feat: Add StructDefinition::add_generic (#5961)

# Description

## Problem\*

Resolves #5957

## Summary\*

Adds a generic to an existing struct.

## Additional Context

Due to the same reasoning in
#5926, existing code referring
to these structs won't see the generics on them. For now I've documented
to try to avoid this use case.

## 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 <[email protected]>

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

@@ -194,7 +194,6 @@ pub enum InterpreterError {
         candidates: Vec<String>,
         location: Location,
     },
-
     Unimplemented {
         item: String,
         location: Location,
@@ -211,6 +210,16 @@ pub enum InterpreterError {
         attribute: String,
         location: Location,
     },
+    GenericNameShouldBeAnIdent {
+        name: Rc<String>,
+        location: Location,
+    },
+    DuplicateGeneric {
+        name: Rc<String>,
+        struct_name: String,
+        duplicate_location: Location,
+        existing_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.
@@ -279,8 +288,10 @@ impl InterpreterError {
             | InterpreterError::FunctionAlreadyResolved { location, .. }
             | InterpreterError::MultipleMatchingImpls { location, .. }
             | InterpreterError::ExpectedIdentForStructField { location, .. }
+            | InterpreterError::InvalidAttribute { location, .. }
+            | InterpreterError::GenericNameShouldBeAnIdent { location, .. }
+            | InterpreterError::DuplicateGeneric { duplicate_location: location, .. }
             | InterpreterError::TypeAnnotationsNeededForMethodCall { location } => *location,
-            InterpreterError::InvalidAttribute { location, .. } => *location,
 
             InterpreterError::FailedToParseMacro { error, file, .. } => {
                 Location::new(error.span(), *file)
@@ -589,6 +600,32 @@ impl<'a> From<&'a InterpreterError> for CustomDiagnostic {
                 let secondary = "Note that this method expects attribute contents, without the leading `#[` or trailing `]`".to_string();
                 CustomDiagnostic::simple_error(msg, secondary, location.span)
             }
+            InterpreterError::GenericNameShouldBeAnIdent { name, location } => {
+                let msg =
+                    "Generic name needs to be a valid identifer (one word beginning with a letter)"
+                        .to_string();
+                let secondary = format!("`{name}` is not a valid identifier");
+                CustomDiagnostic::simple_error(msg, secondary, location.span)
+            }
+            InterpreterError::DuplicateGeneric {
+                name,
+                struct_name,
+                duplicate_location,
+                existing_location,
+            } => {
+                let msg = format!("`{struct_name}` already has a generic named `{name}`");
+                let secondary = format!("`{name}` added here a second time");
+                let mut error =
+                    CustomDiagnostic::simple_error(msg, secondary, duplicate_location.span);
+
+                let existing_msg = format!("`{name}` was previously defined here");
+                error.add_secondary_with_file(
+                    existing_msg,
+                    existing_location.span,
+                    existing_location.file,
+                );
+                error
+            }
         }
     }
 }

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

@@ -39,7 +39,7 @@ use crate::{
     node_interner::{DefinitionKind, TraitImplKind},
     parser::{self},
     token::{Attribute, SecondaryAttribute, Token},
-    QuotedType, Shared, Type,
+    Kind, QuotedType, ResolvedGeneric, Shared, Type, TypeVariable,
 };
 
 use self::builtin_helpers::{get_array, get_str, get_u8};
@@ -139,7 +139,8 @@ impl<'local, 'context> Interpreter<'local, 'context> {
             "slice_push_front" => slice_push_front(interner, arguments, location),
             "slice_remove" => slice_remove(interner, arguments, location, call_stack),
             "str_as_bytes" => str_as_bytes(interner, arguments, location),
-            "struct_def_add_attribute" => struct_def_add_attribute(self, arguments, location),
+            "struct_def_add_attribute" => struct_def_add_attribute(interner, arguments, location),
+            "struct_def_add_generic" => struct_def_add_generic(interner, arguments, location),
             "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),
@@ -280,13 +281,13 @@ fn str_as_bytes(
 
 // fn add_attribute<let N: u32>(self, attribute: str<N>)
 fn struct_def_add_attribute(
-    interpreter: &mut Interpreter,
+    interner: &mut NodeInterner,
     arguments: Vec<(Value, Location)>,
     location: Location,
 ) -> IResult<Value> {
     let (self_argument, attribute) = check_two_arguments(arguments, location)?;
     let attribute_location = attribute.1;
-    let attribute = get_str(interpreter.elaborator.interner, attribute)?;
+    let attribute = get_str(interner, attribute)?;
 
     let mut tokens = Lexer::lex(&format!("#[{}]", attribute)).0 .0;
     if let Some(Token::EOF) = tokens.last().map(|token| token.token()) {
@@ -315,13 +316,68 @@ fn struct_def_add_attribute(
     };
 
     let struct_id = get_struct(self_argument)?;
-    interpreter.elaborator.interner.update_struct_attributes(struct_id, |attributes| {
+    interner.update_struct_attributes(struct_id, |attributes| {
         attributes.push(attribute.clone());
     });
 
     Ok(Value::Unit)
 }
 
+// fn add_generic<let N: u32>(self, generic_name: str<N>)
+fn struct_def_add_generic(
+    interner: &NodeInterner,
+    arguments: Vec<(Value, Location)>,
+    location: Location,
+) -> IResult<Value> {
+    let (self_argument, generic) = check_two_arguments(arguments, location)?;
+    let generic_location = generic.1;
+    let generic = get_str(interner, generic)?;
+
+    let mut tokens = Lexer::lex(&generic).0 .0;
+    if let Some(Token::EOF) = tokens.last().map(|token| token.token()) {
+        tokens.pop();
+    }
+
+    if tokens.len() != 1 {
+        return Err(InterpreterError::GenericNameShouldBeAnIdent {
+            name: generic,
+            location: generic_location,
+        });
+    }
+
+    let Token::Ident(generic_name) = tokens.pop().unwrap().into_token() else {
+        return Err(InterpreterError::GenericNameShouldBeAnIdent {
+            name: generic,
+            location: generic_location,
+        });
+    };
+
+    let struct_id = get_struct(self_argument)?;
+    let the_struct = interner.get_struct(struct_id);
+    let mut the_struct = the_struct.borrow_mut();
+    let name = Rc::new(generic_name);
+
+    for generic in &the_struct.generics {
+        if generic.name == name {
+            return Err(InterpreterError::DuplicateGeneric {
+                name,
+                struct_name: the_struct.name.to_string(),
+                existing_location: Location::new(generic.span, the_struct.location.file),
+                duplicate_location: generic_location,
+            });
+        }
+    }
+
+    let type_var = TypeVariable::unbound(interner.next_type_variable_id());
+    let span = generic_location.span;
+    let kind = Kind::Normal;
+    let typ = Type::NamedGeneric(type_var.clone(), name.clone(), kind.clone());
+    let new_generic = ResolvedGeneric { name, type_var, span, kind };
+    the_struct.generics.push(new_generic);
+
+    Ok(Value::Type(typ))
+}
+
 /// fn as_type(self) -> Type
 fn struct_def_as_type(
     interner: &NodeInterner,

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

@@ -13,6 +13,24 @@ This type corresponds to `struct Name { field1: Type1, ... }` items in the sourc
 
 Adds an attribute to the struct.
 
+### add_generic
+
+#include_code add_generic noir_stdlib/src/meta/struct_def.nr rust
+
+Adds an generic to the struct. Returns the new generic type.
+Errors if the given generic name isn't a single identifier or if
+the struct already has a generic with the same name.
+
+This method should be used carefully, if there is existing code referring
+to the struct type it may be checked before this function is called and
+see the struct with the original number of generics. This method should
+thus be preferred to use on code generated from other macros and structs
+that are not used in function signatures.
+
+Example:
+
+#include_code add-generic-example test_programs/compile_success_empty/comptime_struct_definition/src/main.nr rust
+
 ### as_type
 
 #include_code as_type noir_stdlib/src/meta/struct_def.nr rust

noir_stdlib/src/meta/struct_def.nr

@@ -4,10 +4,15 @@ impl StructDefinition {
     fn add_attribute<let N: u32>(self, attribute: str<N>) {}
     // docs:end:add_attribute
 
+    #[builtin(struct_def_add_generic)]
+    // docs:start:add_generic
+    fn add_generic<let N: u32>(self, generic_name: str<N>) -> Type {}
+    // docs:end:add_generic
+
     /// Return a syntactic version of this struct definition as a type.
     /// For example, `as_type(quote { type Foo<A, B> { ... } })` would return `Foo<A, B>`
     #[builtin(struct_def_as_type)]
-// docs:start:as_type
+    // docs:start:as_type
     fn as_type(self) -> Type {}
     // docs:end:as_type
 
@@ -18,14 +23,14 @@ impl StructDefinition {
 
     /// Return each generic on this struct.
     #[builtin(struct_def_generics)]
-// docs:start:generics
+    // docs:start:generics
     fn generics(self) -> [Type] {}
     // docs:end:generics
 
     /// Returns (name, type) pairs of each field in this struct. Each type is as-is
     /// with any generic arguments unchanged.
     #[builtin(struct_def_fields)]
-// docs:start:fields
+    // docs:start:fields
     fn fields(self) -> [(Quoted, Type)] {}
     // docs:end:fields
 

test_programs/compile_success_empty/comptime_struct_definition/src/main.nr

@@ -28,10 +28,23 @@ mod foo {
     #[attr]
     struct Foo {}
 
-    fn attr(s: StructDefinition) {
+    comptime fn attr(s: StructDefinition) {
         assert_eq(s.module().name(), quote { foo });
     }
+
+    #[add_generic]
+    struct Bar {}
+
+    // docs:start:add-generic-example
+    comptime fn add_generic(s: StructDefinition) {
+        assert_eq(s.generics().len(), 0);
+        let new_generic = s.add_generic("T");
+
+        let generics = s.generics();
+        assert_eq(generics.len(), 1);
+        assert_eq(generics[0], new_generic);
+    }
+    // docs:end:add-generic-example
 }
 
 fn main() {}
-