Assume that docstring code with no lang is julia

By making this assumption when parsing docstrings, we can remove the
assumption that all no-lang Markdown code blocks are in fact Julia
without compromising docstring highlighting.

stdlib/Markdown/src/render/terminal/render.jl

@@ -116,7 +116,7 @@ function term(io::AnnotIO, md::Header{l}, columns) where l
 end
 
 function term(io::IO, md::Code, columns)
-    code = if md.language ∈ ("", "julia")
+    code = if md.language == "julia"
         highlight(md.code)
     elseif md.language == "julia-repl" || Base.startswith(md.language, "jldoctest")
         hl = AnnotatedString(md.code)

stdlib/REPL/src/docview.jl

@@ -81,7 +81,8 @@ function formatdoc(d::DocStr)
     for part in d.text
         formatdoc(buffer, d, part)
     end
-    Markdown.MD(Any[Markdown.parse(seekstart(buffer))])
+    md = Markdown.MD(Any[Markdown.parse(seekstart(buffer))])
+    assume_julia_code!(md)
 end
 @noinline formatdoc(buffer, d, part) = print(buffer, part)
 
@@ -95,6 +96,27 @@ function parsedoc(d::DocStr)
     d.object
 end
 
+"""
+    assume_julia_code!(doc::Markdown.MD) -> doc
+
+Assume that code blocks with no language specified are Julia code.
+"""
+function assume_julia_code!(doc::Markdown.MD)
+    assume_julia_code!(doc.content)
+    doc
+end
+
+function assume_julia_code!(blocks::Vector)
+    for (i, block) in enumerate(blocks)
+        if block isa Markdown.Code && block.language == ""
+            blocks[i] = Markdown.Code("julia", block.code)
+        elseif block isa Vector || block isa Markdown.MD
+            assume_julia_code!(block)
+        end
+    end
+    blocks
+end
+
 ## Trimming long help ("# Extended help")
 
 struct Message  # For direct messages to the terminal