fix command literals with trailing backslashes

`` `\\` `` currently throws an error because it is parsed as `@cmd "\\"`
due to escaping in the parser and `shell_parse` therefore thinks it's a
dangling backslash.

base/cmd.jl

@@ -504,6 +504,7 @@ Process(`echo 1`, ProcessExited(0))
 ```
 """
 macro cmd(str)
+    str = escape_raw_string(str, '`')
     cmd_ex = shell_parse(str, special=shell_special, filename=String(__source__.file))[1]
     return :(cmd_gen($(esc(cmd_ex))))
 end

base/strings/io.jl

@@ -612,14 +612,14 @@ julia> println(raw"\\\\x \\\\\\"")
 macro raw_str(s); s; end
 
 """
-    escape_raw_string(s::AbstractString)
-    escape_raw_string(io, s::AbstractString)
+    escape_raw_string(s::AbstractString, delim='"') -> AbstractString
+    escape_raw_string(io, s::AbstractString, delim='"')
 
 Escape a string in the manner used for parsing raw string literals.
-For each double-quote (`"`) character in input string `s`, this
-function counts the number _n_ of preceding backslash (`\\`) characters,
-and then increases there the number of backslashes from _n_ to 2_n_+1
-(even for _n_ = 0). It also doubles a sequence of backslashes at the end
+For each double-quote (`"`) character in input string `s` (or `delim` if
+specified), this function counts the number _n_ of preceding backslash (`\\`)
+characters, and then increases there the number of backslashes from _n_ to
+2_n_+1 (even for _n_ = 0). It also doubles a sequence of backslashes at the end
 of the string.
 
 This escaping convention is used in raw strings and other non-standard
@@ -629,36 +629,41 @@ command-line string into the argv[] array.)
 
 See also [`escape_string`](@ref).
 """
-function escape_raw_string(io, str::AbstractString)
+function escape_raw_string(io::IO, str::AbstractString, delim::Char='"')
+    total = 0
     escapes = 0
     for c in str
         if c == '\\'
             escapes += 1
         else
-            if c == '"'
+            if c == delim
                 # if one or more backslashes are followed by
                 # a double quote then escape all backslashes
                 # and the double quote
-                escapes = escapes * 2 + 1
-            end
-            while escapes > 0
-                write(io, '\\')
-                escapes -= 1
+                escapes += 1
+                total += escapes
+                while escapes > 0
+                    write(io, '\\')
+                    escapes -= 1
+                end
             end
             escapes = 0
-            write(io, c)
         end
+        write(io, c)
     end
     # also escape any trailing backslashes,
     # so they do not affect the closing quote
+    total += escapes
     while escapes > 0
-        write(io, '\\')
         write(io, '\\')
         escapes -= 1
     end
+    total
+end
+function escape_raw_string(str::AbstractString, delim::Char='"')
+    total = escape_raw_string(devnull, str, delim) # check whether the string even needs to be copied and how much to allocate for it
+    return total == 0 ? str : sprint(escape_raw_string, str, delim; sizehint = sizeof(str) + total)
 end
-escape_raw_string(str::AbstractString) = sprint(escape_raw_string, str;
-                                                sizehint = lastindex(str) + 2)
 
 ## multiline strings ##
 

doc/src/base/strings.md

@@ -95,5 +95,6 @@ Base.isspace
 Base.isuppercase
 Base.isxdigit
 Base.escape_string
+Base.escape_raw_string
 Base.unescape_string
 ```

test/misc.jl

@@ -1402,3 +1402,21 @@ end
     GC.gc(true); yield()
     @test in_fin[]
 end
+
+@testset "cmd literals with escaped backslashes" begin
+    @test ``       == Cmd(String[])
+    @test `\\`     == Cmd(["\\"])
+    @test `\\\\`   == Cmd(["\\\\"])
+    @test `\\\\\\` == Cmd(["\\\\\\"])
+
+    @test `"\\"`     == Cmd(["\\"])
+    @test `"\\\\"`   == Cmd(["\\\\"])
+    @test `"\\\\\\"` == Cmd(["\\\\\\"])
+
+    @test `'\\'`     == Cmd(["\\\\"])
+    @test `'\\\\'`   == Cmd(["\\\\\\\\"])
+
+    @test `\`\\\``     == Cmd(["`\\`"])
+    @test `\`\\\\\``   == Cmd(["`\\\\`"])
+    @test `\`\\\\\\\`` == Cmd(["`\\\\\\`"])
+end