chore!: remove distinct keyword (#5219)

# Description

## Problem\*

Resolves <!-- Link to GitHub Issue -->

## Summary\*

This PR removes the `distinct` keyword and any traces of distinctiveness
from the codebase as they have been deprecated for a couple of releases.

## Additional Context



## Documentation\*

Check one:
- [x] 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_frontend/src/ast/mod.rs

@@ -390,26 +390,3 @@ impl std::fmt::Display for Visibility {
         }
     }
 }
-
-#[derive(Clone, Copy, Debug, PartialEq, Eq, Hash)]
-/// Represents whether the return value should compromise of unique witness indices such that no
-/// index occurs within the program's abi more than once.
-///
-/// This is useful for application stacks that require an uniform abi across across multiple
-/// circuits. When index duplication is allowed, the compiler may identify that a public input
-/// reaches the output unaltered and is thus referenced directly, causing the input and output
-/// witness indices to overlap. Similarly, repetitions of copied values in the output may be
-/// optimized away.
-pub enum Distinctness {
-    Distinct,
-    DuplicationAllowed,
-}
-
-impl std::fmt::Display for Distinctness {
-    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
-        match self {
-            Self::Distinct => write!(f, "distinct"),
-            Self::DuplicationAllowed => write!(f, "duplication-allowed"),
-        }
-    }
-}

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

@@ -40,8 +40,6 @@ pub enum ResolverError {
     UnnecessaryPub { ident: Ident, position: PubPosition },
     #[error("Required 'pub', main function must return public value")]
     NecessaryPub { ident: Ident },
-    #[error("'distinct' keyword can only be used with main method")]
-    DistinctNotAllowed { ident: Ident },
     #[error("Missing expression for declared constant")]
     MissingRhsExpr { name: String, span: Span },
     #[error("Expression invalid in an array length context")]
@@ -220,18 +218,6 @@ impl<'a> From<&'a ResolverError> for Diagnostic {
                 diag.add_note("The `pub` keyword is mandatory for the entry-point function return type because the verifier cannot retrieve private witness and thus the function will not be able to return a 'priv' value".to_owned());
                 diag
             }
-            ResolverError::DistinctNotAllowed { ident } => {
-                let name = &ident.0.contents;
-
-                let mut diag = Diagnostic::simple_error(
-                    format!("Invalid `distinct` keyword on return type of function {name}"),
-                    "Invalid distinct on return type".to_string(),
-                    ident.0.span(),
-                );
-
-                diag.add_note("The `distinct` keyword is only valid when used on the main function of a program, as its only purpose is to ensure that all witness indices that occur in the abi are unique".to_owned());
-                diag
-            }
             ResolverError::MissingRhsExpr { name, span } => Diagnostic::simple_error(
                 format!(
                     "no expression specifying the value stored by the constant variable {name}"

compiler/noirc_frontend/src/lexer/token.rs

@@ -839,7 +839,6 @@ pub enum Keyword {
     Contract,
     Crate,
     Dep,
-    Distinct,
     Else,
     Field,
     Fn,
@@ -884,7 +883,6 @@ impl fmt::Display for Keyword {
             Keyword::Contract => write!(f, "contract"),
             Keyword::Crate => write!(f, "crate"),
             Keyword::Dep => write!(f, "dep"),
-            Keyword::Distinct => write!(f, "distinct"),
             Keyword::Else => write!(f, "else"),
             Keyword::Field => write!(f, "Field"),
             Keyword::Fn => write!(f, "fn"),
@@ -932,7 +930,6 @@ impl Keyword {
             "contract" => Keyword::Contract,
             "crate" => Keyword::Crate,
             "dep" => Keyword::Dep,
-            "distinct" => Keyword::Distinct,
             "else" => Keyword::Else,
             "Field" => Keyword::Field,
             "fn" => Keyword::Fn,

compiler/noirc_frontend/src/noir_parser.lalrpop

@@ -67,7 +67,6 @@ extern {
         "contract" => BorrowedToken::Keyword(noir_token::Keyword::Contract),
         "crate" => BorrowedToken::Keyword(noir_token::Keyword::Crate),
         "dep" => BorrowedToken::Keyword(noir_token::Keyword::Dep),
-        "distinct" => BorrowedToken::Keyword(noir_token::Keyword::Distinct),
         "else" => BorrowedToken::Keyword(noir_token::Keyword::Else),
         "Field" => BorrowedToken::Keyword(noir_token::Keyword::Field),
         "fn" => BorrowedToken::Keyword(noir_token::Keyword::Fn),

compiler/noirc_frontend/src/parser/errors.rs

@@ -30,8 +30,6 @@ pub enum ParserErrorReason {
     TraitImplFunctionModifiers,
     #[error("comptime keyword is deprecated")]
     ComptimeDeprecated,
-    #[error("distinct keyword is deprecated. The `distinct` behavior is now the default.")]
-    DistinctDeprecated,
     #[error("{0} are experimental and aren't fully supported yet")]
     ExperimentalFeature(&'static str),
     #[error(

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

@@ -4,16 +4,12 @@ use super::{
     parameter_name_recovery, parameter_recovery, parenthesized, parse_type, pattern,
     self_parameter, where_clause, NoirParser,
 };
+use crate::ast::{
+    FunctionDefinition, FunctionReturnType, Ident, ItemVisibility, NoirFunction, Param, Visibility,
+};
 use crate::parser::labels::ParsingRuleLabel;
 use crate::parser::spanned;
 use crate::token::{Keyword, Token};
-use crate::{
-    ast::{
-        FunctionDefinition, FunctionReturnType, Ident, ItemVisibility, NoirFunction, Param,
-        Visibility,
-    },
-    parser::{ParserError, ParserErrorReason},
-};
 
 use chumsky::prelude::*;
 
@@ -95,20 +91,9 @@ pub(super) fn generics() -> impl NoirParser<Vec<Ident>> {
         .map(|opt| opt.unwrap_or_default())
 }
 
-#[deprecated = "Distinct keyword is now deprecated. Remove this function after the 0.30.0 release"]
-fn optional_distinctness() -> impl NoirParser<bool> {
-    keyword(Keyword::Distinct).or_not().validate(|opt, span, emit| {
-        if opt.is_some() {
-            emit(ParserError::with_reason(ParserErrorReason::DistinctDeprecated, span));
-        }
-        opt.is_some()
-    })
-}
-
 pub(super) fn function_return_type() -> impl NoirParser<(Visibility, FunctionReturnType)> {
     #[allow(deprecated)]
     just(Token::Arrow)
-        .ignore_then(optional_distinctness())
         .ignore_then(optional_visibility())
         .then(spanned(parse_type()))
         .or_not()
@@ -224,8 +209,6 @@ mod test {
                 // A leading plus is not allowed.
                 "fn func_name<T>(f: Field, y : T) where T: + SomeTrait {}",
                 "fn func_name<T>(f: Field, y : T) where T: TraitX + <Y> {}",
-                // `distinct` is deprecated
-                "fn main(x: pub u8, y: pub u8) -> distinct pub [u8; 2] { [x, y] }",
             ],
         );
     }

tooling/noirc_abi/src/lib.rs

@@ -107,21 +107,6 @@ impl From<&Visibility> for AbiVisibility {
     }
 }
 
-#[derive(Clone, Copy, Debug, PartialEq, Eq, Serialize, Deserialize)]
-#[serde(rename_all = "lowercase")]
-/// Represents whether the return value should compromise of unique witness indices such that no
-/// index occurs within the program's abi more than once.
-///
-/// This is useful for application stacks that require an uniform abi across across multiple
-/// circuits. When index duplication is allowed, the compiler may identify that a public input
-/// reaches the output unaltered and is thus referenced directly, causing the input and output
-/// witness indices to overlap. Similarly, repetitions of copied values in the output may be
-/// optimized away.
-pub enum AbiDistinctness {
-    Distinct,
-    DuplicationAllowed,
-}
-
 #[derive(Clone, Copy, Debug, PartialEq, Eq, Serialize, Deserialize)]
 #[serde(rename_all = "lowercase")]
 pub enum Sign {