RST writer: better image handling.

- An image alone in its paragraph (but not a figure) is now
  rendered as an independent image, with an `alt` attribute
  if a description is supplied.
- An inline image that is not alone in its paragraph will
  be rendered, as before, using a substitution.
  Such an image cannot have a "center", "left", or
  "right" alignment, so the classes `align-center`,
  `align-left`, or `align-right` are ignored.
  However, `align-top`, `align-middle`, `align-bottom`
  will generate a corresponding `align` attribute.

Closes #6948.

3 files changed, 52 insertions, 20 deletions

diff --git a/src/Text/Pandoc/Writers/RST.hs b/src/Text/Pandoc/Writers/RST.hs
index 43bf382b7..8beeef46a 100644
--- a/src/Text/Pandoc/Writers/RST.hs
+++ b/src/Text/Pandoc/Writers/RST.hs
@@ -143,9 +143,12 @@ pictToRST (label, (attr, src, _, mbtarget)) = do
   let (_, cls, _) = attr
       classes = case cls of
                    []               -> empty
-                   ["align-right"]  -> ":align: right"
-                   ["align-left"]   -> ":align: left"
-                   ["align-center"] -> ":align: center"
+                   ["align-top"]    -> ":align: top"
+                   ["align-middle"] -> ":align: middle"
+                   ["align-bottom"] -> ":align: bottom"
+                   ["align-center"] -> empty
+                   ["align-right"]  -> empty
+                   ["align-left"]   -> empty
                    _                -> ":class: " <> literal (T.unwords cls)
   return $ nowrap
          $ ".. |" <> label' <> "| image:: " <> literal src $$ hang 3 empty (classes $$ dims)
@@ -215,19 +218,28 @@ blockToRST (Div (ident,classes,_kvs) bs) = do
            nest 3 contents $$
            blankline
 blockToRST (Plain inlines) = inlineListToRST inlines
--- title beginning with fig: indicates that the image is a figure
-blockToRST (Para [Image attr txt (src,T.stripPrefix "fig:" -> Just tit)]) = do
-  capt <- inlineListToRST txt
+blockToRST (Para [Image attr txt (src, rawtit)]) = do
+  description <- inlineListToRST txt
   dims <- imageDimsToRST attr
-  let fig = "figure:: " <> literal src
-      alt = ":alt: " <> if T.null tit then capt else literal tit
+  -- title beginning with fig: indicates that the image is a figure
+  let (isfig, tit) = case T.stripPrefix "fig:" rawtit of
+                          Nothing   -> (False, rawtit)
+                          Just tit' -> (True, tit')
+  let fig | isfig = "figure:: " <> literal src
+          | otherwise = "image:: " <> literal src
+      alt | isfig = ":alt: " <> if T.null tit then description else literal tit
+          | null txt = empty
+          | otherwise = ":alt: " <> description
+      capt | isfig = description
+           | otherwise = empty
       (_,cls,_) = attr
       classes = case cls of
                    []               -> empty
                    ["align-right"]  -> ":align: right"
                    ["align-left"]   -> ":align: left"
                    ["align-center"] -> ":align: center"
-                   _                -> ":figclass: " <> literal (T.unwords cls)
+                   _ | isfig        -> ":figclass: " <> literal (T.unwords cls)
+                     | otherwise    -> ":class: " <> literal (T.unwords cls)
   return $ hang 3 ".. " (fig $$ alt $$ classes $$ dims $+$ capt) $$ blankline
 blockToRST (Para inlines)
   | LineBreak `elem` inlines =
diff --git a/test/command/4420.md b/test/command/4420.md
index 3e7008935..36d697234 100644
--- a/test/command/4420.md
+++ b/test/command/4420.md
@@ -1,16 +1,5 @@
 ```
 % pandoc -f native -t rst
-[Image ("",["align-right"],[("width","100px")]) [Str "image"] ("foo.png","")]
-^D
-|image|
-
-.. |image| image:: foo.png
-   :align: right
-   :width: 100px
-```
-
-```
-% pandoc -f native -t rst
 [Para [Image ("",["align-right"],[("width","100px")]) [Str "image"] ("foo.png","fig:test")]]
 ^D
 .. figure:: foo.png
diff --git a/test/command/6948.md b/test/command/6948.md
new file mode 100644
index 000000000..8803aebe9
--- /dev/null
+++ b/test/command/6948.md
@@ -0,0 +1,31 @@
+Treat an image alone in its paragraph (but not a figure)
+as an independent image:
+```
+% pandoc -f native -t rst
+[Para [Image ("",["align-center"],[]) [Str "https://pandoc.org/diagram.jpg"] ("https://pandoc.org/diagram.jpg","")]]
+^D
+.. image:: https://pandoc.org/diagram.jpg
+   :alt: https://pandoc.org/diagram.jpg
+   :align: center
+```
+
+Here we just omit the center attribute as it's not valid:
+```
+% pandoc -f native -t rst
+[Para [Str "hi",Space,Image ("",["align-center"],[]) [Str "https://pandoc.org/diagram.jpg"] ("https://pandoc.org/diagram.jpg","")]]
+^D
+hi |https://pandoc.org/diagram.jpg|
+
+.. |https://pandoc.org/diagram.jpg| image:: https://pandoc.org/diagram.jpg
+```
+
+But we can use top, middle, or bottom alignment:
+```
+% pandoc -f native -t rst
+[Para [Str "hi",Space,Image ("",["align-top"],[]) [Str "https://pandoc.org/diagram.jpg"] ("https://pandoc.org/diagram.jpg","")]]
+^D
+hi |https://pandoc.org/diagram.jpg|
+
+.. |https://pandoc.org/diagram.jpg| image:: https://pandoc.org/diagram.jpg
+   :align: top
+```