Cause a compilation error to occur if using @extern with is_dll_import in a comptime scope.

Add a note about thread local / dll import being the cause.

src/Sema.zig

@@ -876,6 +876,7 @@ const InferredAlloc = struct {
 
 const NeededComptimeReason = struct {
     needed_comptime_reason: []const u8,
+    value_comptime_reason: ?[]const u8 = null,
     block_comptime_reason: ?*const Block.ComptimeReason = null,
 };
 
@@ -2251,7 +2252,7 @@ fn resolveValueAllowVariables(sema: *Sema, inst: Air.Inst.Ref) CompileError!?Val
         }
     };
     const val = Value.fromInterned(ip_index);
-    if (val.isPtrToThreadLocal(pt.zcu)) return null;
+    if (val.isPtrRuntimeValue(pt.zcu)) return null;
     return val;
 }
 
@@ -2277,8 +2278,14 @@ pub fn resolveFinalDeclValue(
     const zcu = sema.pt.zcu;
 
     const val = try sema.resolveValueAllowVariables(air_ref) orelse {
+        const value_comptime_reason: ?[]const u8 = if (air_ref.toInterned()) |_|
+            "thread local and dll imported variables have runtime-known addresses"
+        else
+            null;
+
         return sema.failWithNeededComptime(block, src, .{
             .needed_comptime_reason = "global variable initializer must be comptime-known",
+            .value_comptime_reason = value_comptime_reason,
         });
     };
     if (val.isGenericPoison()) return error.GenericPoison;
@@ -2296,6 +2303,9 @@ fn failWithNeededComptime(sema: *Sema, block: *Block, src: LazySrcLoc, reason: N
         const msg = try sema.errMsg(src, "unable to resolve comptime value", .{});
         errdefer msg.destroy(sema.gpa);
         try sema.errNote(src, msg, "{s}", .{reason.needed_comptime_reason});
+        if (reason.value_comptime_reason) |value_comptime_reason| {
+            try sema.errNote(src, msg, "{s}", .{value_comptime_reason});
+        }
 
         if (reason.block_comptime_reason) |block_comptime_reason| {
             try block_comptime_reason.explain(sema, msg);

src/Value.zig

@@ -1340,11 +1340,11 @@ pub fn isLazySize(val: Value, zcu: *Zcu) bool {
     };
 }
 
-pub fn isPtrToThreadLocal(val: Value, zcu: *Zcu) bool {
+pub fn isPtrRuntimeValue(val: Value, zcu: *Zcu) bool {
     const ip = &zcu.intern_pool;
     const nav = ip.getBackingNav(val.toIntern()).unwrap() orelse return false;
     return switch (ip.indexToKey(ip.getNav(nav).status.resolved.val)) {
-        .@"extern" => |e| e.is_threadlocal,
+        .@"extern" => |e| e.is_threadlocal or e.is_dll_import,
         .variable => |v| v.is_threadlocal,
         else => false,
     };

src/codegen/llvm.zig

@@ -3237,10 +3237,10 @@ pub const Object = struct {
         const ip = &zcu.intern_pool;
         const nav = ip.getNav(nav_index);
         const resolved = nav.status.resolved;
-        const is_extern, const is_threadlocal, const is_weak_linkage = switch (ip.indexToKey(resolved.val)) {
-            .variable => |variable| .{ false, variable.is_threadlocal, variable.is_weak_linkage },
-            .@"extern" => |@"extern"| .{ true, @"extern".is_threadlocal, @"extern".is_weak_linkage },
-            else => .{ false, false, false },
+        const is_extern, const is_threadlocal, const is_weak_linkage, const is_dll_import = switch (ip.indexToKey(resolved.val)) {
+            .variable => |variable| .{ false, variable.is_threadlocal, variable.is_weak_linkage, false },
+            .@"extern" => |@"extern"| .{ true, @"extern".is_threadlocal, @"extern".is_weak_linkage, @"extern".is_dll_import },
+            else => .{ false, false, false, false },
         };
 
         const variable_index = try o.builder.addVariable(
@@ -3257,6 +3257,7 @@ pub const Object = struct {
             if (is_threadlocal and !zcu.navFileScope(nav_index).mod.single_threaded)
                 variable_index.setThreadLocal(.generaldynamic, &o.builder);
             if (is_weak_linkage) variable_index.setLinkage(.extern_weak, &o.builder);
+            if (is_dll_import) variable_index.setDllStorageClass(.dllimport, &o.builder);
         } else {
             variable_index.setLinkage(.internal, &o.builder);
             variable_index.setUnnamedAddr(.unnamed_addr, &o.builder);

src/codegen/llvm/Builder.zig

@@ -2528,6 +2528,10 @@ pub const Variable = struct {
             return self.ptrConst(builder).global.setLinkage(linkage, builder);
         }
 
+        pub fn setDllStorageClass(self: Index, class: DllStorageClass, builder: *Builder) void {
+            return self.ptrConst(builder).global.setDllStorageClass(class, builder);
+        }
+
         pub fn setUnnamedAddr(self: Index, unnamed_addr: UnnamedAddr, builder: *Builder) void {
             return self.ptrConst(builder).global.setUnnamedAddr(unnamed_addr, builder);
         }