tamer: nir::tplshort: Desugar nested template applications

I'm happy with how this ended up turning out---I was able to accomplish this
without having to introduce any additional state to the parser (I _removed_
a state, actually) by tweaking NIR a bit in a previous commit.

We can't update the system test yet, though, because nested templates are
not yet supported by asg::air::tpl; that'll come next.  If you try, you'll
be greeted with this error presently (which is worth showing since you'll
never see it unless you're hacking TAMER):

,=====[ ./tests/xmli/template/ logs ]======
|
| thread 'main' panicked at 'not yet implemented: internal error:
| note: nested tpl open
|    --> ./tests/xmli/template/src.xml:129:5
|     |
| 129 |     <t:inner-short />
|     |     -------------- note: for this template
|
|
| !!! ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ !!!
| !!!        THIS IS AN UNFINISHED FEATURE IN TAMER         !!!
| !!! ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ !!!
| !!! This message means that TAMER has encountered an      !!!
| !!! unrecoverable error that forced it to terminate       !!!
| !!! processing.                                           !!!
| !!!                                                       !!!
| !!! TAMER has attempted to provide you with contextual    !!!
| !!! information above that might allow you to work around !!!
| !!! this problem until it can be fixed.                   !!!
| !!!                                                       !!!
| !!! Please report this error, including the above         !!!
| !!! diagnostic output beginning with 'internal error:'.   !!!
| !!! ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ !!!
| ', src/asg/air/tpl.rs:207:55
| note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
| Command exited with non-zero status 101
| 0/165fault 0/8io 3528rss 14/2ctx
| /home/[...]/tame/tamer/target/debug/tamec -o ./tests/xmli/template/out.xmli --emit xmlo ./tests/xmli/template/src.xml
|
`====[ end ./tests/xmli/template/ logs ]====

DEV-13708

tamer/src/nir/tplshort.rs

@@ -19,6 +19,8 @@
 
 //! Shorthand template application desugaring for NIR.
 //!
+//! Desugaring is performed by [`TplShortDesugar`].
+//!
 //! A shorthand template application looks something like this,
 //!   in XML form:
 //!
@@ -90,21 +92,27 @@ use std::convert::Infallible;
 use Nir::*;
 use NirEntity::*;
 
+/// Desugar shorthand template applications into long-form template
+///   applications.
+///
+/// This parser is able to handle nested applications without having to
+///   track nesting by taking advantage of the parsing that NIR has already
+///   performed.
+///
+/// See the [module-level documentation](super) for more information.
 #[derive(Debug, PartialEq, Eq, Default)]
 pub enum TplShortDesugar {
     /// Waiting for shorthand template application,
     ///   passing tokens along in the meantime.
+    ///
+    /// This state is also used when parsing the body of a shorthand
+    ///   template application.
     #[default]
     Scanning,
 
     /// A shorthand template application associated with the provided
     ///   [`Span`] was encountered and shorthand params are being desugared.
     DesugaringParams(Span),
-
-    /// A child element was encountered while desugaring params,
-    ///   indicating a body of the shorthand application that needs
-    ///   desugaring into `@values@`.
-    DesugaringBody,
 }
 
 impl Display for TplShortDesugar {
@@ -116,9 +124,6 @@ impl Display for TplShortDesugar {
             Self::DesugaringParams(_) => {
                 write!(f, "desugaring shorthand template application params")
             }
-            Self::DesugaringBody => {
-                write!(f, "desugaring shorthand template application body")
-            }
         }
     }
 }
@@ -192,7 +197,6 @@ impl ParseState for TplShortDesugar {
                 //   yet-to-be-defined means.
                 //
                 // note: reversed (stack)
-                stack.push(tok);
                 stack.push(BindIdent(SPair(gen_name, ospan)));
                 stack.push(Open(Tpl, ospan));
 
@@ -203,13 +207,23 @@ impl ParseState for TplShortDesugar {
                 stack.push(Close(TplParam, ospan));
                 stack.push(Text(SPair(gen_name, ospan)));
                 stack.push(BindIdent(SPair(L_TPLP_VALUES, ospan)));
-                Transition(DesugaringBody).ok(Open(TplParam, ospan))
+
+                // Note that we must have `tok` as lookahead instead of
+                //   pushing directly on the stack in case it's a
+                //   `TplApplyShort`.
+                Transition(Scanning)
+                    .ok(Open(TplParam, ospan))
+                    .with_lookahead(tok)
             }
 
-            (DesugaringBody, Close(TplApplyShort(_), span)) => {
+            // If we're scanning and find a closing shorthand application,
+            //   then we must have just finished with a shorthand body.
+            (Scanning, Close(TplApplyShort(_), span)) => {
                 Transition(Scanning).ok(Close(Tpl, span))
             }
 
+            // If we complete the shorthand template during param parsing,
+            //   then we have no body and must close the application.
             (DesugaringParams(_), Close(TplApplyShort(_), span)) => {
                 Transition(Scanning).ok(Close(TplApply, span))
             }

tamer/src/nir/tplshort/test.rs

@@ -162,6 +162,54 @@ fn desugars_body_into_tpl_with_ref_in_values_param() {
     );
 }
 
+// Shorthand within a shorthand.
+#[test]
+fn desugar_nested_apply() {
+    let qname_outer = ("t", "outer").unwrap_into();
+    let name_outer = SPair("_outer_".into(), S1);
+
+    let qname_inner = ("t", "inner").unwrap_into();
+    let name_inner = SPair("_inner_".into(), S2);
+
+    #[rustfmt::skip]
+    let toks = vec![
+        Open(TplApplyShort(qname_outer), S1),
+          // Body is a second shorthand template application.
+          Open(TplApplyShort(qname_inner), S2),
+          Close(TplApplyShort(qname_inner), S3),
+        Close(TplApplyShort(qname_outer), S4),
+    ];
+
+    let gen_name_outer = gen_tpl_name_at_offset(S1);
+
+    #[rustfmt::skip]
+    assert_eq!(
+        Ok(vec![
+            O(Open(TplApply, S1)),
+              O(Ref(name_outer)),
+
+              // @values@
+              O(Open(TplParam, S1)),
+                O(BindIdent(SPair(L_TPLP_VALUES, S1))),
+                O(Text(SPair(gen_name_outer, S1))),    //:-.
+              O(Close(TplParam, S1)),                  //   |
+            O(Close(TplApply, S1)),                    //   |
+                                                       //   |
+            O(Open(Tpl, S1)),                          //  /
+              O(BindIdent(SPair(gen_name_outer, S1))), //<`
+
+              // And within this template,
+              //   we generate another application.
+              // This one has no body and so no `@values@`.
+              O(Open(TplApply, S2)),
+                O(Ref(name_inner)),
+              O(Close(TplApply, S3)),
+            O(Close(Tpl, S4)),
+        ]),
+        Sut::parse(toks.into_iter()).collect(),
+    );
+}
+
 // Don't parse what we desugar into!
 #[test]
 fn does_not_desugar_long_form() {