Enforce rule that `export`/`extern` (non cpp) must be `const` (#7113)

* Enforce rule that `export`/`extern` (non cpp) must be `const`

fixes: #5570

Problem:
1. we allow non-const-link-time-var to be linked to a const-link-time-var.
2. problem is that: module use site has const var, so, we emit OpStore %Ptr %Const in IR, this is expected, this is good. We fail because we in reality have a OpStore %Ptr %Var (fails since we need a OpLoad in-between) in IR since the module with our link-time-variable-value is a regular variable.
3. We loose the float_litteral talked about inside the github issue since, we technically don't use our variable "VAL" (we never OpLoad from it), so spirv-opt removes the float_litteral, this is a byproduct of the actual issue.

Solution:
* `export`/`extern` variables must always be `const`. This excludes `__extern_cpp` since `cpp` does not exhibit this issue and works differently.

* format code

* changel logic and tests to only ensure `static const` with `export`/`extern`

* changing the rules: only reqirement is that if we have const we must have static

* remove a spirrious change made

* fix merge

---------

Co-authored-by: slangbot <186143334+slangbot@users.noreply.github.com>
Co-authored-by: Yong He <yonghe@outlook.com>

2 files changed, 31 insertions, 1 deletions

diff --git a/source/slang/slang-check-decl.cpp b/source/slang/slang-check-decl.cpp
index 79482ea6e..daa761d3d 100644
--- a/source/slang/slang-check-decl.cpp
+++ b/source/slang/slang-check-decl.cpp
@@ -42,6 +42,31 @@ struct SemanticsDeclModifiersVisitor : public SemanticsDeclVisitorBase,
 
     void visitDeclGroup(DeclGroup*) {}
 
+    void visitVarDecl(VarDecl* decl)
+    {
+        visitDecl(decl);
+
+        // Export'd/Extern'd variables must be `const`, otherwise we may have a mismatch
+        // causing errors.
+        bool hasConst = false;
+        bool hasExportOrExtern = false;
+        bool hasStatic = false;
+        for (auto m : decl->modifiers)
+        {
+            if (as<ExternModifier>(m) || as<HLSLExportModifier>(m))
+                hasExportOrExtern = true;
+            else if (as<ConstModifier>(m))
+                hasConst = true;
+            else if (as<HLSLStaticModifier>(m))
+                hasStatic = true;
+        }
+        if (hasExportOrExtern && hasConst != hasStatic)
+            getSink()->diagnose(
+                decl,
+                Diagnostics::ExternAndExportVarDeclMustBeConst,
+                decl->getName());
+    }
+
     void visitDecl(Decl* decl) { checkModifiers(decl); }
 
     void visitStructDecl(StructDecl* structDecl);
diff --git a/source/slang/slang-diagnostic-defs.h b/source/slang/slang-diagnostic-defs.h
index 6bedcfe3e..cafd61062 100644
--- a/source/slang/slang-diagnostic-defs.h
+++ b/source/slang/slang-diagnostic-defs.h
@@ -1366,13 +1366,18 @@ DIAGNOSTIC(
     Error,
     variableCannotBePushAndSpecializationConstant,
     "'$0' cannot be a push constant and a specialization constant at the same time")
-
 DIAGNOSTIC(31221, Error, invalidHLSLRegisterName, "invalid HLSL register name '$0'.")
 DIAGNOSTIC(
     31222,
     Error,
     invalidHLSLRegisterNameForType,
     "invalid HLSL register name '$0' for type '$1'.")
+DIAGNOSTIC(
+    31223,
+    Error,
+    ExternAndExportVarDeclMustBeConst,
+    "extern and export variables must be static const: '$0'")
+
 // Enums
 
 DIAGNOSTIC(32000, Error, invalidEnumTagType, "invalid tag type for 'enum': '$0'")