Support multi-line `if-then-else`

[jdunkerley]

Currently we can do:

if a==b then action else
    anotheraction

or

if a==b then
    action

It would be good to allow:

if a==b then
    action
else
    anotheraction

(we currently do this by using case a=b of to achieve similar results).

[jdunkerley]

Will need some help from @kazcw

[JaroslavTulach]

To debug the tests one has to:

Change to debug = 2 in [profile.test] section of the root Cargo.toml to see the local variables.

The handling of if_then and also _else is happening in fn if_body(segments: NonEmptyVec<MatchedSegment>) in built_in.rs. Simple one line if True then 0 else 1 sends in three segments. However trying to:

enso$ git diff lib/rust/parser/debug/tests/parse.rs
diff --git lib/rust/parser/debug/tests/parse.rs lib/rust/parser/debug/tests/parse.rs
index 4a22c36cc5..e6470a4eae 100644
--- lib/rust/parser/debug/tests/parse.rs
+++ lib/rust/parser/debug/tests/parse.rs
@@ -102,6 +102,23 @@ fn else_block() {
                           ((Ident else) (BodyBlock #((Ident False))))))]);
 }
 
+#[test]
+fn true_else_blocks() {
+    #[rustfmt::skip]
+    let lines = vec![
+        "if True then",
+        "    0",
+        "else",
+        "    1",
+    ];
+    let text = &lines.join("\n");
+    let expected = block![
+        (MultiSegmentApp #(((Ident if) (Ident True))
+                           ((Ident then) (BodyBlock #((Number () "0" ()))))
+                           ((Ident else) (BodyBlock #((Number () "0" ()))))))];
+    test(text, expected);
+}
+
 
 // === Comments ===
 

triggers just the if_then macro (and sends just two segments) as macro processing is terminated by resolvers.rs finish_current_line which has no idea there is going to be else somewhere below. else is then treated as a separate identifier invoked with the 1 block:

(BodyBlock #((MultiSegmentApp #(((Ident if) (Ident True)) ((Ident then)
(BodyBlock #((Number () \"0\" ()))))))
(ArgumentBlockApplication (Ident else) #((Number () \"1\" ())))))

I got an idea to disable the if_then macro by

enso$ git diff lib/rust/parser/src/macros/built_in.rs
diff --git lib/rust/parser/src/macros/built_in.rs lib/rust/parser/src/macros/built_in.rs
index 1f789687af..b6eecb4c8c 100644
--- lib/rust/parser/src/macros/built_in.rs
+++ lib/rust/parser/src/macros/built_in.rs
@@ -19,7 +19,7 @@ pub fn all() -> resolver::MacroMap {
 /// Built-in macro definitions that match anywhere in an expression.
 fn expression() -> resolver::SegmentMap<'static> {
     let mut macro_map = resolver::SegmentMap::default();
-    macro_map.register(if_then());
+//    macro_map.register(if_then());
     macro_map.register(if_then_else());
     macro_map.register(group());
     macro_map.register(lambda());

that prevents the first line to be recognized as if_then macro, but of course, it doesn't recognize multi line if_then_else either.