1.6 bug: nested string interpolation is broken

[NHDaly]

Hello. :) I believe we have encountered a bug in the 1.6 release: nested string interpolation appears to be broken.

Consider this simple example:

julia> VERSION
v"1.6.0"

julia> v = "a $("b $("c")")"
"a b "

julia> length(v)
4

Where the inner nested string is dropped. Compare this with the prior release which produces the correct answer:

julia> VERSION
v"1.5.4"

julia> v = "a $("b $("c")")"
"a b c"

julia> length(v)
5

I am running on macOS:

julia> versioninfo()
Julia Version 1.6.0
Commit f9720dc2eb (2021-03-24 12:55 UTC)
Platform Info:
  OS: macOS (x86_64-apple-darwin19.6.0)
  CPU: Intel(R) Core(TM) i9-8950HK CPU @ 2.90GHz
  WORD_SIZE: 64
  LIBM: libopenlibm
  LLVM: libLLVM-11.0.1 (ORCJIT, skylake)

---

In case this is relevant, we came across this when a macro that includes string interpolation broke. As a very simple example, consider something like this:

macro pstring(str)
    @info str
    return quote
        println("str1:", $(str))
        println("str2: $($(str))")
    end
end

Interestingly, the @macroexpand is different between 1.5 and 1.6, but as far as I can tell, both are equally valid expansions, so I don't think anything is wrong with the new expansion in 1.6. However, the 1.6 expansion creates nested string interpolation, which doesn't work in 1.6, even though it did work in 1.5:

julia> VERSION
v"1.6.0"

julia> @pstring "hello $("nathan")"
[ Info: "hello $("nathan")"
str1:hello nathan
str2: hello 

julia> @macroexpand @pstring "hello $("nathan")"
[ Info: "hello $("nathan")"
quote
    #= /Users/nathandaly/work/Delve/test/compilation_times/lib.jl:7 =#
    Main.println("str1:", "hello $("nathan")")
    #= /Users/nathandaly/work/Delve/test/compilation_times/lib.jl:8 =#
    Main.println("str2: $("hello $("nathan")")")
end

julia> VERSION
v"1.5.4"

julia> @pstring "hello $("nathan")"
[ Info: "hello nathan"
str1:hello nathan
str2: hello nathan

julia> @macroexpand @pstring "hello $("nathan")"
[ Info: "hello nathan"
quote
    #= /Users/nathandaly/work/Delve/test/compilation_times/lib.jl:7 =#
    Main.println("str1:", "hello nathan")
    #= /Users/nathandaly/work/Delve/test/compilation_times/lib.jl:8 =#
    Main.println("str2: $("hello nathan")")
end

[NHDaly]

And just to confirm, the above is all true on master as well:

julia> VERSION
v"1.7.0-DEV.717"

julia> v = "a $("b $("c")")"
"a b "

julia> length(v)
4

julia> macro pstring(str)
           @info str
           return quote
               println("str1:", $(str))
               println("str2: $($(str))")
           end
       end
@pstring (macro with 1 method)

julia> VERSION
v"1.7.0-DEV.717"

julia> @pstring "hello $("nathan")"
[ Info: "hello $("nathan")"
str1:hello nathan
str2: hello

julia> @macroexpand @pstring "hello $("nathan")"
[ Info: "hello $("nathan")"
quote
    #= REPL[4]:4 =#
    Main.println("str1:", "hello $("nathan")")
    #= REPL[4]:5 =#
    Main.println("str2: $("hello $("nathan")")")
end

[NHDaly]

added parser bug labels

FWIW I'm not sure if this is indeed related to the parser, since it seems to parse correctly (EDIT: or at least, reasonably), and rather it fails during the eval:

julia> e = Meta.parse(raw""" "str2: $("hello $("nathan")")" """)
:("str2: $("hello $("nathan")")")

julia> dump(e)
Expr
  head: Symbol string
  args: Array{Any}((2,))
    1: String "str2: "
    2: Expr
      head: Symbol string
      args: Array{Any}((2,))
        1: String "hello "
        2: Expr
          head: Symbol string
          args: Array{Any}((1,))
            1: String "nathan"

julia> eval(e)
"str2: hello "

[NHDaly]

Although I take that back, since it parses differently on 1.5, so the parser might indeed be the problem:

julia> e = Meta.parse(raw""" "str2: $("hello $("nathan")")" """)
:("str2: $("hello nathan")")

julia> dump(e)
Expr
  head: Symbol string
  args: Array{Any}((2,))
    1: String "str2: "
    2: Expr
      head: Symbol string
      args: Array{Any}((2,))
        1: String "hello "
        2: String "nathan"

julia> VERSION
v"1.5.4"

[KristofferC]

It is completel gone after lowering:

julia> f() = "a $("b $("c")")"
f (generic function with 1 method)

julia> @code_lowered f()
CodeInfo(
1 ─ %1 = Base.string("a ", "b ")
└──      return %1
)