hpc markup error: hGetContents: invalid argument (invalid byte sequence)

Summary

hpc depends on the system locale when reading source files, while ghc expects those files to be encoded in utf-8

Steps to reproduce

When compiling a simple Main.hs file with some unicode characters:

module Main where

main :: IO ()
main = putStrLn "Добрый день"

we will not have a problem, but when running hpc markup we do get an error, unless our locale is set to unicode:

$ rm -fr .hpc Main.tix && LANG=ASCII ghc -fhpc Main.hs -fforce-recomp && ./Main && hpc report Main && LANG=ASCII hpc markup Main
[1 of 1] Compiling Main             ( Main.hs, Main.o )
Linking Main ...
Добрый день
100% expressions used (2/2)
100% boolean coverage (0/0)
     100% guards (0/0)
     100% 'if' conditions (0/0)
     100% qualifiers (0/0)
100% alternatives used (0/0)
100% local declarations used (0/0)
100% top-level declarations used (1/1)
Writing: Main.hs.html
hpc: ./Main.hs: hGetContents: invalid argument (invalid byte sequence)

Expected behavior

hpc should not rely on system locale for source file encoding, since they are expected to be in utf-8, and will not be compiled otherwise. Moreover, while generating the html files with hpc markup system locale is queried with System.IO.localeEncoding and set as a charset in the Content-Type header, which is again incorrect, since .hs files will be in utf-8. Expected outcome:

$ LANG=en_US.UTF-8 hpc markup Main
Writing: Main.hs.html
Writing: hpc_index.html
Writing: hpc_index_fun.html
Writing: hpc_index_alt.html
Writing: hpc_index_exp.html

Environment

• GHC version used: 8.6.5 and others

Optional:

• Operating System: Ubuntu 18.04LTS
• System Architecture: x86_64