Detect incorrect chaining of if and if let conditions and recover

[chenyukang]

Fixes #103381

[rust-highfive]

r? @petrochenkov

(rust-highfive has picked a reviewer for you, use r? to override)

[rustbot]

rustc_error_messages was changed

cc @davidtwco, @compiler-errors, @JohnTitor, @estebank, @TaKO8Ki

[chenyukang]

r? @estebank

[compiler-errors]

This breaks existing code -- if expressions are allowed to be their own conditions within other if statements.

fn main() {
    if true && if let 1 = 1 { true } else { true } {}
}

Before this PR: compiles

After this PR:

error: unexpected `if` in the condition expression
 --> /home/gh-compiler-errors/test.rs:2:16
  |
2 |     if true && if let 1 = 1 { true } else { true } {}
  |                ^^^
  |
help: remove the `if`
  |
2 -     if true && if let 1 = 1 { true } else { true } {}
2 +     if true && let 1 = 1 { true } else { true } {}
  |

error[E0658]: `let` expressions in this position are unstable
 --> /home/gh-compiler-errors/test.rs:2:19
  |
2 |     if true && if let 1 = 1 { true } else { true } {}
  |                   ^^^^^^^^^
  |
  = note: see issue #53667 <https://github.com/rust-lang/rust/issues/53667> for more information
  = help: add `#![feature(let_chains)]` to the crate attributes to enable

error[E0308]: mismatched types
 --> /home/gh-compiler-errors/test.rs:2:31
  |
2 |     if true && if let 1 = 1 { true } else { true } {}
  |     --------------------------^^^^----------------
  |     |                         |
  |     |                         expected `()`, found `bool`
  |     expected this to be `()`

error[E0308]: mismatched types
 --> /home/gh-compiler-errors/test.rs:2:45
  |
2 |     if true && if let 1 = 1 { true } else { true } {}
  |     ----------------------------------------^^^^--
  |     |                                       |
  |     |                                       expected `()`, found `bool`
  |     expected this to be `()`

error: aborting due to 4 previous errors

Some errors have detailed explanations: E0308, E0658.
For more information about an error, try `rustc --explain E0308`.

[chenyukang]

This breaks existing code -- if expressions are allowed to be their own conditions within other if statements.

This code will also compile in current compiler:

fn should_ok() {
    if true && if let x = 1 { true } else { true } {}
}

With this warning:

warning: irrefutable `if let` pattern
  --> /home/cat/code/rust/src/test/ui/parser/issue-103381.rs:13:19
   |
13 |     if true && if let x = 1 { true } else { true } {}
   |                   ^^^^^^^^^
   |
   = note: `#[warn(irrefutable_let_patterns)]` on by default
   = note: this pattern will always match, so the `if let` is useless
   = help: consider replacing the `if let` with a `let`

warning: 1 warning emitted

[compiler-errors]

Just noting that "irrefutable if let pattern" warning is unrelated to this issue, and is just because let x = 1 is a "irrefutable" binding, i.e. not actually conditional.

[chenyukang]

The job x86_64-gnu-llvm-13 failed! Check out the build log: (web) (plain)

Click to see the possible cause of the failure (guessed by this bot)

From the UI testing result, we have this syntax：

fn shoule_match_ok() {
    #[cfg(feature = "full")]
    {
        let a = 1;
        let b = 2;
        if match a {
            1 if b == 1 => true,
            _ => false,
        } && if a > 1 { true } else { false }
        {
            true
        }
    }
}

I think it's not a readable code, but it compiled and runnable 😂
Anyway, I plan to restrict this suggestion to limited scenarios:

            // recovery from `if b && if let Some(x)`
            if op == AssocOp::LAnd
                && self.token.is_keyword(kw::If)
                && self.look_ahead(1, |t| t.is_bool_lit())
                || (self.look_ahead(1, |t| t.is_keyword(kw::Let))
                    && self.look_ahead(2, |t| {
                        t.is_ident_named(sym::Some) || t.is_ident_named(sym::None)
                    }))
            {
                // eat the `if`
                self.bump();
                self.sess.emit_err(UnexpectedIfWithIf(
                    self.prev_token.span.to(self.token.span.shrink_to_lo()),
                ));
            }

Not sure whether there is a better way.

[compiler-errors]

@chenyukang, this approach still has issues:

fn main() {
    if true && if true { true } else { false } {}
}

This fails to compile with your new changes:

error: unexpected `if` in the condition expression
 --> /home/gh-compiler-errors/test.rs:2:16
  |
2 |     if true && if true { true } else { false } {}
  |                ^^^
  |
help: remove the `if`
  |
2 -     if true && if true { true } else { false } {}
2 +     if true && true { true } else { false } {}
  |

error[E0308]: mismatched types
 --> /home/gh-compiler-errors/test.rs:2:26
  |
2 |     if true && if true { true } else { false } {}
  |     ---------------------^^^^-----------------
  |     |                    |
  |     |                    expected `()`, found `bool`
  |     expected this to be `()`

error[E0308]: mismatched types
 --> /home/gh-compiler-errors/test.rs:2:40
  |
2 |     if true && if true { true } else { false } {}
  |     -----------------------------------^^^^^--
  |     |                                  |
  |     |                                  expected `()`, found `bool`
  |     expected this to be `()`

error: aborting due to 3 previous errors

---

I'm pretty sure it's impossible to detect #103381 inside of parse_assoc_expr_with -- you need to add a suggestion to remove if only after the parser has already detected an error, probably by passing the correct state down to parse_if_after_cond or some similar function that is involved in parsing if statements.

This may be hard to do, or not worth the changes to the parser to allow this state to be tracked.

[chenyukang]

Ok, I will have look later Michael Goulet ***@***.***> 于 2022年10月23日周日 14:06写道：

…

@chenyukang <https://github.com/chenyukang>, this approach still has issues: fn main() { if true && if true { true } else { false } {}} This fails to compile with your new changes: error: unexpected `if` in the condition expression --> /home/gh-compiler-errors/test.rs:2:16 | 2 | if true && if true { true } else { false } {} | ^^^ | help: remove the `if` | 2 - if true && if true { true } else { false } {} 2 + if true && true { true } else { false } {} | error[E0308]: mismatched types --> /home/gh-compiler-errors/test.rs:2:26 | 2 | if true && if true { true } else { false } {} | ---------------------^^^^----------------- | | | | | expected `()`, found `bool` | expected this to be `()` error[E0308]: mismatched types --> /home/gh-compiler-errors/test.rs:2:40 | 2 | if true && if true { true } else { false } {} | -----------------------------------^^^^^-- | | | | | expected `()`, found `bool` | expected this to be `()` error: aborting due to 3 previous errors ------------------------------ I'm pretty sure it's impossible to detect #103381 <#103381> inside of parse_assoc_expr_with -- you need to add a suggestion to remove if only *after* the parser has already detected an error, probably by passing the correct state down to parse_if_after_cond or some similar function that is involved in parsing if statements. — Reply to this email directly, view it on GitHub <#103405 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AABYJ5VZ7VWUZIXRMEQRUX3WETIUXANCNFSM6AAAAAARL5BSCU> . You are receiving this because you were mentioned.Message ID: ***@***.***>

[Noratrieb]

Hi, I've seen you changed some diagnostic structs in your PR. After #103345, the way we refer to fluent messages changed. They are now in a flat namespace with the same identifier as in the fluent file. For example, parser::cool_thing is now parser_cool_thing and parser::suggestion just suggestion.
You should rebase to the latest master and change your fluent message references as described above. Thanks!

[chenyukang]

This may be hard to do, or not worth the changes to the parser to allow this state to be tracked.

Take some time to investigate it.
Yes, seems it is not easy since we need to rollback too much.
I think we'd better don't try to suggest and fix this case right now, it's easy for developer fix without diagnostics:

 let _ = if true && if true { true } else { false };
 let _ = if true && if false { true } else { false };

Only add check for && if let Option...

[chenyukang]

@rustbot ready

[camsteffen]

The following code is perfectly valid:

if true
    && if let Some(1) = Some(1) { true } else { true }
{
    todo!()
}

In general, it is always possible to write valid code containing if <some condition> && if let <some pattern>.

I don't think it's possible to detect the scenario you're looking for until after you parse the inner if let .. expression.

[compiler-errors]

@chenyukang, camsteffen is correct, and as I noted in my previous comment, I'm pretty sure it's impossible to detect #103381 inside of parse_assoc_expr_with. Marking as S-waiting-on-author until this PR is reformulated.

This check needs to live in another function using a different strategy, after parsing has detected a failure. Be very careful about adding parser recoveries before we've detected an error, since it's very¹ easy² to accidentally introduce changes in parser behavior while improving error messages during parsing...

[chenyukang]

I think we'd better don't recover from these scenarios, but give proper diagnostics 😁
Code updated.

[estebank]

r? @compiler-errors as you're already familiar with the changes :)

[estebank]

What happens for if true && if true { true } else { false } { true } else { false };?

Given where you call the recovery it should succeed, right? In that case I believe the code changes look fine because the error is only ever precluding another, so there are no changes of regressions.

[compiler-errors]

Given where you call the recovery it should succeed, right?

yeah, this is my thinking. I'll give this a better review later, but I much prefer this approach since it's already deep in the "bad path".

[chenyukang]

Yes, this code will compile successfully, I have added it as a UI.

[compiler-errors]

@rustbot ready

I will get around to reviewing this soon.

[bors]

☔ The latest upstream changes (presumably #104418) made this pull request unmergeable. Please resolve the merge conflicts.

[chenyukang]

@rustbot ready

conflict resolved.

[compiler-errors]

@bors r+

[bors]

📌 Commit cd2bde1 has been approved by compiler-errors

It is now in the queue for this repository.

[bors]

🌲 The tree is currently closed for pull requests below priority 1. This pull request will be tested once the tree is reopened.