feat: Add StructDefinition::set_fields (#5931)

# Description

## Problem\*

Resolves #5911

## Summary\*

Adds a function to set the fields on a struct type

## Additional Context

Hyper-specific error messages can be fun. Here's the error issued when
one of the field names isn't a valid identifier:

```
error: Quoted value in index 1 of this slice is not a valid field name
   ┌─ src/main.nr:25:18
   │
25 │     s.set_fields(fields);
   │                  ------ `quote { foo bar }` is not a valid field name for `set_fields`
   │
```

## Documentation\*

Check one:
- [ ] No documentation needed.
- [x] Documentation included in this PR.
- [ ] **[For Experimental Features]** 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.

---------

Co-authored-by: Ary Borenszweig <asterite@gmail.com>

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

@@ -202,6 +202,11 @@ pub enum InterpreterError {
     TypeAnnotationsNeededForMethodCall {
         location: Location,
     },
+    ExpectedIdentForStructField {
+        value: String,
+        index: usize,
+        location: Location,
+    },
 
     // These cases are not errors, they are just used to prevent us from running more code
     // until the loop can be resumed properly. These cases will never be displayed to users.
@@ -269,6 +274,7 @@ impl InterpreterError {
             | InterpreterError::FailedToResolveTraitBound { location, .. }
             | InterpreterError::FunctionAlreadyResolved { location, .. }
             | InterpreterError::MultipleMatchingImpls { location, .. }
+            | InterpreterError::ExpectedIdentForStructField { location, .. }
             | InterpreterError::TypeAnnotationsNeededForMethodCall { location } => *location,
 
             InterpreterError::FailedToParseMacro { error, file, .. } => {
@@ -566,6 +572,13 @@ impl<'a> From<&'a InterpreterError> for CustomDiagnostic {
                 error.add_note(message.to_string());
                 error
             }
+            InterpreterError::ExpectedIdentForStructField { value, index, location } => {
+                let msg = format!(
+                    "Quoted value in index {index} of this slice is not a valid field name"
+                );
+                let secondary = format!("`{value}` is not a valid field name for `set_fields`");
+                CustomDiagnostic::simple_error(msg, secondary, location.span)
+            }
         }
     }
 }

compiler/noirc_frontend/src/hir/comptime/interpreter/builtin.rs

@@ -33,7 +33,7 @@ use crate::{
     },
     hir_def::function::FunctionBody,
     lexer::Lexer,
-    macros_api::{HirExpression, HirLiteral, ModuleDefId, NodeInterner, Signedness},
+    macros_api::{HirExpression, HirLiteral, Ident, ModuleDefId, NodeInterner, Signedness},
     node_interner::{DefinitionKind, TraitImplKind},
     parser::{self},
     token::Token,
@@ -133,6 +133,7 @@ impl<'local, 'context> Interpreter<'local, 'context> {
             "struct_def_as_type" => struct_def_as_type(interner, arguments, location),
             "struct_def_fields" => struct_def_fields(interner, arguments, location),
             "struct_def_generics" => struct_def_generics(interner, arguments, location),
+            "struct_def_set_fields" => struct_def_set_fields(interner, arguments, location),
             "to_le_radix" => to_le_radix(arguments, return_type, location),
             "trait_constraint_eq" => trait_constraint_eq(interner, arguments, location),
             "trait_constraint_hash" => trait_constraint_hash(interner, arguments, location),
@@ -326,6 +327,64 @@ fn struct_def_fields(
     Ok(Value::Slice(fields, typ))
 }
 
+/// fn set_fields(self, new_fields: [(Quoted, Type)]) {}
+/// Returns (name, type) pairs of each field of this StructDefinition
+fn struct_def_set_fields(
+    interner: &mut NodeInterner,
+    arguments: Vec<(Value, Location)>,
+    location: Location,
+) -> IResult<Value> {
+    let (the_struct, fields) = check_two_arguments(arguments, location)?;
+    let struct_id = get_struct(the_struct)?;
+
+    let struct_def = interner.get_struct(struct_id);
+    let mut struct_def = struct_def.borrow_mut();
+
+    let field_location = fields.1;
+    let fields = get_slice(interner, fields)?.0;
+
+    let new_fields = fields
+        .into_iter()
+        .flat_map(|field_pair| get_tuple(interner, (field_pair, field_location)))
+        .enumerate()
+        .map(|(index, mut field_pair)| {
+            if field_pair.len() == 2 {
+                let typ = field_pair.pop().unwrap();
+                let name_value = field_pair.pop().unwrap();
+
+                let name_tokens = get_quoted((name_value.clone(), field_location))?;
+                let typ = get_type((typ, field_location))?;
+
+                match name_tokens.first() {
+                    Some(Token::Ident(name)) if name_tokens.len() == 1 => {
+                        Ok((Ident::new(name.clone(), field_location.span), typ))
+                    }
+                    _ => {
+                        let value = name_value.display(interner).to_string();
+                        let location = field_location;
+                        Err(InterpreterError::ExpectedIdentForStructField {
+                            value,
+                            index,
+                            location,
+                        })
+                    }
+                }
+            } else {
+                let type_var = interner.next_type_variable();
+                let expected = Type::Tuple(vec![type_var.clone(), type_var]);
+
+                let actual =
+                    Type::Tuple(vecmap(&field_pair, |value| value.get_type().into_owned()));
+
+                Err(InterpreterError::TypeMismatch { expected, actual, location })
+            }
+        })
+        .collect::<Result<Vec<_>, _>>()?;
+
+    struct_def.set_fields(new_fields);
+    Ok(Value::Unit)
+}
+
 fn slice_remove(
     interner: &mut NodeInterner,
     arguments: Vec<(Value, Location)>,

compiler/noirc_frontend/src/hir_def/types.rs

@@ -259,7 +259,6 @@ impl StructType {
     /// created. Therefore, this method is used to set the fields once they
     /// become known.
     pub fn set_fields(&mut self, fields: Vec<(Ident, Type)>) {
-        assert!(self.fields.is_empty());
         self.fields = fields;
     }
 

docs/docs/noir/standard_library/meta/struct_def.md

@@ -43,3 +43,32 @@ comptime fn example(foo: StructDefinition) {
 #include_code fields noir_stdlib/src/meta/struct_def.nr rust
 
 Returns each field of this struct as a pair of (field name, field type).
+
+### set_fields
+
+#include_code set_fields noir_stdlib/src/meta/struct_def.nr rust
+
+Sets the fields of this struct to the given fields list where each element
+is a pair of the field's name and the field's type. Expects each field name
+to be a single identifier. Note that this will override any previous fields
+on this struct. If those should be preserved, use `.fields()` to retrieve the
+current fields on the struct type and append the new fields from there.
+
+Example:
+
+```rust
+// Change this struct to:
+// struct Foo {
+//     a: u32,
+//     b: i8,
+// }
+#[mangle_fields]
+struct Foo { x: Field }
+
+comptime fn mangle_fields(s: StructDefinition) {
+    s.set_fields(&[
+        (quote { a }, quote { u32 }.as_type()),
+        (quote { b }, quote { i8 }.as_type()),
+    ]);
+}
+```

noir_stdlib/src/meta/struct_def.nr

@@ -18,4 +18,13 @@ impl StructDefinition {
 // docs:start:fields
     fn fields(self) -> [(Quoted, Type)] {}
     // docs:end:fields
+
+    /// Sets the fields of this struct to the given fields list.
+    /// All existing fields of the struct will be overridden with the given fields.
+    /// Each element of the fields list corresponds to the name and type of a field.
+    /// Each name is expected to be a single identifier.
+    #[builtin(struct_def_set_fields)]
+    // docs:start:set_fields
+    fn set_fields(self, new_fields: [(Quoted, Type)]) {}
+    // docs:end:set_fields
 }

test_programs/compile_success_empty/comptime_type_definition/src/main.nr

@@ -6,8 +6,21 @@ struct MyType<A, B, C> {
     field2: (B, C),
 }
 
+#[mutate_struct_fields]
+struct I32AndField {
+    z: i8,
+}
+
 comptime fn my_comptime_fn(typ: StructDefinition) {
     let _ = typ.as_type();
     assert_eq(typ.generics().len(), 3);
     assert_eq(typ.fields().len(), 2);
 }
+
+comptime fn mutate_struct_fields(s: StructDefinition) {
+    let fields = &[
+        (quote[x], quote[i32].as_type()),
+        (quote[y], quote[Field].as_type())
+    ];
+    s.set_fields(fields);
+}