show(::String): elide long strings (close #40724)

base/strings/io.jl

@@ -188,7 +188,41 @@ string(a::Symbol) = String(a)
 #       write uses an encoding determined by `s` (UTF-8 for `String`)
 print(io::IO, s::AbstractString) = for c in s; print(io, c); end
 write(io::IO, s::AbstractString) = (len = 0; for c in s; len += Int(write(io, c))::Int; end; len)
-show(io::IO, s::AbstractString) = print_quoted(io, s)
+
+# show string elided if more than `limit` characters
+function show(
+    io    :: IO,
+    str   :: AbstractString;
+    limit :: Integer = 5 * max(20, displaysize(io)[2]), # five lines
+)
+    # early out for short strings
+    len = ncodeunits(str)
+    len ≤ limit - 2 && # quotes
+        return print_quoted(io, str)
+
+    # these don't depend on string data
+    units = codeunit(str) == UInt8 ? "bytes" : "code units"
+    skip_text(skip) = " ⋯ $skip $units ⋯ "
+    chars = limit - length(skip_text("")) - 4 # quotes
+
+    # figure out how many characters to print in elided case
+    chars -= d = ndigits(len - chars) # first adjustment
+    chars += d - ndigits(len - chars) # second if needed
+
+    # find head & tail, avoiding O(length(str)) computation
+    head = nextind(str, 0, (chars + 1) ÷ 2)
+    tail = prevind(str, len + 1, chars ÷ 2)
+
+    # decide whether to elide or not
+    if limit - chars ≤ tail - head
+        skip = skip_text(tail - head - 1)
+        print_quoted(io, SubString(str, 1, head))
+        print(io, skip) # TODO: bold styled
+        print_quoted(io, SubString(str, tail))
+    else
+        print_quoted(io, str)
+    end
+end
 
 # optimized methods to avoid iterating over chars
 write(io::IO, s::Union{String,SubString{String}}) =