introduce dstringt::starts_with

[kroening]

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[NlightNFotis]

There's a weird build failure on Travis, on osx, but aside from that, this is good stuff!

I very much like the readability of this compared to has_prefix. Does it mean that has_prefix is now deprecated? If yes, should it be marked so?

[hannes-steffenhagen-diffblue]

Does that mean we no longer care about being able to swap in std::string for dstringt? No objection to that personally, but if we do that we should also remove USE_STD_STRING (that’d have to happen before this PR because we do actually test if cbmc builds with USE_STD_STRING).

[tautschnig]

Does that mean we no longer care about being able to swap in std::string for dstringt? No objection to that personally, but if we do that we should also remove USE_STD_STRING (that’d have to happen before this PR because we do actually test if cbmc builds with USE_STD_STRING).

I would believe we maintain full compatibility: with this patch, .starts_with() can be used the same way with dstringt and std::string.

@kroening I'd +1 the suggestion to deprecate has_prefix, and in a lot of files the #include <util/prefix.h> should be removed.

[kroening]

We can either wait for C++20 (which adds .starts_with() to std::string) or remove USE_STD_STRING.

[codecov]

Codecov Report

Merging #5488 (e5d77fd) into develop (a1ecd93) will decrease coverage by 9.34%.
The diff coverage is 60.52%.

❗ Current head e5d77fd differs from pull request most recent head f44cac4. Consider uploading reports for the commit f44cac4 to get more accurate results

@@             Coverage Diff             @@
##           develop    #5488      +/-   ##
===========================================
- Coverage    77.88%   68.53%   -9.35%     
===========================================
  Files         1569     1187     -382     
  Lines       180894    98262   -82632     
===========================================
- Hits        140882    67340   -73542     
+ Misses       40012    30922    -9090     

Flag	Coverage Δ
cproversmt2	42.96% <53.84%> (?)
regression	65.68% <60.52%> (?)
unit	32.25% <39.34%> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/analyses/goto_check.cpp	88.20% <0.00%> (ø)
src/goto-instrument/branch.cpp	0.00% <0.00%> (ø)
src/goto-instrument/function.cpp	0.00% <0.00%> (ø)
src/goto-instrument/race_check.cpp	0.00% <0.00%> (ø)
src/goto-instrument/wmm/shared_buffers.cpp	0.00% <0.00%> (-77.80%)	⬇️
src/goto-programs/show_goto_functions_xml.cpp	0.00% <0.00%> (ø)
src/jsil/jsil_typecheck.cpp	0.00% <0.00%> (ø)
src/goto-programs/graphml_witness.cpp	56.00% <20.00%> (+2.91%)	⬆️
src/ansi-c/c_preprocess.cpp	32.31% <33.33%> (-2.75%)	⬇️
src/goto-instrument/goto_program2code.cpp	65.62% <42.85%> (-3.22%)	⬇️
... and 1615 more

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[kroening]

I've dropped the second commit until we do C++20. The first commit is useful in preparation.