feat: add support for wildcard types (#5275)

# Description

## Problem\*

Resolves #5183 

## Summary\*
Add support for wildcard type: `_`


## 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.

compiler/noirc_frontend/src/elaborator/types.rs

@@ -11,7 +11,7 @@ use crate::{
         def_map::ModuleDefId,
         resolution::{
             errors::ResolverError,
-            resolver::{verify_mutable_reference, SELF_TYPE_NAME},
+            resolver::{verify_mutable_reference, SELF_TYPE_NAME, WILDCARD_TYPE},
         },
         type_check::{Source, TypeCheckError},
     },
@@ -146,6 +146,8 @@ impl<'context> Elaborator<'context> {
                     }
                     return self_type;
                 }
+            } else if name == WILDCARD_TYPE {
+                return self.interner.next_type_variable();
             }
         }
 

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

@@ -64,6 +64,7 @@ use super::errors::{PubPosition, ResolverError};
 use super::import::PathResolution;
 
 pub const SELF_TYPE_NAME: &str = "Self";
+pub const WILDCARD_TYPE: &str = "_";
 
 type Scope = GenericScope<String, ResolverMeta>;
 type ScopeTree = GenericScopeTree<String, ResolverMeta>;

docs/docs/noir/concepts/data_types/index.md

@@ -105,6 +105,14 @@ type Bad2 = Bad1;
 //   ^^^^^^^^^^^ 'Bad2' recursively depends on itself: Bad2 -> Bad1 -> Bad2
 ```
 
+## Wildcard Type
+Noir can usually infer the type of the variable from the context, so specifying the type of a variable is only required when it cannot be inferred. However, specifying a complex type can be tedious, especially when it has multiple generic arguments. Often some of the generic types can be inferred from the context, and Noir only needs a hint to properly infer the other types. We can partially specify a variable's type by using `_` as a marker, indicating where we still want the compiler to infer the type.
+
+```rust
+let a: [_; 4] = foo(b);
+```
+
+
 ### BigInt
 
 You can achieve BigInt functionality using the [Noir BigInt](https://github.com/shuklaayush/noir-bigint) library.

test_programs/execution_success/wildcard_type/Nargo.toml

@@ -0,0 +1,6 @@
+[package]
+name = "wildcard_type"
+type = "bin"
+authors = [""]
+
+[dependencies]

test_programs/execution_success/wildcard_type/Prover.toml

@@ -0,0 +1 @@
+enable = [4,7]

test_programs/execution_success/wildcard_type/src/main.nr

@@ -0,0 +1,23 @@
+struct bar {
+    enable: [bool; 4],
+    data: [Field; 2],
+    pad: u32,
+}
+
+fn main(enable: [Field; 2]) -> pub [Field; 4] {
+    let mut result = [0; 4];
+    let a: [_; 4] = foo(enable[1]);
+    for i in 0..4 {
+        result[i] = a[i].data[i % 2];
+    }
+    result
+}
+
+fn foo(x: Field) -> [bar; 4] {
+    [
+        bar { enable: [true, true, false, false], data: [x, x + 1], pad: 0 },
+        bar { enable: [true, false, false, false], data: [x + 2, x + 7], pad: 0 },
+        bar { enable: [true, true, false, true], data: [x + 3, x + 5], pad: 0 },
+        bar { enable: [false, false, false, false], data: [x + 4, x - 1], pad: 0 }
+    ]
+}

tooling/nargo_cli/build.rs

@@ -58,10 +58,10 @@ const IGNORED_BRILLIG_TESTS: [&str; 11] = [
     "is_unconstrained",
 ];
 
-/// Certain comptime features are only available in the elaborator.
+/// Certain features are only available in the elaborator.
 /// We skip these tests for non-elaborator code since they are not
 /// expected to work there. This can be removed once the old code is removed.
-const IGNORED_COMPTIME_TESTS: [&str; 1] = ["macros"];
+const IGNORED_NEW_FEATURE_TESTS: [&str; 2] = ["macros", "wildcard_type"];
 
 fn generate_execution_success_tests(test_file: &mut File, test_data_dir: &Path) {
     let test_sub_dir = "execution_success";
@@ -82,11 +82,16 @@ fn generate_execution_success_tests(test_file: &mut File, test_data_dir: &Path)
 
         let brillig_ignored =
             if IGNORED_BRILLIG_TESTS.contains(&test_name.as_str()) { "\n#[ignore]" } else { "" };
+        let new_features_ignored = if IGNORED_NEW_FEATURE_TESTS.contains(&test_name.as_str()) {
+            "\n#[ignore]"
+        } else {
+            ""
+        };
 
         write!(
             test_file,
             r#"
-#[test]
+#[test]{new_features_ignored}
 fn execution_success_legacy_{test_name}() {{
     let test_program_dir = PathBuf::from("{test_dir}");
 
@@ -286,8 +291,11 @@ fn generate_compile_success_empty_tests(test_file: &mut File, test_data_dir: &Pa
         };
         let test_dir = &test_dir.path();
 
-        let comptime_ignored =
-            if IGNORED_COMPTIME_TESTS.contains(&test_name.as_str()) { "\n#[ignore]" } else { "" };
+        let comptime_ignored = if IGNORED_NEW_FEATURE_TESTS.contains(&test_name.as_str()) {
+            "\n#[ignore]"
+        } else {
+            ""
+        };
 
         write!(
             test_file,