feat: add Type::as_string (#5871)

# Description

## Problem

Part of #5668

## Summary


## Additional Context



## 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: jfecher <jake@aztecprotocol.com>

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

@@ -144,6 +144,7 @@ impl<'local, 'context> Interpreter<'local, 'context> {
             "type_as_constant" => type_as_constant(arguments, return_type, location),
             "type_as_integer" => type_as_integer(arguments, return_type, location),
             "type_as_slice" => type_as_slice(arguments, return_type, location),
+            "type_as_str" => type_as_str(arguments, return_type, location),
             "type_as_struct" => type_as_struct(arguments, return_type, location),
             "type_as_tuple" => type_as_tuple(arguments, return_type, location),
             "type_eq" => type_eq(arguments, location),
@@ -543,6 +544,21 @@ fn type_as_slice(
     })
 }
 
+// fn as_str(self) -> Option<Type>
+fn type_as_str(
+    arguments: Vec<(Value, Location)>,
+    return_type: Type,
+    location: Location,
+) -> IResult<Value> {
+    type_as(arguments, return_type, location, |typ| {
+        if let Type::String(n) = typ {
+            Some(Value::Type(*n))
+        } else {
+            None
+        }
+    })
+}
+
 // fn as_struct(self) -> Option<(StructDefinition, [Type])>
 fn type_as_struct(
     arguments: Vec<(Value, Location)>,

docs/docs/noir/standard_library/meta/typ.md

@@ -45,6 +45,12 @@ if the type is signed, as well as the number of bits of this integer type.
 
 If this is a slice type, return the element type of the slice.
 
+### as_str
+
+#include_code as_str noir_stdlib/src/meta/typ.nr rust
+
+If this is a `str<N>` type, returns the length `N` as a type.
+
 ### as_struct
 
 #include_code as_struct noir_stdlib/src/meta/typ.nr rust

noir_stdlib/src/meta/typ.nr

@@ -22,6 +22,11 @@ impl Type {
     fn as_slice(self) -> Option<Type> {}
     // docs:end:as_slice
 
+    #[builtin(type_as_str)]
+// docs:start:as_str
+    fn as_str(self) -> Option<Type> {}
+    // docs:end:as_str
+
     #[builtin(type_as_struct)]
 // docs:start:as_struct
     fn as_struct(self) -> Option<(StructDefinition, [Type])> {}

test_programs/compile_success_empty/comptime_type/src/main.nr

@@ -110,6 +110,11 @@ fn main() {
         assert(!struct_does_not_implement_some_trait.implements(some_trait_field));
 
         let _trait_impl = struct_implements_some_trait.get_trait_impl(some_trait_i32).unwrap();
+
+        // Check Type::as_str
+        let str_type = quote { str<10> }.as_type();
+        let constant = str_type.as_str().unwrap();
+        assert_eq(constant.as_constant().unwrap(), 10);
     }
 }