display something useful for text/plain output of invalid String (#18296)

(cherry picked from commit 2b2894c)

Author: stevengj

base/replutil.jl

@@ -153,6 +153,16 @@ end
 show(io::IO, ::MIME"text/plain", X::AbstractArray) = showarray(io, X, false)
 show(io::IO, ::MIME"text/plain", r::Range) = show(io, r) # always use the compact form for printing ranges
 
+# display something useful even for strings containing arbitrary
+# (non-UTF8) binary data:
+function show(io::IO, ::MIME"text/plain", s::String)
+    if isvalid(s)
+        show(io, s)
+    else
+        println(io, sizeof(s), "-byte String of invalid UTF-8 data:")
+        showarray(io, s.data, false; header=false)
+    end
+end
 
 # showing exception objects as descriptive error messages
 

test/show.jl

@@ -572,3 +572,6 @@ end
 @test repr(:(f.(X,Y))) == ":(f.(X,Y))"
 @test repr(:(f.(X))) == ":(f.(X))"
 @test repr(:(f.())) == ":(f.())"
+
+# Test that REPL/mime display of invalid UTF-8 data doesn't throw an exception:
+@test isa(stringmime("text/plain", String(UInt8[0x00:0xff;])), String)