Bad code generated in Rust for recursive function having a local variable with the same name as a parameter.

[ajewellamz]

Dafny version

latest feat-rust

Code to produce this issue

function method {:tailrecursion} GetMaps(
    opts : string,
    shortMap : map<char, string> := map[])
    : map<char, string> 
  {
    if |opts| == 0 then
      shortMap
    else
      var shortMap := shortMap[opts[0] := "spoo"];
      GetMaps(opts[1..], shortMap)
  }

Command to run and resulting output

dafny -noVerify -compileTarget:rs -compile:0 -functionSyntax:3 -spillTargetCode:3 mapfail.dfy

     1  #![allow(warnings, unconditional_panic)]
     2  #![allow(nonstandard_style)]
     3  
     4  pub mod _module {
     5    pub struct _default {}
     6  
     7    impl _default {
     8      pub fn GetMaps(opts: &::dafny_runtime::Sequence<::dafny_runtime::DafnyChar>, shortMap: &::dafny_runtime::Map<::dafny_runtime::DafnyChar, ::dafny_runtime::Sequence<::dafny_runtime::DafnyChar>>) -> ::dafny_runtime::Map<::dafny_runtime::DafnyChar, ::dafny_runtime::Sequence<::dafny_runtime::DafnyChar>> {
     9        let mut opts = opts.clone();
    10        let mut shortMap = shortMap.clone();
    11        'TAIL_CALL_START: loop {
    12          if opts.cardinality() == ::dafny_runtime::int!(0) {
    13            return shortMap.clone();
    14          } else {
    15            let mut shortMap: ::dafny_runtime::Map<::dafny_runtime::DafnyChar, ::dafny_runtime::Sequence<::dafny_runtime::DafnyChar>> = shortMap.update_index(&opts.get(&::dafny_runtime::int!(0)), &::dafny_runtime::string_of("spoo"));
    16            let mut _in0: ::dafny_runtime::Sequence<::dafny_runtime::DafnyChar> = opts.drop(&::dafny_runtime::int!(1));
    17            let mut _in1: ::dafny_runtime::Map<::dafny_runtime::DafnyChar, ::dafny_runtime::Sequence<::dafny_runtime::DafnyChar>> = shortMap.clone();
    18            opts = _in0.clone();
    19            shortMap = _in1.clone();
    20            continue 'TAIL_CALL_START;
    21          }
    22        }
    23      }
    24    }
    25  }

What happened?

Line 10 declares shortMap, which is the return value.

Line 15 declares a shadowing shortMap.

Line 19 updates the shadow defined on line 15, not the "real" one defined on line 10.

Then the function returns an empty map, instead of one with data in it.

What type of operating system are you experiencing the problem on?

Mac

[ssomayyajula]

Thanks for the report; do you mind showing the output without the {:tailrecursion} attribute? The variable capture may happen due to that optimization.

[ajewellamz]

The output is exactly the same. This is expected, because all {:tailrecursion} does is throw an error if tail recursion isn't possible.

[ssomayyajula]

(Documenting for anyone watching) workaround: assign a fresh name to shortMap in the else branch

[MikaelMayer]

Among all possible solutions, I found that this one works pretty straightforward. We change the recursive name variable to unique names independent of the parameter names, and then we assign the parameter names at the beginning of the loop.

     1  #![allow(warnings, unconditional_panic)]
     2  #![allow(nonstandard_style)]
     3  
     4  pub mod _module {
     5    pub struct _default {}
     6  
     7    impl _default {
     8      pub fn GetMaps(opts: &::dafny_runtime::Sequence<::dafny_runtime::DafnyChar>, shortMap: &::dafny_runtime::Map<::dafny_runtime::DafnyChar, ::dafny_runtime::Sequence<::dafny_runtime::DafnyChar>>) -> ::dafny_runtime::Map<::dafny_runtime::DafnyChar, ::dafny_runtime::Sequence<::dafny_runtime::DafnyChar>> {
     9        let mut _r0 = opts;
    10        let mut _r1 = shortMap;
    11        'TAIL_CALL_START: loop {
    11bis       let opts = _r0;
    11ter       let shortMap = _r1;
    12          if opts.cardinality() == ::dafny_runtime::int!(0) {
    13            return shortMap.clone();
    14          } else {
    15            let mut shortMap: ::dafny_runtime::Map<::dafny_runtime::DafnyChar, ::dafny_runtime::Sequence<::dafny_runtime::DafnyChar>> = shortMap.update_index(&opts.get(&::dafny_runtime::int!(0)), &::dafny_runtime::string_of("spoo"));
    16            let mut _in0: ::dafny_runtime::Sequence<::dafny_runtime::DafnyChar> = opts.drop(&::dafny_runtime::int!(1));
    17            let mut _in1: ::dafny_runtime::Map<::dafny_runtime::DafnyChar, ::dafny_runtime::Sequence<::dafny_runtime::DafnyChar>> = shortMap.clone();
    18            _r0 = _in0.clone();
    19            _r1 = _in1.clone();
    20            continue 'TAIL_CALL_START;
    21          }
    22        }
    23      }
    24    }
    25  }

The rationale is the following:

• Normally parameters are read-only in Dafny, so the fact that we can assign to them is just a convenience provided by many programming languages. However, Rust, like Dafny, does not allow assigning to parameters.
• Hence, it's ok for recursion variables to not have the same name as parameters.
• Recursion variables do take ownership of the content of the parameters
• Recursion variables transfer ownership to locally scoped variables
• Recursion variables are assigned before any jump tail call to the start