Trim trailing whitespace in comments

This patch adds trimming of trailing whitespace inside of comments in
addition to the trimming of trailing whitespace in code. Note that
trailing whitespace inside of multiline is not trimmed since doing so
would change the content of the string.

Closes #50.

README.md

@@ -657,10 +657,14 @@ Braces are consistently used around the right hand side of `where` expressions.
 
 #### Trailing spaces
 
-Trailing spaces are removed. Example:
+Trailing spaces are removed in code and comments (but not inside of multiline strings where
+doing so would change the meaning of the code). Examples:
 ```diff
 -1 + 1 
 +1 + 1
+
+-x = 2 # x is two 
++x = 2 # x is two
 ```
 
 #### Tabs

src/runestone.jl

@@ -3,16 +3,24 @@
 # This is the runestone where all the formatting transformations are implemented.
 
 function trim_trailing_whitespace(ctx::Context, node::Node)
-    kind(node) === K"NewlineWs" || return nothing
+    kind(node) in KSet"NewlineWs Comment" || return nothing
     @assert is_leaf(node)
     str = String(read_bytes(ctx, node))
-    str′ = replace(str, r"\h*(\r\n|\r|\n)" => '\n')
-    # If the next sibling is also a NewlineWs we can trim trailing
-    # whitespace from this node too
-    next_kind = next_sibling_kind(ctx)
-    if next_kind === K"NewlineWs"
-        # str′ = replace(str′, r"(\r\n|\r|\n)\h*" => '\n')
-        str′ = replace(str′, r"\n\h*" => '\n')
+    local str′::String
+    if kind(node) === K"NewlineWs"
+        # Strip all whitespace up until the newline while normalizing line endings to \njK:w
+        str′ = replace(str, r"\h*(\r\n|\r|\n)" => '\n')
+        # If the next sibling is also a NewlineWs we can trim trailing
+        # whitespace from this node too
+        next_kind = next_sibling_kind(ctx)
+        if next_kind === K"NewlineWs"
+            # str′ = replace(str′, r"(\r\n|\r|\n)\h*" => '\n')
+            str′ = replace(str′, r"\n\h*" => '\n')
+        end
+    else
+        @assert kind(node) === K"Comment"
+        # Strip trailing spaces and tabs from comments
+        str′ = rstrip(str, (' ', '\t'))
     end
     if str == str′
         return nothing

test/runtests.jl

@@ -70,6 +70,10 @@ end
     # Trailing whitespace just before closing indent token
     @test format_string("begin\n    a = 1 \nend") == "begin\n    a = 1\nend"
     @test format_string("let\n    a = 1 \nend") == "let\n    a = 1\nend"
+    # Trailing whitespace in comments
+    @test format_string("# comment ") == format_string("# comment  ") ==
+        format_string("# comment\t") == format_string("# comment\t\t") ==
+        format_string("# comment \t ") == format_string("# comment\t \t") == "# comment"
 end
 
 @testset "Hex/oct/bin literal integers" begin