false
+            }
+            Type::Tuple(types) => {
+                for typ in types.iter() {
+                    if typ.contains_slice() {
+                        return true;
+                    }
+                }
+                false
+            }
+            _ => false,
+        }
+    }
 }
 
 /// A list of TypeVariableIds to bind to a type. Storing the

docs/docs/noir/concepts/data_types/arrays.md

@@ -70,6 +70,10 @@ You can define multidimensional arrays:
 let array : [[Field; 2]; 2];
 let element = array[0][0];
 ```
+However, multidimensional slices are not supported. For example, the following code will error at compile time:
+```rust
+let slice : [[Field]] = [];
+```
 
 ## Types