strip spaces after backslash-escaped newline (JuliaLang#41245)

This was discussed on Slack with @StefanKarpinski and @BioTurboNick and
the general consensus was that stripping all spaces following a
backslash-escaped newline would make more sense than the current
behavior. Stripping the newline and spaces after a backslash now happens
before detenting for triple-quoted srings, so in cases like

```julia
"""
  a\
b
  c"""
```

the `b` doesn't affect detenting and the spaces before `a` and `c` are
removed.

base/shell.jl

@@ -94,6 +94,10 @@ function shell_parse(str::AbstractString, interpolate::Bool=true;
                         i += 1
                         popfirst!(st)
                     end
+                    while !isempty(st) && peek(st)[2] in (' ', '\t')
+                        i = nextind(str, i)
+                        _ = popfirst!(st)
+                    end
                 elseif in_double_quotes
                     isempty(st) && error("unterminated double quote")
                     k, c′ = peek(st)

src/julia-parser.scm

@@ -2188,16 +2188,24 @@
 (define (unescape-parsed-string-literal strs)
   (map-at even? unescape-string strs))
 
+(define (strip-escaped-newline s raw)
+  (if raw s (map (lambda (s)
+                   (if (string? s) (strip-escaped-newline- s) s))
+                 s)))
+
 ;; remove `\` followed by a newline
-(define (strip-escaped-newline s)
+(define (strip-escaped-newline- s)
   (let ((in  (open-input-string s))
         (out (open-output-string)))
     (define (loop preceding-backslash?)
           (let ((c (read-char in)))
             (cond ((eof-object? c))
                   (preceding-backslash?
                    (if (not (eqv? c #\newline))
-                       (begin (write-char #\\ out) (write-char c out)))
+                       (begin (write-char #\\ out) (write-char c out))
+                       ((define (loop-)
+                          (if (memv (peek-char in) '(#\space #\tab))
+                              (begin (take-char in) (loop-))))))
                    (loop #f))
                   ((eqv? c #\\) (loop #t))
                   (else (write-char c out) (loop #f)))))
@@ -2210,13 +2218,14 @@
                   (if (eqv? (peek-char (take-char p)) delim)
                       (map-first strip-leading-newline
                                  (dedent-triplequoted-string
-                                   (parse-string-literal- 2 (take-char p) s delim raw)))
+                                   (strip-escaped-newline
+                                     (parse-string-literal- 2 (take-char p) s delim raw)
+                                     raw)))
                       (list ""))
-                  (parse-string-literal- 0 p s delim raw))))
-    (if raw str (unescape-parsed-string-literal
-                  (map (lambda (s)
-                         (if (string? s) (strip-escaped-newline s) s))
-                       str)))))
+                  (strip-escaped-newline
+                    (parse-string-literal- 0 p s delim raw)
+                    raw))))
+    (if raw str (unescape-parsed-string-literal str))))
 
 (define (strip-leading-newline s)
   (let ((n (sizeof s)))

test/syntax.jl

@@ -2837,7 +2837,7 @@ end
     @test "a\
 b" == "ab"
     @test "a\
-    b" == "a    b"
+    b" == "ab"
     @test raw"a\
 b" == "a\\\nb"
     @test "a$c\
@@ -2851,10 +2851,10 @@ b" == "acb"
           b""" == "ab"
     @test """
           a\
-            b""" == "a  b"
+            b""" == "ab"
     @test """
             a\
-          b""" == "  ab"
+          b""" == "ab"
     @test raw"""
           a\
           b""" == "a\\\nb"
@@ -2894,7 +2894,7 @@ b" == "acb"
     @test `a\
 b` == `ab`
     @test `a\
-    b` == `a    b`
+    b` == `ab`
     @test `a$c\
 b` == `acb`
     @test `"a\
@@ -2910,7 +2910,7 @@ b'` == `$("a\\\nb")`
           b``` == `ab`
     @test ```
           a\
-            b``` == `a  b`
+            b``` == `ab`
     @test ```
             a\
           b``` == `  ab`