frontend: fix reference tracking through coerced function bodies

This bug was manifesting for user as a nasty link error because they
were calling their application's main entry point as a coerced function,
which essentially broke reference tracking for the entire ZCU, causing
exported symbols to silently not get exported.

I've been a little unsure about how coerced functions should interact
with the unit graph before, but the solution is actually really obvious
now: they shouldn't! `Sema` is now responsible for unwrapping
possibly-coerced functions *before* queuing analysis or marking unit
references. This makes the reference graph optimal (there are no
redundant edges representing coerced versions of the same function) and
simplifies logic elsewhere at the expense of just a few lines in Sema.

src/Sema.zig

@@ -4376,8 +4376,9 @@ fn zirResolveInferredAlloc(sema: *Sema, block: *Block, inst: Zir.Inst.Index) Com
             if (zcu.intern_pool.isFuncBody(val)) {
                 const ty: Type = .fromInterned(zcu.intern_pool.typeOf(val));
                 if (try ty.fnHasRuntimeBitsSema(pt)) {
-                    try sema.addReferenceEntry(block, src, AnalUnit.wrap(.{ .func = val }));
+                    const orig_fn_index = zcu.intern_pool.unwrapCoercedFunc(val);
-                    try zcu.ensureFuncBodyAnalysisQueued(val);
+                    try sema.addReferenceEntry(block, src, .wrap(.{ .func = orig_fn_index }));
+                    try zcu.ensureFuncBodyAnalysisQueued(orig_fn_index);
                 }
             }
 
@@ -5589,16 +5590,21 @@ fn zirPanic(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!void
     }
 
     try sema.ensureMemoizedStateResolved(src, .panic);
-    try zcu.ensureFuncBodyAnalysisQueued(zcu.builtin_decl_values.get(.@"panic.call"));
+    const panic_fn_index = zcu.builtin_decl_values.get(.@"panic.call");
-
-    const panic_fn = Air.internedToRef(zcu.builtin_decl_values.get(.@"panic.call"));
-
     const opt_usize_ty = try pt.optionalType(.usize_type);
     const null_ret_addr = Air.internedToRef((try pt.intern(.{ .opt = .{
         .ty = opt_usize_ty.toIntern(),
         .val = .none,
     } })));
-    try sema.callBuiltin(block, src, panic_fn, .auto, &.{ coerced_msg, null_ret_addr }, .@"@panic");
+    // `callBuiltin` also calls `addReferenceEntry` to the function body for us.
+    try sema.callBuiltin(
+        block,
+        src,
+        .fromIntern(panic_fn_index),
+        .auto,
+        &.{ coerced_msg, null_ret_addr },
+        .@"@panic",
+    );
 }
 
 fn zirTrap(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!void {
@@ -7567,8 +7573,9 @@ fn analyzeCall(
         ref_func: {
             const runtime_func_val = try sema.resolveValue(runtime_func) orelse break :ref_func;
             if (!ip.isFuncBody(runtime_func_val.toIntern())) break :ref_func;
-            try sema.addReferenceEntry(block, call_src, .wrap(.{ .func = runtime_func_val.toIntern() }));
+            const orig_fn_index = ip.unwrapCoercedFunc(runtime_func_val.toIntern());
-            try zcu.ensureFuncBodyAnalysisQueued(runtime_func_val.toIntern());
+            try sema.addReferenceEntry(block, call_src, .wrap(.{ .func = orig_fn_index }));
+            try zcu.ensureFuncBodyAnalysisQueued(orig_fn_index);
         }
 
         const call_tag: Air.Inst.Tag = switch (modifier) {
@@ -26383,23 +26390,27 @@ fn explainWhyTypeIsNotPacked(
 /// instructions. This function ensures the panic function will be available to
 /// be called during that time.
 fn preparePanicId(sema: *Sema, src: LazySrcLoc, panic_id: Zcu.SimplePanicId) !void {
+    const zcu = sema.pt.zcu;
+
     // If the backend doesn't support `.panic_fn`, it doesn't want us to lower the panic handlers.
     // The backend will transform panics into traps instead.
-    if (sema.pt.zcu.backendSupportsFeature(.panic_fn)) {
+    if (!zcu.backendSupportsFeature(.panic_fn)) return;
-        _ = try sema.getPanicIdFunc(src, panic_id);
+
-    }
+    const fn_index = try sema.getPanicIdFunc(src, panic_id);
+    const orig_fn_index = zcu.intern_pool.unwrapCoercedFunc(fn_index);
+    try sema.addReferenceEntry(null, src, .wrap(.{ .func = orig_fn_index }));
+    try zcu.ensureFuncBodyAnalysisQueued(orig_fn_index);
 }
 
 fn getPanicIdFunc(sema: *Sema, src: LazySrcLoc, panic_id: Zcu.SimplePanicId) !InternPool.Index {
     const zcu = sema.pt.zcu;
     try sema.ensureMemoizedStateResolved(src, .panic);
-    const panic_func = zcu.builtin_decl_values.get(panic_id.toBuiltin());
+    const panic_fn_index = zcu.builtin_decl_values.get(panic_id.toBuiltin());
-    try zcu.ensureFuncBodyAnalysisQueued(panic_func);
     switch (sema.owner.unwrap()) {
         .@"comptime", .nav_ty, .nav_val, .type, .memoized_state => {},
         .func => |owner_func| zcu.intern_pool.funcSetHasErrorTrace(owner_func, true),
     }
-    return panic_func;
+    return panic_fn_index;
 }
 
 fn addSafetyCheck(
@@ -31143,6 +31154,11 @@ fn addReferenceEntry(
     referenced_unit: AnalUnit,
 ) !void {
     const zcu = sema.pt.zcu;
+    const ip = &zcu.intern_pool;
+    switch (referenced_unit.unwrap()) {
+        .func => |f| assert(ip.unwrapCoercedFunc(f) == f), // for `.{ .func = f }`, `f` must be uncoerced
+        else => {},
+    }
     if (!zcu.comp.incremental and zcu.comp.reference_trace == 0) return;
     const gop = try sema.references.getOrPut(sema.gpa, referenced_unit);
     if (gop.found_existing) return;
@@ -31329,8 +31345,9 @@ fn maybeQueueFuncBodyAnalysis(sema: *Sema, block: *Block, src: LazySrcLoc, nav_i
     const nav_val = zcu.navValue(nav_index);
     if (!ip.isFuncBody(nav_val.toIntern())) return;
 
-    try sema.addReferenceEntry(block, src, AnalUnit.wrap(.{ .func = nav_val.toIntern() }));
+    const orig_fn_index = ip.unwrapCoercedFunc(nav_val.toIntern());
-    try zcu.ensureFuncBodyAnalysisQueued(nav_val.toIntern());
+    try sema.addReferenceEntry(block, src, .wrap(.{ .func = orig_fn_index }));
+    try zcu.ensureFuncBodyAnalysisQueued(orig_fn_index);
 }
 
 fn analyzeRef(
@@ -34963,8 +34980,9 @@ fn resolveInferredErrorSet(
         }
         // In this case we are dealing with the actual InferredErrorSet object that
         // corresponds to the function, not one created to track an inline/comptime call.
-        try sema.addReferenceEntry(block, src, AnalUnit.wrap(.{ .func = func_index }));
+        const orig_func_index = ip.unwrapCoercedFunc(func_index);
-        try pt.ensureFuncBodyUpToDate(func_index);
+        try sema.addReferenceEntry(block, src, .wrap(.{ .func = orig_func_index }));
+        try pt.ensureFuncBodyUpToDate(orig_func_index);
     }
 
     // This will now have been resolved by the logic at the end of `Zcu.analyzeFnBody`

src/Zcu.zig

@@ -3451,8 +3451,11 @@ pub fn mapOldZirToNew(
 /// will be analyzed when it returns: for that, see `ensureFuncBodyAnalyzed`.
 pub fn ensureFuncBodyAnalysisQueued(zcu: *Zcu, func_index: InternPool.Index) !void {
     const ip = &zcu.intern_pool;
+
     const func = zcu.funcInfo(func_index);
 
+    assert(func.ty == func.uncoerced_ty); // analyze the body of the original function, not a coerced one
+
     if (zcu.func_body_analysis_queued.contains(func_index)) return;
 
     if (func.analysisUnordered(ip).is_analyzed) {

src/Zcu/PerThread.zig

@@ -1571,7 +1571,7 @@ fn analyzeNavType(pt: Zcu.PerThread, nav_id: InternPool.Nav.Index) Zcu.CompileEr
     return .{ .type_changed = true };
 }
 
-pub fn ensureFuncBodyUpToDate(pt: Zcu.PerThread, maybe_coerced_func_index: InternPool.Index) Zcu.SemaError!void {
+pub fn ensureFuncBodyUpToDate(pt: Zcu.PerThread, func_index: InternPool.Index) Zcu.SemaError!void {
     dev.check(.sema);
 
     const tracy = trace(@src());
@@ -1581,15 +1581,15 @@ pub fn ensureFuncBodyUpToDate(pt: Zcu.PerThread, maybe_coerced_func_index: Inter
     const gpa = zcu.gpa;
     const ip = &zcu.intern_pool;
 
-    _ = zcu.func_body_analysis_queued.swapRemove(maybe_coerced_func_index);
+    _ = zcu.func_body_analysis_queued.swapRemove(func_index);
 
-    // We only care about the uncoerced function.
-    const func_index = ip.unwrapCoercedFunc(maybe_coerced_func_index);
     const anal_unit: AnalUnit = .wrap(.{ .func = func_index });
 
     log.debug("ensureFuncBodyUpToDate {f}", .{zcu.fmtAnalUnit(anal_unit)});
 
-    const func = zcu.funcInfo(maybe_coerced_func_index);
+    const func = zcu.funcInfo(func_index);
+
+    assert(func.ty == func.uncoerced_ty); // analyze the body of the original function, not a coerced one
 
     const was_outdated = zcu.outdated.swapRemove(anal_unit) or
         zcu.potentially_outdated.swapRemove(anal_unit);

test/cases/export_from_body_of_coerced_fn.zig

@@ -0,0 +1,19 @@
+fn original() usize {
+    _ = struct {
+        export const val: u32 = 123;
+    };
+    return 0;
+}
+
+pub fn main() void {
+    const coerced: fn () u64 = original;
+    _ = coerced();
+
+    const S = struct {
+        extern const val: u32;
+    };
+    if (S.val != 123) @panic("wrong value");
+}
+
+// run
+// target=x86_64-linux