feat: builtins ADR and REF now have a CONSTANT return specifier (#1326)

* feat: builtins ADR and REF now have a CONSTANT return specifier so they can be used in initializers

Allows parsing of the `CONSTANT` pragma when parsing POUs, however for now only the builtins are actually allowed to make use of this.
This enables the const-evaluator to check for constant builtins when trying to evaluate call-statements in variable initializers.

Author: mhasel

compiler/plc_ast/src/ast.rs

@@ -42,6 +42,7 @@ pub struct Pou {
     pub generics: Vec<GenericBinding>,
     pub linkage: LinkageType,
     pub super_class: Option<String>,
+    pub is_const: bool,
 }
 
 #[derive(Debug, PartialEq, Eq)]

compiler/plc_diagnostics/src/diagnostics.rs

@@ -265,6 +265,12 @@ impl Diagnostic {
             location,
         )
     }
+
+    pub fn const_pragma_is_not_allowed(location: SourceLocation) -> Diagnostic {
+        Diagnostic::new("Pragma {constant} is not allowed in POU declarations")
+            .with_location(location)
+            .with_error_code("E105")
+    }
 }
 
 // CFC related diagnostics

compiler/plc_diagnostics/src/diagnostics/diagnostics_registry.rs

@@ -206,6 +206,7 @@ lazy_static! {
         E102,   Error,      include_str!("./error_codes/E102.md"),  // Template variable without hardware binding
         E103,   Error,      include_str!("./error_codes/E103.md"),  // Immutable Hardware Binding
         E104,   Error,      include_str!("./error_codes/E104.md"),  // Config Variable With Incomplete Address
+        E105,   Error,      include_str!("./error_codes/E105.md"),  // CONSTANT keyword in POU
     );
 }
 

compiler/plc_diagnostics/src/diagnostics/error_codes/E105.md

@@ -0,0 +1,12 @@
+# CONSTANT keyword in POU
+
+The `CONSTANT` keyword is not allowed for POU declarations, only variables can be `CONSTANT`
+
+Erroneous code example:
+```
+FUNCTION FOO : BOOL CONSTANT 
+VAR_INPUT
+END_VAR
+    // ...
+END_FUNCTION
+```

src/builtins.rs

@@ -35,7 +35,9 @@ lazy_static! {
         (
             "ADR",
             BuiltIn {
-                decl: "FUNCTION ADR<U: ANY> : LWORD
+                decl: "
+                {constant}
+                FUNCTION ADR<U: ANY> : LWORD
                 VAR_INPUT
                     in : U;
                 END_VAR
@@ -63,7 +65,9 @@ lazy_static! {
         (
             "REF",
             BuiltIn {
-                decl: "FUNCTION REF<U: ANY> : REF_TO U
+                decl: "
+                {constant}
+                FUNCTION REF<U: ANY> : REF_TO U
                 VAR_INPUT
                     in : U;
                 END_VAR

src/index.rs

@@ -477,6 +477,7 @@ pub enum PouIndexEntry {
         is_variadic: bool,
         location: SourceLocation,
         is_generated: bool, // true if this entry was added automatically (e.g. by generics)
+        is_const: bool,
     },
     Class {
         name: String,
@@ -549,6 +550,7 @@ impl PouIndexEntry {
         linkage: LinkageType,
         is_variadic: bool,
         location: SourceLocation,
+        is_const: bool,
     ) -> PouIndexEntry {
         PouIndexEntry::Function {
             name: name.into(),
@@ -558,6 +560,7 @@ impl PouIndexEntry {
             is_variadic,
             location,
             is_generated: false,
+            is_const,
         }
     }
 
@@ -570,6 +573,7 @@ impl PouIndexEntry {
         linkage: LinkageType,
         is_variadic: bool,
         location: SourceLocation,
+        is_const: bool,
     ) -> PouIndexEntry {
         PouIndexEntry::Function {
             name: name.into(),
@@ -579,6 +583,7 @@ impl PouIndexEntry {
             is_variadic,
             location,
             is_generated: true,
+            is_const,
         }
     }
 
@@ -772,6 +777,10 @@ impl PouIndexEntry {
         matches!(self, PouIndexEntry::Method { .. })
     }
 
+    pub(crate) fn is_constant(&self) -> bool {
+        matches!(self, PouIndexEntry::Function { is_const: true, .. })
+    }
+
     pub fn get_location(&self) -> &SourceLocation {
         match self {
             PouIndexEntry::Program { location, .. }

src/index/tests/builtin_tests.rs

@@ -7,7 +7,6 @@ fn builtin_functions_added_to_index() {
     let provider = IdProvider::default();
     let builtins = builtins::parse_built_ins(provider);
     let index = crate::index::visitor::visit(&builtins);
-
     assert!(index.find_member("ADR", "in").is_some());
     assert!(index.find_member("REF", "in").is_some());
     assert!(index.find_member("MUX", "K").is_some());

src/index/tests/index_tests.rs

@@ -1379,6 +1379,7 @@ fn a_program_pou_is_indexed() {
             is_variadic: false,
             location: source_location_factory.create_range(65..75),
             is_generated: false,
+            is_const: false,
         }),
         index.find_pou("myFunction"),
     );

src/index/visitor.rs

@@ -185,6 +185,7 @@ pub fn visit_pou(index: &mut Index, pou: &Pou) {
                 pou.linkage,
                 has_varargs,
                 pou.name_location.clone(),
+                pou.is_const,
             ));
             index.register_pou_type(datatype);
         }

src/lexer/tokens.rs

@@ -19,6 +19,10 @@ pub enum Token {
     #[token("{ref}")]
     PropertyByRef,
 
+    #[token("@CONSTANT")]
+    #[token("{constant}")]
+    PropertyConstant,
+
     #[token("{sized}")]
     PropertySized,