TAMER: Virtual symbol override

7 files changed, 165 insertions, 10 deletions

diff --git a/.rev-xmle b/.rev-xmle
index 2253abf..0a9afa0 100644
--- a/.rev-xmle
+++ b/.rev-xmle
@@ -1,4 +1,4 @@
 # This number is incremented for every linker change to force rebuilding
 # of xmle files.
-2
+3
 
diff --git a/src/current/include/preproc/symtable.xsl b/src/current/include/preproc/symtable.xsl
index 9d21906..700f930 100644
--- a/src/current/include/preproc/symtable.xsl
+++ b/src/current/include/preproc/symtable.xsl
@@ -514,10 +514,18 @@
           <!-- overridden; we're obsolete :( -->
         </when>
 
-        <!-- if we've gotten this far, then the override is good; clear it -->
+        <!-- if we've gotten this far, then the override is good; clear it
+             so as not to trigger override errors  -->
         <when test="@override='true'">
           <copy>
-            <sequence select="@*[ not( name()='override' ) ], *" />
+            <sequence select="@*[ not( name()='override' ) ]" />
+
+            <!-- mark this has having been overridden for the linker (see
+                 TAMER; we'll hopefully be getting rid of overrides in the
+                 future) -->
+            <attribute name="isoverride" select="'true'" />
+
+            <sequence select="*" />
           </copy>
         </when>
 
diff --git a/tamer/src/ir/asg/base.rs b/tamer/src/ir/asg/base.rs
index f1868c3..ed490f7 100644
--- a/tamer/src/ir/asg/base.rs
+++ b/tamer/src/ir/asg/base.rs
@@ -141,13 +141,29 @@ where
     ) -> AsgResult<ObjectRef<Ix>> {
         // TODO: src check
         if let Some(existing) = self.lookup(name) {
-            match self.graph.node_weight_mut(existing.0) {
-                Some(node @ Some(Object::Missing(_))) => {
+            let node = self.graph.node_weight_mut(existing.0).unwrap();
+
+            match node {
+                Some(Object::Missing(_)) => {
                     node.replace(Object::Ident(name, kind, src));
                     return Ok(existing);
                 }
-                Some(_) => return Ok(existing),
-                _ => (),
+                // TODO: no override-override
+                Some(Object::Ident(_, _, orig_src))
+                    if orig_src.virtual_ && src.override_ =>
+                {
+                    *orig_src = src;
+                    return Ok(existing);
+                }
+                // TODO: no override-override
+                Some(Object::IdentFragment(_, _, orig_src, _))
+                    if orig_src.virtual_ && src.override_ =>
+                {
+                    // clears fragment, which is no longer applicable
+                    node.replace(Object::Ident(name, kind, src));
+                    return Ok(existing);
+                }
+                _ => return Ok(existing),
             }
         }
 
@@ -414,6 +430,81 @@ mod test {
         Ok(())
     }
 
+    // TODO: incompatible
+    #[test]
+    fn declare_override_virtual_ident() -> AsgResult<()> {
+        let mut sut = Sut::with_capacity(0, 0);
+
+        let sym = Symbol::new_dummy(SymbolIndex::from_u32(1), "virtual");
+        let over_src = Symbol::new_dummy(SymbolIndex::from_u32(2), "src");
+
+        let virt_node = sut.declare(
+            &sym,
+            IdentKind::Meta,
+            Source {
+                virtual_: true,
+                ..Default::default()
+            },
+        )?;
+
+        let over_src = Source {
+            override_: true,
+            src: Some(&over_src),
+            ..Default::default()
+        };
+
+        let over_node = sut.declare(&sym, IdentKind::Meta, over_src.clone())?;
+
+        assert_eq!(virt_node, over_node);
+
+        assert_eq!(
+            sut.get(over_node),
+            Some(&Object::Ident(&sym, IdentKind::Meta, over_src,))
+        );
+
+        Ok(())
+    }
+
+    // TODO: incompatible
+    #[test]
+    fn declare_override_virtual_ident_fragment() -> AsgResult<()> {
+        let mut sut = Sut::with_capacity(0, 0);
+
+        let sym = Symbol::new_dummy(SymbolIndex::from_u32(1), "virtual");
+        let over_src = Symbol::new_dummy(SymbolIndex::from_u32(2), "src");
+
+        let virt_node = sut.declare(
+            &sym,
+            IdentKind::Meta,
+            Source {
+                virtual_: true,
+                ..Default::default()
+            },
+        )?;
+
+        sut.set_fragment(virt_node, FragmentText::from("remove me"))?;
+
+        let over_src = Source {
+            override_: true,
+            src: Some(&over_src),
+            ..Default::default()
+        };
+
+        let over_node = sut.declare(&sym, IdentKind::Meta, over_src.clone())?;
+
+        assert_eq!(virt_node, over_node);
+
+        // The act of overriding the node should have cleared any existing
+        // fragment, making way for a new fragment to take its place as soon
+        // as it is discovered.  (So, back to an Object::Ident.)
+        assert_eq!(
+            sut.get(over_node),
+            Some(&Object::Ident(&sym, IdentKind::Meta, over_src,))
+        );
+
+        Ok(())
+    }
+
     #[test]
     fn add_fragment_to_ident() -> AsgResult<()> {
         let mut sut = Sut::with_capacity(0, 0);
diff --git a/tamer/src/ir/asg/graph.rs b/tamer/src/ir/asg/graph.rs
index 7be7615..e2135e4 100644
--- a/tamer/src/ir/asg/graph.rs
+++ b/tamer/src/ir/asg/graph.rs
@@ -61,6 +61,12 @@ pub trait Asg<'i, Ix: IndexType> {
     ///       [`Object::Extern`] into a [`Object::Ident`].
     /// When this happens,
     ///   the extern is said to be _resolved_.
+    ///
+    /// If a virtual identifier of type [`Object::IdentFragment`] is
+    ///   overridden,
+    ///     then its fragment is cleared
+    ///     (it returns to a [`Object::Ident`])
+    ///     to make way for the fragment of the override.
     fn declare(
         &mut self,
         name: &'i Symbol<'i>,
diff --git a/tamer/src/ir/asg/object.rs b/tamer/src/ir/asg/object.rs
index 1921111..282834a 100644
--- a/tamer/src/ir/asg/object.rs
+++ b/tamer/src/ir/asg/object.rs
@@ -33,9 +33,9 @@ use crate::sym::Symbol;
 ///      /         \            \
 ///     /           v            v
 /// ((Empty)) -> (Extern) -> ((Ident)) -> ((IdentFragment)).
-///     \                        ^
-///      \                      /
-///       `--------------------`
+///     \                        ^               /
+///      \                      / \             /
+///       `--------------------`   `-----------'
 /// ```
 ///
 /// The [`Empty`][Object::Empty] state is never directly accessable
@@ -149,6 +149,22 @@ pub struct Source<'i> {
     /// TODO: We have `parent`, `yields`, and `from`.
     ///   We should begin to consolodate.
     pub from: Option<Vec<&'i Symbol<'i>>>,
+
+    /// Whether identifier is virtual (can be overridden).
+    ///
+    /// This feature adds complexity and will ideally be removed in the
+    ///   future.
+    ///
+    /// See also [`override`][Source::override_].
+    pub virtual_: bool,
+
+    /// Whether identifier overrides a virtual identifier.
+    ///
+    /// This feature adds complexity and will ideally be removed in the
+    ///   future.
+    ///
+    /// See also [`virtual_`][Source::virtual_].
+    pub override_: bool,
 }
 
 impl<'i> From<SymAttrs<'i>> for Source<'i> {
@@ -164,6 +180,8 @@ impl<'i> From<SymAttrs<'i>> for Source<'i> {
             yields: attrs.yields,
             desc: attrs.desc,
             from: attrs.from,
+            virtual_: attrs.virtual_,
+            override_: attrs.override_,
         }
     }
 }
@@ -189,6 +207,8 @@ mod test {
             yields: Some(&ysym),
             desc: Some("sym desc".to_string()),
             from: Some(vec![&fsym]),
+            virtual_: true,
+            override_: true,
             ..Default::default()
         };
 
@@ -201,6 +221,8 @@ mod test {
                 yields: attrs.yields,
                 desc: Some("sym desc".to_string()),
                 from: Some(vec![&fsym]),
+                virtual_: true,
+                override_: true,
             },
             attrs.into(),
         );
diff --git a/tamer/src/ir/legacyir.rs b/tamer/src/ir/legacyir.rs
index be318b2..bdf33a8 100644
--- a/tamer/src/ir/legacyir.rs
+++ b/tamer/src/ir/legacyir.rs
@@ -158,6 +158,16 @@ pub struct SymAttrs<'i> {
     ///  - [`SymType::Func`] lists params in order (so that the compiler
     ///    knows application order).
     pub from: Option<Vec<&'i Symbol<'i>>>,
+
+    /// Whether symbol can be overridden.
+    ///
+    /// See also [`override`][SymAttrs::override_].
+    pub virtual_: bool,
+
+    /// Whether symbol is an override of a virtual symbol.
+    ///
+    /// See also [`virtual`][SymAttrs::virtual_].
+    pub override_: bool,
 }
 
 /// Legacy symbol types.
diff --git a/tamer/src/obj/xmlo/reader.rs b/tamer/src/obj/xmlo/reader.rs
index 8264b14..dfc8d34 100644
--- a/tamer/src/obj/xmlo/reader.rs
+++ b/tamer/src/obj/xmlo/reader.rs
@@ -469,6 +469,14 @@ impl<'i, B: BufRead, I: Interner<'i>> XmloReader<'i, B, I> {
                     });
                 }
 
+                b"virtual" => {
+                    sym_attrs.virtual_ = &*attr.value == b"true";
+                }
+
+                b"isoverride" => {
+                    sym_attrs.override_ = &*attr.value == b"true";
+                }
+
                 // As this reader evolves, we may wish to provide an error
                 // for unknown attributes so that we can be sure that we've
                 // handled them all.
@@ -1751,6 +1759,16 @@ mod test {
             ..Default::default()
         }
 
+        r#virtual: [virtual="true"] => SymAttrs {
+            virtual_: true,
+            ..Default::default()
+        }
+
+        r#override: [isoverride="true"] => SymAttrs {
+            override_: true,
+            ..Default::default()
+        }
+
         // Multiple attributes at once
         multi: [src="foo", type="class", dim="1", dtype="float", extern="true"]
             => SymAttrs {