feat: add de-sugaring for impl Trait in function parameters (#4919)

# Description

## Problem\*

Resolves #4540

## Summary\*

In the resolver, when `x: impl Trait` is encountered as a function
parameter,
it is desugared to:
```rust
fn function_name<T_impl_trait>(x: T_impl_trait) where T_impl_trait: Trait
```

## Additional Context

In the current iteration of this PR, `T[func_id]_impl_[trait_name]` is
checked for collisions and
incremented (`T[func_id]_impl_[trait_name]_N`) until there's no
collision.

Note that this PR adds a few ~unrelated cases where `TraitAsType` could
have arguments that were previously skipped:

https://github.com/noir-lang/noir/pull/4919/files#diff-52c7ae61478bab09a4d23320128eead3a88cde6a7be16fb8b070b5512d690bbdR1203

## Documentation\*

Check one:
- [ ] No documentation needed.
- [ ] 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.

compiler/noirc_arena/src/lib.rs

@@ -3,6 +3,8 @@
 #![warn(unreachable_pub)]
 #![warn(clippy::semicolon_if_nothing_returned)]
 
+use std::fmt;
+
 #[derive(Clone, Copy, Debug, Eq, Ord, PartialEq, PartialOrd, Hash)]
 pub struct Index(usize);
 
@@ -25,6 +27,12 @@ impl Index {
     }
 }
 
+impl fmt::Display for Index {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        self.0.fmt(f)
+    }
+}
+
 #[derive(Clone, Debug)]
 pub struct Arena<T> {
     pub vec: Vec<T>,

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

@@ -29,7 +29,7 @@ use crate::ast::{
     ArrayLiteral, BinaryOpKind, BlockExpression, Distinctness, Expression, ExpressionKind,
     ForRange, FunctionDefinition, FunctionKind, FunctionReturnType, Ident, ItemVisibility, LValue,
     LetStatement, Literal, NoirFunction, NoirStruct, NoirTypeAlias, Param, Path, PathKind, Pattern,
-    Statement, StatementKind, UnaryOp, UnresolvedGenerics, UnresolvedTraitConstraint,
+    Statement, StatementKind, TraitBound, UnaryOp, UnresolvedGenerics, UnresolvedTraitConstraint,
     UnresolvedType, UnresolvedTypeData, UnresolvedTypeExpression, Visibility, ERROR_IDENT,
 };
 use crate::graph::CrateId;
@@ -202,23 +202,70 @@ impl<'a> Resolver<'a> {
         self.errors.push(err);
     }
 
+    /// This turns function parameters of the form:
+    /// fn foo(x: impl Bar)
+    ///
+    /// into
+    /// fn foo<T0_impl_Bar>(x: T0_impl_Bar) where T0_impl_Bar: Bar
+    fn desugar_impl_trait_args(&mut self, func: &mut NoirFunction, func_id: FuncId) {
+        let mut impl_trait_generics = HashSet::new();
+        let mut counter: usize = 0;
+        for parameter in func.def.parameters.iter_mut() {
+            if let UnresolvedTypeData::TraitAsType(path, args) = &parameter.typ.typ {
+                let mut new_generic_ident: Ident =
+                    format!("T{}_impl_{}", func_id, path.as_string()).into();
+                let mut new_generic_path = Path::from_ident(new_generic_ident.clone());
+                while impl_trait_generics.contains(&new_generic_ident)
+                    || self.lookup_generic_or_global_type(&new_generic_path).is_some()
+                {
+                    new_generic_ident =
+                        format!("T{}_impl_{}_{}", func_id, path.as_string(), counter).into();
+                    new_generic_path = Path::from_ident(new_generic_ident.clone());
+                    counter += 1;
+                }
+                impl_trait_generics.insert(new_generic_ident.clone());
+
+                let is_synthesized = true;
+                let new_generic_type_data =
+                    UnresolvedTypeData::Named(new_generic_path, vec![], is_synthesized);
+                let new_generic_type =
+                    UnresolvedType { typ: new_generic_type_data.clone(), span: None };
+                let new_trait_bound = TraitBound {
+                    trait_path: path.clone(),
+                    trait_id: None,
+                    trait_generics: args.to_vec(),
+                };
+                let new_trait_constraint = UnresolvedTraitConstraint {
+                    typ: new_generic_type,
+                    trait_bound: new_trait_bound,
+                };
+
+                parameter.typ.typ = new_generic_type_data;
+                func.def.generics.push(new_generic_ident);
+                func.def.where_clause.push(new_trait_constraint);
+            }
+        }
+        self.add_generics(&impl_trait_generics.into_iter().collect());
+    }
+
     /// Resolving a function involves interning the metadata
     /// interning any statements inside of the function
     /// and interning the function itself
     /// We resolve and lower the function at the same time
     /// Since lowering would require scope data, unless we add an extra resolution field to the AST
     pub fn resolve_function(
         mut self,
-        func: NoirFunction,
+        mut func: NoirFunction,
         func_id: FuncId,
     ) -> (HirFunction, FuncMeta, Vec<ResolverError>) {
         self.scopes.start_function();
         self.current_item = Some(DependencyId::Function(func_id));
 
         // Check whether the function has globals in the local module and add them to the scope
         self.resolve_local_globals();
-
         self.add_generics(&func.def.generics);
+
+        self.desugar_impl_trait_args(&mut func, func_id);
         self.trait_bounds = func.def.where_clause.clone();
 
         let is_low_level_or_oracle = func
@@ -1150,10 +1197,15 @@ impl<'a> Resolver<'a> {
             | Type::TypeVariable(_, _)
             | Type::Constant(_)
             | Type::NamedGeneric(_, _)
-            | Type::TraitAsType(..)
             | Type::Code
             | Type::Forall(_, _) => (),
 
+            Type::TraitAsType(_, _, args) => {
+                for arg in args {
+                    Self::find_numeric_generics_in_type(arg, found);
+                }
+            }
+
             Type::Array(length, element_type) => {
                 if let Type::NamedGeneric(type_variable, name) = length.as_ref() {
                     found.insert(name.to_string(), type_variable.clone());

compiler/noirc_frontend/src/hir_def/types.rs

@@ -643,9 +643,11 @@ impl Type {
             | Type::Constant(_)
             | Type::NamedGeneric(_, _)
             | Type::Forall(_, _)
-            | Type::Code
-            | Type::TraitAsType(..) => false,
+            | Type::Code => false,
 
+            Type::TraitAsType(_, _, args) => {
+                args.iter().any(|generic| generic.contains_numeric_typevar(target_id))
+            }
             Type::Array(length, elem) => {
                 elem.contains_numeric_typevar(target_id) || named_generic_id_matches_target(length)
             }
@@ -1591,11 +1593,17 @@ impl Type {
                 element.substitute_helper(type_bindings, substitute_bound_typevars),
             )),
 
+            Type::TraitAsType(s, name, args) => {
+                let args = vecmap(args, |arg| {
+                    arg.substitute_helper(type_bindings, substitute_bound_typevars)
+                });
+                Type::TraitAsType(*s, name.clone(), args)
+            }
+
             Type::FieldElement
             | Type::Integer(_, _)
             | Type::Bool
             | Type::Constant(_)
-            | Type::TraitAsType(..)
             | Type::Error
             | Type::Code
             | Type::Unit => self.clone(),
@@ -1613,7 +1621,9 @@ impl Type {
                 let field_occurs = fields.occurs(target_id);
                 len_occurs || field_occurs
             }
-            Type::Struct(_, generic_args) | Type::Alias(_, generic_args) => {
+            Type::Struct(_, generic_args)
+            | Type::Alias(_, generic_args)
+            | Type::TraitAsType(_, _, generic_args) => {
                 generic_args.iter().any(|arg| arg.occurs(target_id))
             }
             Type::Tuple(fields) => fields.iter().any(|field| field.occurs(target_id)),
@@ -1637,7 +1647,6 @@ impl Type {
             | Type::Integer(_, _)
             | Type::Bool
             | Type::Constant(_)
-            | Type::TraitAsType(..)
             | Type::Error
             | Type::Code
             | Type::Unit => false,
@@ -1689,16 +1698,14 @@ impl Type {
 
             MutableReference(element) => MutableReference(Box::new(element.follow_bindings())),
 
+            TraitAsType(s, name, args) => {
+                let args = vecmap(args, |arg| arg.follow_bindings());
+                TraitAsType(*s, name.clone(), args)
+            }
+
             // Expect that this function should only be called on instantiated types
             Forall(..) => unreachable!(),
-            TraitAsType(..)
-            | FieldElement
-            | Integer(_, _)
-            | Bool
-            | Constant(_)
-            | Unit
-            | Code
-            | Error => self.clone(),
+            FieldElement | Integer(_, _) | Bool | Constant(_) | Unit | Code | Error => self.clone(),
         }
     }
 

compiler/noirc_frontend/src/node_interner.rs

@@ -1,5 +1,6 @@
 use std::borrow::Cow;
 use std::collections::HashMap;
+use std::fmt;
 use std::ops::Deref;
 
 use fm::FileId;
@@ -314,6 +315,12 @@ impl FuncId {
     }
 }
 
+impl fmt::Display for FuncId {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        self.0.fmt(f)
+    }
+}
+
 #[derive(Debug, Eq, PartialEq, Hash, Copy, Clone, PartialOrd, Ord)]
 pub struct StructId(ModuleId);
 

compiler/noirc_frontend/src/parser/parser/function.rs

@@ -193,6 +193,11 @@ mod test {
                 "fn func_name<T>(f: Field, y : T) where T: SomeTrait + {}",
                 // The following should produce compile error on later stage. From the parser's perspective it's fine
                 "fn func_name<A>(f: Field, y : Field, z : Field) where T: SomeTrait {}",
+                // TODO: this fails with known EOF != EOF error
+                // https://github.com/noir-lang/noir/issues/4763
+                // fn func_name(x: impl Eq) {} with error Expected an end of input but found end of input
+                // "fn func_name(x: impl Eq) {}",
+                "fn func_name<T>(x: impl Eq, y : T) where T: SomeTrait + Eq {}",
             ],
         );
 

compiler/noirc_frontend/src/tests.rs

@@ -746,6 +746,70 @@ mod test {
         }
     }
 
+    #[test]
+    fn check_trait_as_type_as_fn_parameter() {
+        let src = "
+        trait Eq {
+            fn eq(self, other: Self) -> bool;
+        }
+
+        struct Foo {
+            a: u64,
+        }
+
+        impl Eq for Foo {
+            fn eq(self, other: Foo) -> bool { self.a == other.a } 
+        }
+
+        fn test_eq(x: impl Eq) -> bool {
+            x.eq(x)
+        }
+
+        fn main(a: Foo) -> pub bool {
+            test_eq(a)
+        }";
+
+        let errors = get_program_errors(src);
+        errors.iter().for_each(|err| println!("{:?}", err));
+        assert!(errors.is_empty());
+    }
+
+    #[test]
+    fn check_trait_as_type_as_two_fn_parameters() {
+        let src = "
+        trait Eq {
+            fn eq(self, other: Self) -> bool;
+        }
+
+        trait Test {
+            fn test(self) -> bool;
+        }
+
+        struct Foo {
+            a: u64,
+        }
+
+        impl Eq for Foo {
+            fn eq(self, other: Foo) -> bool { self.a == other.a } 
+        }
+
+        impl Test for u64 {
+            fn test(self) -> bool { self == self } 
+        }
+
+        fn test_eq(x: impl Eq, y: impl Test) -> bool {
+            x.eq(x) == y.test()
+        }
+
+        fn main(a: Foo, b: u64) -> pub bool {
+            test_eq(a, b)
+        }";
+
+        let errors = get_program_errors(src);
+        errors.iter().for_each(|err| println!("{:?}", err));
+        assert!(errors.is_empty());
+    }
+
     fn get_program_captures(src: &str) -> Vec<Vec<String>> {
         let (program, context, _errors) = get_program(src);
         let interner = context.def_interner;

tooling/nargo_fmt/tests/expected/impl_trait_fn_parameter.nr

@@ -0,0 +1,3 @@
+fn func_name(x: impl Eq) {}
+
+fn func_name<T>(x: impl Eq, y: T) where T: SomeTrait + Eq {}

tooling/nargo_fmt/tests/input/impl_trait_fn_parameter.nr

@@ -0,0 +1,3 @@
+fn func_name(x: impl Eq) {}
+
+fn func_name<T>(x: impl Eq, y: T) where T: SomeTrait + Eq {}