Invalid expression discarding when using semicolons

[CohenArthur]

rustc does not produce any warnings for the following code:

fn main() {
    8;
}

(which is very different from this code, which does not compile with rustc either, obviously)

fn main() {
    8
}

What's happening is that, despite 8 being an "expression", the use of a semicolon afterwards allows not using it. This would be similar to something like

fn not_void() -> i32 {
    8
}

fn main() {
    not_void();
}

where discarding the return value of a function is fine, unless said function or type is marked with #[must_use].

However, gccrs does not seem to be able to discard the value of an expression (and thus transforming said expression into a "statement"). Therefore, the snippets of code above fail with the following error:

/tmp/main.rs:1:1: error: expected [()] got [<integer>] # or `got [i32]` if we use the `not_void()` function
    1 | fn main() {
      | ^

I haven't found any issues about the subject, but might not have looked hard enough! If so, I apologize

[github-actions]

Thanks for your contribution fellow Rustacean

[yizhepku]

I checked with a debugger. The syntax is parsed correctly(8 is parsed as ExprStmtWithoutBlock, not as the final expression of the block). The error happened in the stage of type resolution.

[yizhepku]

The offending code is at rust-hir-type-check.cc:82. The implemented logic is that, if no trailing expression exists, the type of the last statement will be used as the type of the block, which is incorrect. The correct behavior is to use () as the type of the block whenever there's no trailing expression.

Excerpt of the offending code:

void
TypeCheckExpr::visit (HIR::BlockExpr &expr)
{
  TyTy::BaseType *block_tyty
    = new TyTy::TupleType (expr.get_mappings ().get_hirid ());

  expr.iterate_stmts ([&] (HIR::Stmt *s) mutable -> bool {
    bool is_final_stmt = expr.is_final_stmt (s);
    bool has_final_expr = expr.has_expr () && expr.tail_expr_reachable ();
    bool stmt_is_final_expr = is_final_stmt && !has_final_expr;

    auto resolved = TypeCheckStmt::Resolve (s, inside_loop);
    if (resolved == nullptr)
      {
	rust_error_at (s->get_locus_slow (), "failure to resolve type");
	return false;
      }

    if (stmt_is_final_expr)
      {
	delete block_tyty;
	block_tyty = resolved;
      }
    else if (!resolved->is_unit ())
      {
	rust_error_at (s->get_locus_slow (), "expected () got %s",
		       resolved->as_string ().c_str ());
      }

    return true;
  });

  if (expr.has_expr ())
    {
      delete block_tyty;

      block_tyty
	= TypeCheckExpr::Resolve (expr.get_final_expr ().get (), inside_loop);
    }

  infered = block_tyty->clone ();
}

@CohenArthur Maybe you'll be interested in implementing a fix? It should help you get comfortable with the codebase too.

[CohenArthur]

Wow thanks for the headstart @yizhepku ! I hadn't even started to research the issue yet. I'd love to implement that fix

[philberty]

I've known about this bug but haven't decided if the bug is in the parser to make the ExprStmtWithoutBlock the final expression or if this can be handled as part of HIR lowering. The other side of this problem is you can have an ExprStmtWithBlock but this is the final expression such as:

fn main() -> i32 {
    let a = 1;
    if a > 10 {
        123
    } else {
       456
    }
}

[philberty]

@SimplyTheOther do you have any opinions on this?

[CohenArthur]

I've known about this bug but haven't decided if the bug is in the parser to make the ExprStmtWithoutBlock the final expression or if this can be handled as part of HIR lowering. The other side of this problem is you can have an ExprStmtWithBlock but this is the final expression such as:

fn main() -> i32 {
    let a = 1;
    if a > 10 {
        123
    } else {
       456
    }
}

In that case, wouldn't the type of the ExprStmtWithBlock resolve as an integer?

[philberty]

Yes I am just showing that the final expr can be an ExprStmtWithBlock. Another more awkward example is this one that is difficult:

This is a testcase deadcode1.rs

fn test2(x: i32) -> i32 {
    if x > 1 {
        return 5;
    } else {
        return 0;
    }
    return 1;
}

Usually anything that is not the final expr should ensure is UnitType but because the final return is unreachable it is not required.

[yizhepku]

Related discussion from rustc: rust-lang/rust#61733

[yizhepku]

After reading the reference and various discussions, here's my understanding:

• There're two types of expressions: ExprWithoutBlock and ExprWithBlock
• Every expression can be made into a statement by appending a semicolon
• However, in case of ExprWithBlock, the semicolon can be omitted for convenience if
  • Its result has type ()
  • It is not the last expression of a block(in this case if you omit the semicolon it becomes the trailing expression)
• If an expression(either ExprWithoutBlock or ExprWithBlock) appears at the end of a block, it is treated as the trailing expression
  • If you terminate it with a semicolon it's no longer an expression, so this rule doesn't apply

rustc currently handles this by distinguishing AST statements with and without semicolons(doc). This information is then removed during the AST-to-HIR lowering process. We could follow rustc, but our current approaching of handling this in the parser seems cleaner.

[SimplyTheOther]

My understanding of it is that code like this:

fn main() {
    8;
}

should be parsed as ExprStmtWithoutBlock, and that the return type of the function should be ().

For code like this:

fn main() -> i32 {
    let a = 1;
    if a > 10 {
        123
    } else {
       456
    }
}

My understanding was that this was technically not allowed. According to the Rust reference, only ExprWithoutBlock is allowed as the "final expression" - if expressions should be parsed as ExprStmtWithBlock in this context, and hence would not contribute to implicit returns or the function return type.

On the other hand, I tested that code in Rust Playground, and apparently it works fine. So this actually seems to me to be a type resolution error as a result of a parser error as a result of an error in the Rust reference.

EDIT: there is an open issue on the Rust reference regarding this. The issue implies that poor wording may have caused a misinterpretation on my part.

[CohenArthur]

It seems that rustc does not represent the unit type directly, but rather considers it as a tuple with 0 elements.
Here is the TyKind enum in the AST, with the is_unit method which basically checks if the type is a Tuple and is empty. This makes sense, considering that a tuple (i32, u8, T) is indeed the unit type () when there are no elements. However, I believe that it would be clearer to have a clearly defined unit type in the TypeKind enum. What would you rather the approach be? Check against the emptiness of the TUPLE type or add a new UNIT type?