Define text/latex printing of HorizontalRule with the latex function, fix #29995.

Author: fredrikekre

stdlib/Markdown/src/render/latex.jl

@@ -101,6 +101,9 @@ function latex(io::IO, md::List)
 end
 
 function show(io::IO, ::MIME"text/latex", md::HorizontalRule)
+    latex(io, md)
+end
+function latex(io::IO, md::HorizontalRule)
     println(io, "\\rule{\\textwidth}{1pt}")
 end
 

stdlib/Markdown/test/runtests.jl

@@ -1108,3 +1108,9 @@ let
     @test v.content[5].loose
     @test !v.content[7].loose
 end
+
+# issue #29995
+let m = Markdown.parse("---"), io = IOBuffer()
+    show(io, "text/latex", m)
+    @test String(take!(io)) == "\\rule{\\textwidth}{1pt}\n"
+end