Broken MIR with if let chain

Recently I played with the newly stabilized if let chain (tracker for issue #53667, stabilized in Rust 1.64 #94927) and found that in some cases it gives internal compiler error: broken MIR . Below is a minimal example. It compiles fine with cargo build or cargo run, but does not compile with cargo build --release. It also compiles if you replace if let chain with nested if let.

Code

struct TupleIter<T, I: Iterator<Item = T>> {
    inner: I,
}

impl<T, I: Iterator<Item = T>> Iterator for TupleIter<T, I> {
    type Item = (T, T, T);

    fn next(&mut self) -> Option<Self::Item> {
        let inner = &mut self.inner;

        // Error with if_let_chain
        if let Some(first) = inner.next()
            && let Some(second) = inner.next()
            && let Some(third) = inner.next()
        {
            Some((first, second, third))
        } else {
            None
        }

        // Ok we use nested if_let
        // if let Some(first) = inner.next() {
        //     if let Some(second) = inner.next() {
        //         if let Some(third) = inner.next() {
        //             Some((first, second, third))
        //         } else {
        //             None
        //         }
        //     } else {
        //         None
        //     }
        // } else {
        //     None
        // }
    }
}

fn main() {
    let vec: Vec<u8> = Vec::new();
    let mut tup_iter = TupleIter {
        inner: vec.into_iter(),
    };
    tup_iter.next();
}

Error output

cargo build --release

error: internal compiler error: no errors encountered even though `delay_span_bug` issued                                 

error: internal compiler error: broken MIR in Item(WithOptConstParam { did: DefId(0:11 ~ if_let_chain[af92]::{impl#0}::next), const_param_did: None }) (end of phase transition to Optimized) at bb22[0]:
                                use of local _14, which has no storage here
  --> src\main.rs:17:9
   |
17 |         } else {
   |         ^
   |
   = note: delayed at compiler\rustc_const_eval\src\transform\validate.rs:129:36

thread 'rustc' panicked at 'Box<dyn Any>', compiler\rustc_errors\src\lib.rs:1426:13
stack backtrace:
   0:     0x7ff92c1e9c5f - <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt::h22e7b784ac716ba6
   1:     0x7ff92c2255ba - core::fmt::write::h710985228ff64dc4
   2:     0x7ff92c1dcada - <std::io::IoSliceMut as core::fmt::Debug>::fmt::h7a965e27de439674
   3:     0x7ff92c1ed65b - std::panicking::default_hook::h59e772df96aabd4c
   4:     0x7ff92c1ed287 - std::panicking::default_hook::h59e772df96aabd4c
   5:     0x7ff915f26b81 - <rustc_session[9ee3cdee11a5ff9c]::options::WasiExecModel as rustc_session[9ee3cdee11a5ff9c]::config::dep_tracking::DepTrackingHash>::hash
   6:     0x7ff92c1ee03c - std::panicking::rust_panic_with_hook::h38636381814ab6ea
   7:     0x7ff918147cf3 - <rustc_errors[76a7f7b80221f39a]::annotate_snippet_emitter_writer::AnnotateSnippetEmitterWriter>::ui_testing
   8:     0x7ff918147a99 - <rustc_errors[76a7f7b80221f39a]::annotate_snippet_emitter_writer::AnnotateSnippetEmitterWriter>::ui_testing
   9:     0x7ff91813d329 - <rustc_errors[76a7f7b80221f39a]::emitter::FileWithAnnotatedLines as core[9a2c4b0c2bb38f26]::fmt::Debug>::fmt
  10:     0x7ff9181316b9 - <rustc_feature[b39b46621b392478]::builtin_attrs::AttributeType as core[9a2c4b0c2bb38f26]::fmt::Debug>::fmt
  11:     0x7ff9147a943d - <rustc_errors[76a7f7b80221f39a]::HandlerInner as core[9a2c4b0c2bb38f26]::ops::drop::Drop>::drop  12:     0x7ff913919b9b - <rustc_middle[d6f72711bda4db03]::ty::SymbolName as core[9a2c4b0c2bb38f26]::fmt::Display>::fmt  
  13:     0x7ff91391cabf - <rustc_middle[d6f72711bda4db03]::ty::SymbolName as core[9a2c4b0c2bb38f26]::fmt::Display>::fmt  
  14:     0x7ff91390aced - <unknown>
  15:     0x7ff9138f73e7 - <unknown>
  16:     0x7ff91392e937 - rustc_driver[197bfed8d57a407b]::args::arg_expand_all
  17:     0x7ff913917cf9 - <unknown>
  18:     0x7ff9139187fd - <rustc_middle[d6f72711bda4db03]::ty::SymbolName as core[9a2c4b0c2bb38f26]::fmt::Display>::fmt  
  19:     0x7ff92c1ff15c - std::sys::windows::thread::Thread::new::hec2508d69eebdd0a
  20:     0x7ff993187034 - BaseThreadInitThunk
  21:     0x7ff9934c2651 - RtlUserThreadStart

note: the compiler unexpectedly panicked. this is a bug.

note: we would appreciate a bug report: https://github.com/rust-lang/rust/issues/new?labels=C-bug%2C+I-ICE%2C+T-compiler&template=ice.md

note: rustc 1.64.0-nightly (4d6d601c8 2022-07-26) running on x86_64-pc-windows-msvc

note: compiler flags: --crate-type bin -C opt-level=3 -C embed-bitcode=no

note: some of the compiler flags provided by cargo are hidden

query stack during panic:
end of query stack
error: could not compile `if_let_chain`

Thank you for you report, but this is probably a duplicate of #99852

Can confirm this no longer ICEs on most recent nightly:

https://play.rust-lang.org/?version=nightly&mode=release&edition=2021&gist=ca82d8f6583d900c67af8e1784a882f1

@rustbot

@rustbot label +E-needs-test

Yeah likely this was fixed by #102394 . I wonder if it's worth adding a test for this, or it can just be closed as a dupe of #99852.

We just hit this during the Clippy sync in #104688 (comment)

The offending code was this:

rust/src/tools/clippy/src/driver.rs

Lines 93 to 98 in 2422802

    
           if cfg!(debug_assertions) 
        
               && let Ok(current_exe) = env::current_exe() 
        
               && let Some(current_exe) = current_exe.to_str() 
        
           { 
        
               file_depinfo.insert(Symbol::intern(current_exe)); 
        
           }

Pulling the cfg! if line out of the chain fixed the issue, see 5e6a9a4, so this works:

rust/src/tools/clippy/src/driver.rs

Lines 93 to 99 in 5e6a9a4

    
           if cfg!(debug_assertions) { 
        
               if let Ok(current_exe) = env::current_exe() 
        
                   && let Some(current_exe) = current_exe.to_str() 
        
               { 
        
                   file_depinfo.insert(Symbol::intern(current_exe)); 
        
               } 
        
           }

@flip1995

@flip1995 your issue is different, can you file a bug for it? This issue does not reproduce any more on latest nightly, it should be closed (or a test added for it before closure, but #102394 had tests already.

Hmm, I can do this, but I think that is actually the same issue. While the above reproducer might be fixed, the fix doesn't seem to generally apply. The above code produced the exact same ICE/error message as posted in this issue.

@flip1995

@flip1995 I think this might be some MIR optimization gone wrong, maybe caused by wrong codegen, but your issue requires optimizations to be turned on, it can't just be reproduced by -Zvalidate-mir.

The original ICE was also reproducible by -Zvalidate-mir (as well as with -O, as well as with both params), I've just verified using nightly-2022-07-30. I would thus recommend you file a new issue, but your choice.

Some additional info to your issue, this is a reproducer:

#![feature(let_chains)]

pub fn foo() {
    if false
        && let Ok(ce) = std::env::current_exe()
        && let Some(ce) = ce.to_str()
    {
        let _ = ce.len();
    }
}

Interestingly, the issue is a regression, as it didn't ICE on nightly-2022-07-30.

Edit: I very much appreciate the notion of not wanting to file multiple issues, after all this very issue is a dupe, but the clippy bug is separate.

This isn't because of MIR opts, it only happens under --release because storage markers, which are needed to get this ICE, are only emitted in optimized builds.

@Nilstrieb is this storage markers emitting only in release a new feature? Because I can reproduce the ICE with the original reproducer from this issue's author, with nightly-2022-07-30 and -Zvalidate-mir and no -O flag whatsoever.

Rollup merge of rust-lang#104771 - est31:if_let_chain_broken_mir_test…

Auto merge of rust-lang#104809 - matthiaskrgr:rollup-8abjdwh, r=matth…

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	if cfg!(debug_assertions)
	&& let Ok(current_exe) = env::current_exe()
	&& let Some(current_exe) = current_exe.to_str()
	{
	file_depinfo.insert(Symbol::intern(current_exe));
	}

	if cfg!(debug_assertions) {
	if let Ok(current_exe) = env::current_exe()
	&& let Some(current_exe) = current_exe.to_str()
	{
	file_depinfo.insert(Symbol::intern(current_exe));
	}
	}

Broken MIR with if let chain #99938

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Noratrieb commented on Jul 30, 2022

Lee-Janggun commented on Oct 15, 2022

Rageking8 commented on Oct 15, 2022

est31 commented on Oct 15, 2022

flip1995 commented on Nov 22, 2022

est31 commented on Nov 22, 2022

flip1995 commented on Nov 22, 2022

est31 commented on Nov 22, 2022

Noratrieb commented on Nov 22, 2022

est31 commented on Nov 22, 2022

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Broken MIR with `if let chain` #99938