[Fix] Codegen for futures.

[d0cd]

Closes #3065

[codecov]

Codecov Report

Merging #3064 (9d11d05) into testnet3 (04c198a) will increase coverage by 0.00%.
The diff coverage is 69.56%.

@@            Coverage Diff            @@
##           testnet3    #3064   +/-   ##
=========================================
  Coverage     78.89%   78.89%           
=========================================
  Files           164      164           
  Lines          5514     5520    +6     
  Branches       5514     5520    +6     
=========================================
+ Hits           4350     4355    +5     
- Misses         1164     1165    +1     

Files	Coverage Δ
...er/passes/src/code_generation/visit_expressions.rs	72.37% <50.00%> (ø)
compiler/passes/src/flattening/flattener.rs	80.21% <80.00%> (+1.15%)	⬆️
tests/test-framework/src/error.rs	25.00% <0.00%> (ø)
...mpiler/passes/src/code_generation/visit_program.rs	76.92% <73.33%> (+0.45%)	⬆️

... and 2 files with indirect coverage changes

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[acoglio]

Looks good, but I'm not super-familiar with this part of the code.