Docstrings with backslashes yield "invalid escape sequence" upon precompilation

[timholy]

Non-contaminating reproducer: start julia with

$ DEPOT_PATH=/tmp/pkgs julia

and then

(v1.0) pkg> generate Backslash
Generating project Backslash:
    Backslash/Project.toml
    Backslash/src/Backslash.jl

julia> edit("Backslash/src/Backslash.jl")

Then add a docstring to greet:

"""
This is a docstring with a backslash: x = A \ b
"""
greet() = print("Hello World!")

(I inserted the spaces between backticks to prevent confusing the markdown renderer here, but you should take them out when you try this.)

And then finally:

julia> using Backslash
[ Info: Precompiling Backslash [f5b45752-b8ef-11e8-19fb-2fec647fc92b]
ERROR: LoadError: syntax: invalid escape sequence
Stacktrace:
 [1] include at ./boot.jl:317 [inlined]
 [2] include_relative(::Module, ::String) at ./loading.jl:1041
 [3] include(::Module, ::String) at ./sysimg.jl:29
 [4] top-level scope at none:2
 [5] eval at ./boot.jl:319 [inlined]
 [6] eval(::Expr) at ./client.jl:389
 [7] top-level scope at ./none:3
in expression starting at /tmp/Backslash/src/Backslash.jl:5
ERROR: Failed to precompile Backslash [f5b45752-b8ef-11e8-19fb-2fec647fc92b] to /tmp/pkgs/compiled/v1.0/Backslash/JC5Xv.ji.
Stacktrace:
 [1] error(::String) at ./error.jl:33
 [2] macro expansion at ./logging.jl:313 [inlined]
 [3] compilecache(::Base.PkgId, ::String) at ./loading.jl:1187
 [4] _require(::Base.PkgId) at ./logging.jl:311
 [5] require(::Base.PkgId) at ./loading.jl:855
 [6] macro expansion at ./logging.jl:311 [inlined]
 [7] require(::Module, ::Symbol) at ./loading.jl:837

Now, perhaps this is deliberate (you can fix it by changing \ to \\), but it seems a little strange to have precompilation fail due to the formatting of the docstring.

[yuyichao]

But "A \ b" isn't a valid string and the doc string is just a string. Why would invalid string as docstring not cause an error?

Also, no need to create a package, REPL is enough...

julia> """
       This is a docstring with a backslash: x = A \ b
       """
       greet() = print("Hello World!")

ERROR: syntax: invalid escape sequence

[timholy]

Fair enough. There isn't a depwarn for 0.7 or FemtoCleaner fix, but I'll close.

[fredrikekre]

See #22800

You can use

using Markdown
@doc doc"""
This is a docstring with a backslash: x = A \ b
"""
greet() = print("Hello World!")