feat(experimental): Implement comptime mut globals (#4926)

# Description

## Problem\*

Resolves #4917 

## Summary\*

Implements mutable comptime globals

## Additional Context

Changes should be relatively straightforward - mostly parser changes to
add parsing `mut` to globals, resolution changes to track this and error
if the global is not also comptime, etc. The one bug that was fixed is
that globals were being re-evaluated in the interpreter each time which
would reset their value whenever they were referenced. This has been
fixed with a lookup check.

## Documentation\*

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

# PR Checklist\*

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

---------

Co-authored-by: Tom French <15848336+TomAFrench@users.noreply.github.com>

aztec_macros/src/utils/hir_utils.rs

@@ -228,6 +228,7 @@ pub fn inject_global(
         module_id,
         file_id,
         global.attributes.clone(),
+        false,
     );
 
     // Add the statement to the scope so its path can be looked up later

compiler/noirc_frontend/src/ast/statement.rs

@@ -486,7 +486,8 @@ impl Pattern {
     pub fn name_ident(&self) -> &Ident {
         match self {
             Pattern::Identifier(name_ident) => name_ident,
-            _ => panic!("only the identifier pattern can return a name"),
+            Pattern::Mutable(pattern, ..) => pattern.name_ident(),
+            _ => panic!("Only the Identifier or Mutable patterns can return a name"),
         }
     }
 

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

@@ -336,9 +336,14 @@ impl<'a> Interpreter<'a> {
             DefinitionKind::Local(_) => self.lookup(&ident),
             DefinitionKind::Global(global_id) => {
                 // Don't need to check let_.comptime, we can evaluate non-comptime globals too.
-                let let_ = self.interner.get_global_let_statement(*global_id).unwrap();
-                self.evaluate_let(let_)?;
-                self.lookup(&ident)
+                // Avoid resetting the value if it is already known
+                if let Ok(value) = self.lookup(&ident) {
+                    Ok(value)
+                } else {
+                    let let_ = self.interner.get_global_let_statement(*global_id).unwrap();
+                    self.evaluate_let(let_)?;
+                    self.lookup(&ident)
+                }
             }
             DefinitionKind::GenericType(type_variable) => {
                 let value = match &*type_variable.borrow() {

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

@@ -76,6 +76,20 @@ fn mutating_arrays() {
     assert_eq!(result, Value::U8(22));
 }
 
+#[test]
+fn mutate_in_new_scope() {
+    let program = "fn main() -> pub u8 {
+        let mut x = 0;
+        x += 1;
+        {
+            x += 1;
+        }
+        x
+    }";
+    let result = interpret(program, vec!["main".into()]);
+    assert_eq!(result, Value::U8(2));
+}
+
 #[test]
 fn for_loop() {
     let program = "fn main() -> pub u8 {

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

@@ -6,7 +6,8 @@ use noirc_errors::Location;
 
 use crate::ast::{
     FunctionDefinition, Ident, ItemVisibility, LetStatement, ModuleDeclaration, NoirFunction,
-    NoirStruct, NoirTrait, NoirTraitImpl, NoirTypeAlias, TraitImplItem, TraitItem, TypeImpl,
+    NoirStruct, NoirTrait, NoirTraitImpl, NoirTypeAlias, Pattern, TraitImplItem, TraitItem,
+    TypeImpl,
 };
 use crate::{
     graph::CrateId,
@@ -109,6 +110,7 @@ impl<'a> ModCollector<'a> {
                 self.module_id,
                 self.file_id,
                 global.attributes.clone(),
+                matches!(global.pattern, Pattern::Mutable { .. }),
             );
 
             // Add the statement to the scope so its path can be looked up later
@@ -463,6 +465,7 @@ impl<'a> ModCollector<'a> {
                             trait_id.0.local_id,
                             self.file_id,
                             vec![],
+                            false,
                         );
 
                         if let Err((first_def, second_def)) = self.def_collector.def_map.modules

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

@@ -86,6 +86,8 @@ pub enum ResolverError {
     JumpInConstrainedFn { is_break: bool, span: Span },
     #[error("break/continue are only allowed within loops")]
     JumpOutsideLoop { is_break: bool, span: Span },
+    #[error("Only `comptime` globals can be mutable")]
+    MutableGlobal { span: Span },
     #[error("Self-referential structs are not supported")]
     SelfReferentialStruct { span: Span },
     #[error("#[inline(tag)] attribute is only allowed on constrained functions")]
@@ -346,6 +348,13 @@ impl<'a> From<&'a ResolverError> for Diagnostic {
                     *span,
                 )
             },
+            ResolverError::MutableGlobal { span } => {
+                Diagnostic::simple_error(
+                    "Only `comptime` globals may be mutable".into(),
+                    String::new(),
+                    *span,
+                )
+            },
             ResolverError::SelfReferentialStruct { span } => {
                 Diagnostic::simple_error(
                     "Self-referential structs are not supported".into(),

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

@@ -1222,15 +1222,18 @@ impl<'a> Resolver<'a> {
     ) -> HirStatement {
         self.current_item = Some(DependencyId::Global(global_id));
         let expression = self.resolve_expression(let_stmt.expression);
-        let global_id = self.interner.next_global_id();
         let definition = DefinitionKind::Global(global_id);
 
         if !self.in_contract
             && let_stmt.attributes.iter().any(|attr| matches!(attr, SecondaryAttribute::Abi(_)))
         {
-            self.push_err(ResolverError::AbiAttributeOusideContract {
-                span: let_stmt.pattern.span(),
-            });
+            let span = let_stmt.pattern.span();
+            self.push_err(ResolverError::AbiAttributeOusideContract { span });
+        }
+
+        if !let_stmt.comptime && matches!(let_stmt.pattern, Pattern::Mutable(..)) {
+            let span = let_stmt.pattern.span();
+            self.push_err(ResolverError::MutableGlobal { span });
         }
 
         HirStatement::Let(HirLetStatement {

compiler/noirc_frontend/src/node_interner.rs

@@ -653,11 +653,13 @@ impl NodeInterner {
         let_statement: StmtId,
         file: FileId,
         attributes: Vec<SecondaryAttribute>,
+        mutable: bool,
     ) -> GlobalId {
         let id = GlobalId(self.globals.len());
         let location = Location::new(ident.span(), file);
         let name = ident.to_string();
-        let definition_id = self.push_definition(name, false, DefinitionKind::Global(id), location);
+        let definition_id =
+            self.push_definition(name, mutable, DefinitionKind::Global(id), location);
 
         self.globals.push(GlobalInfo {
             id,
@@ -682,9 +684,13 @@ impl NodeInterner {
         local_id: LocalModuleId,
         file: FileId,
         attributes: Vec<SecondaryAttribute>,
+        mutable: bool,
     ) -> GlobalId {
         let statement = self.push_stmt(HirStatement::Error);
-        self.push_global(name, local_id, statement, file, attributes)
+        let span = name.span();
+        let id = self.push_global(name, local_id, statement, file, attributes, mutable);
+        self.push_statement_location(statement, span, file);
+        id
     }
 
     /// Intern an empty function.

compiler/noirc_frontend/src/parser/parser.rs

@@ -235,16 +235,27 @@ fn implementation() -> impl NoirParser<TopLevelStatement> {
 fn global_declaration() -> impl NoirParser<TopLevelStatement> {
     let p = attributes::attributes()
         .then(maybe_comp_time())
+        .then(spanned(keyword(Keyword::Mut)).or_not())
         .then_ignore(keyword(Keyword::Global).labelled(ParsingRuleLabel::Global))
         .then(ident().map(Pattern::Identifier));
 
     let p = then_commit(p, optional_type_annotation());
     let p = then_commit_ignore(p, just(Token::Assign));
     let p = then_commit(p, expression());
-    p.validate(|((((attributes, comptime), pattern), r#type), expression), span, emit| {
-        let global_attributes = attributes::validate_secondary_attributes(attributes, span, emit);
-        LetStatement { pattern, r#type, comptime, expression, attributes: global_attributes }
-    })
+    p.validate(
+        |(((((attributes, comptime), mutable), mut pattern), r#type), expression), span, emit| {
+            let global_attributes =
+                attributes::validate_secondary_attributes(attributes, span, emit);
+
+            // Only comptime globals are allowed to be mutable, but we always parse the `mut`
+            // and throw the error in name resolution.
+            if let Some((_, mut_span)) = mutable {
+                let span = mut_span.merge(pattern.span());
+                pattern = Pattern::Mutable(Box::new(pattern), span, false);
+            }
+            LetStatement { pattern, r#type, comptime, expression, attributes: global_attributes }
+        },
+    )
     .map(TopLevelStatement::Global)
 }
 

compiler/noirc_frontend/src/tests.rs

@@ -1284,6 +1284,15 @@ fn lambda$f1(mut env$l1: (Field)) -> Field {
     }
 
     #[test]
+    fn ban_mutable_globals() {
+        // Mutable globals are only allowed in a comptime context
+        let src = r#"
+            mut global FOO: Field = 0;
+            fn main() {}
+        "#;
+        assert_eq!(get_program_errors(src).len(), 1);
+    }
+
     fn deny_inline_attribute_on_unconstrained() {
         let src = r#"
             #[inline(never)]

test_programs/compile_success_empty/comptime_mut_global/Nargo.toml

@@ -0,0 +1,7 @@
+[package]
+name = "comptime_mut_global"
+type = "bin"
+authors = [""]
+compiler_version = ">=0.28.0"
+
+[dependencies]

test_programs/compile_success_empty/comptime_mut_global/src/main.nr

@@ -0,0 +1,30 @@
+comptime mut global COUNTER: Field = 0;
+
+comptime fn get_unique_id() -> Field {
+    let id = COUNTER;
+    COUNTER += 1;
+    id
+}
+
+fn id1() -> Field {
+    comptime
+    {
+        get_unique_id()
+    }
+}
+
+fn id2() -> Field {
+    comptime
+    {
+        get_unique_id()
+    }
+}
+
+fn main() {
+    // Order of comptime evaluation between functions isn't guaranteed
+    // so we don't know if (id1 == 0 && id2 == 1) or if (id1 == 1 && id2 == 0).
+    // we only know they are not equal
+    let id1 = id1();
+    let id2 = id2();
+    assert(id1 != id2);
+}