Improve annotation *-concatenation behaviour

SubStrings have been overlooked, and thanks to a few compiler
quirks (relating to inlining and effect analysis), adding support for
them is unfortunately a little more complicated than adding a
"|| s isa SubString{<:AnnotatedString}" clause thanks to the new
generated runtime-checks.

To maintain the zero-overhead non-annotated code path, we need to
implement a separate function _isannotated, which we also make use of to
simplify the current join method.

Author: tecosaur

base/strings/basic.jl

@@ -257,9 +257,7 @@ julia> 'j' * "ulia"
 ```
 """
 function (*)(s1::Union{AbstractChar, AbstractString}, ss::Union{AbstractChar, AbstractString}...)
-    isannotated = s1 isa AnnotatedString || s1 isa AnnotatedChar ||
-        any(s -> s isa AnnotatedString || s isa AnnotatedChar, ss)
-    if isannotated
+    if _isannotated(s1) || any(_isannotated, ss)
         annotatedstring(s1, ss...)
     else
         string(s1, ss...)
@@ -268,6 +266,12 @@ end
 
 one(::Union{T,Type{T}}) where {T<:AbstractString} = convert(T, "")
 
+# This could be written as a single statement with three ||-clauses, however then effect
+# analysis thinks it may throw and runtime checks are added.
+# Also see `substring.jl` for the `::SubString{T}` method.
+_isannotated(S::Type) = S != Union{} && (S <: AnnotatedString || S <: AnnotatedChar)
+_isannotated(s) = _isannotated(typeof(s))
+
 ## generic string comparison ##
 
 """

base/strings/io.jl

@@ -365,10 +365,7 @@ function join_annotated(iterator, delim="", last=delim)
 end
 
 function _join_maybe_annotated(args...)
-    if any(function (arg)
-               t = eltype(arg)
-               !(t == Union{}) && (t <: AnnotatedString || t <: AnnotatedChar)
-           end, args)
+    if any(_isannotated ∘ eltype, args)
         join_annotated(args...)
     else
         sprint(join, args...)

base/strings/substring.jl

@@ -140,6 +140,8 @@ function hash(s::SubString{String}, h::UInt)
     ccall(memhash, UInt, (Ptr{UInt8}, Csize_t, UInt32), s, sizeof(s), h % UInt32) + h
 end
 
+_isannotated(::SubString{T}) where {T} = _isannotated(T)
+
 """
     reverse(s::AbstractString) -> AbstractString
 

base/strings/util.jl

@@ -459,7 +459,7 @@ function lpad(
     n::Integer,
     p::Union{AbstractChar,AbstractString}=' ',
 )
-    stringfn = if any(isa.((s, p), Union{AnnotatedString, AnnotatedChar, SubString{<:AnnotatedString}}))
+    stringfn = if _isannotated(s) || _isannotated(p)
         annotatedstring else string end
     n = Int(n)::Int
     m = signed(n) - Int(textwidth(s))::Int
@@ -491,7 +491,7 @@ function rpad(
     n::Integer,
     p::Union{AbstractChar,AbstractString}=' ',
 )
-    stringfn = if any(isa.((s, p), Union{AnnotatedString, AnnotatedChar, SubString{<:AnnotatedString}}))
+    stringfn = if _isannotated(s) || _isannotated(p)
         annotatedstring else string end
     n = Int(n)::Int
     m = signed(n) - Int(textwidth(s))::Int