compiler: split Decl into Nav and Cau

The type `Zcu.Decl` in the compiler is problematic: over time it has
gained many responsibilities. Every source declaration, container type,
generic instantiation, and `@extern` has a `Decl`. The functions of
these `Decl`s are in some cases entirely disjoint.

After careful analysis, I determined that the two main responsibilities
of `Decl` are as follows:
* A `Decl` acts as the "subject" of semantic analysis at comptime. A
  single unit of analysis is either a runtime function body, or a
  `Decl`. It registers incremental dependencies, tracks analysis errors,
  etc.
* A `Decl` acts as a "global variable": a pointer to it is consistent,
  and it may be lowered to a specific symbol by the codegen backend.

This commit eliminates `Decl` and introduces new types to model these
responsibilities: `Cau` (Comptime Analysis Unit) and `Nav` (Named
Addressable Value).

Every source declaration, and every container type requiring resolution
(so *not* including `opaque`), has a `Cau`. For a source declaration,
this `Cau` performs the resolution of its value. (When #131 is
implemented, it is unsolved whether type and value resolution will share
a `Cau` or have two distinct `Cau`s.) For a type, this `Cau` is the
context in which type resolution occurs.

Every non-`comptime` source declaration, every generic instantiation,
and every distinct `extern` has a `Nav`. These are sent to codegen/link:
the backends by definition do not care about `Cau`s.

This commit has some minor technically-breaking changes surrounding
`usingnamespace`. I don't think they'll impact anyone, since the changes
are fixes around semantics which were previously inconsistent (the
behavior changed depending on hashmap iteration order!).

Aside from that, this changeset has no significant user-facing changes.
Instead, it is an internal refactor which makes it easier to correctly
model the responsibilities of different objects, particularly regarding
incremental compilation. The performance impact should be negligible,
but I will take measurements before merging this work into `master`.

Co-authored-by: Jacob Young <jacobly0@users.noreply.github.com>
Co-authored-by: Jakub Konka <kubkon@jakubkonka.com>

1 files changed, 37 insertions, 48 deletions

diff --git a/src/Value.zig b/src/Value.zig
index 7aead8ae7d..8ae3b58458 100644
--- a/src/Value.zig
+++ b/src/Value.zig
@@ -227,13 +227,6 @@ pub fn getFunction(val: Value, mod: *Module) ?InternPool.Key.Func {
     };
 }
 
-pub fn getExternFunc(val: Value, mod: *Module) ?InternPool.Key.ExternFunc {
-    return switch (mod.intern_pool.indexToKey(val.toIntern())) {
-        .extern_func => |extern_func| extern_func,
-        else => null,
-    };
-}
-
 pub fn getVariable(val: Value, mod: *Module) ?InternPool.Key.Variable {
     return switch (mod.intern_pool.indexToKey(val.toIntern())) {
         .variable => |variable| variable,
@@ -319,17 +312,8 @@ pub fn toBool(val: Value) bool {
     };
 }
 
-fn ptrHasIntAddr(val: Value, mod: *Module) bool {
-    var check = val;
-    while (true) switch (mod.intern_pool.indexToKey(check.toIntern())) {
-        .ptr => |ptr| switch (ptr.base_addr) {
-            .decl, .comptime_alloc, .comptime_field, .anon_decl => return false,
-            .int => return true,
-            .eu_payload, .opt_payload => |base| check = Value.fromInterned(base),
-            .arr_elem, .field => |base_index| check = Value.fromInterned(base_index.base),
-        },
-        else => unreachable,
-    };
+fn ptrHasIntAddr(val: Value, zcu: *Zcu) bool {
+    return zcu.intern_pool.getBackingAddrTag(val.toIntern()).? == .int;
 }
 
 /// Write a Value's contents to `buffer`.
@@ -1058,7 +1042,7 @@ pub fn orderAgainstZeroAdvanced(
         .bool_true => .gt,
         else => switch (pt.zcu.intern_pool.indexToKey(lhs.toIntern())) {
             .ptr => |ptr| if (ptr.byte_offset > 0) .gt else switch (ptr.base_addr) {
-                .decl, .comptime_alloc, .comptime_field => .gt,
+                .nav, .comptime_alloc, .comptime_field => .gt,
                 .int => .eq,
                 else => unreachable,
             },
@@ -1130,11 +1114,11 @@ pub fn compareHeteroAdvanced(
     pt: Zcu.PerThread,
     comptime strat: ResolveStrat,
 ) !bool {
-    if (lhs.pointerDecl(pt.zcu)) |lhs_decl| {
-        if (rhs.pointerDecl(pt.zcu)) |rhs_decl| {
+    if (lhs.pointerNav(pt.zcu)) |lhs_nav| {
+        if (rhs.pointerNav(pt.zcu)) |rhs_nav| {
             switch (op) {
-                .eq => return lhs_decl == rhs_decl,
-                .neq => return lhs_decl != rhs_decl,
+                .eq => return lhs_nav == rhs_nav,
+                .neq => return lhs_nav != rhs_nav,
                 else => {},
             }
         } else {
@@ -1144,7 +1128,7 @@ pub fn compareHeteroAdvanced(
                 else => {},
             }
         }
-    } else if (rhs.pointerDecl(pt.zcu)) |_| {
+    } else if (rhs.pointerNav(pt.zcu)) |_| {
         switch (op) {
             .eq => return false,
             .neq => return true,
@@ -1252,12 +1236,12 @@ pub fn canMutateComptimeVarState(val: Value, zcu: *Zcu) bool {
             .payload => |payload| Value.fromInterned(payload).canMutateComptimeVarState(zcu),
         },
         .ptr => |ptr| switch (ptr.base_addr) {
-            .decl => false, // The value of a Decl can never reference a comptime alloc.
+            .nav => false, // The value of a Nav can never reference a comptime alloc.
             .int => false,
             .comptime_alloc => true, // A comptime alloc is either mutable or references comptime-mutable memory.
             .comptime_field => true, // Comptime field pointers are comptime-mutable, albeit only to the "correct" value.
             .eu_payload, .opt_payload => |base| Value.fromInterned(base).canMutateComptimeVarState(zcu),
-            .anon_decl => |anon_decl| Value.fromInterned(anon_decl.val).canMutateComptimeVarState(zcu),
+            .uav => |uav| Value.fromInterned(uav.val).canMutateComptimeVarState(zcu),
             .arr_elem, .field => |base_index| Value.fromInterned(base_index.base).canMutateComptimeVarState(zcu),
         },
         .slice => |slice| return Value.fromInterned(slice.ptr).canMutateComptimeVarState(zcu),
@@ -1273,16 +1257,17 @@ pub fn canMutateComptimeVarState(val: Value, zcu: *Zcu) bool {
     };
 }
 
-/// Gets the decl referenced by this pointer.  If the pointer does not point
-/// to a decl, or if it points to some part of a decl (like field_ptr or element_ptr),
-/// this function returns null.
-pub fn pointerDecl(val: Value, mod: *Module) ?InternPool.DeclIndex {
+/// Gets the `Nav` referenced by this pointer.  If the pointer does not point
+/// to a `Nav`, or if it points to some part of one (like a field or element),
+/// returns null.
+pub fn pointerNav(val: Value, mod: *Module) ?InternPool.Nav.Index {
     return switch (mod.intern_pool.indexToKey(val.toIntern())) {
-        .variable => |variable| variable.decl,
-        .extern_func => |extern_func| extern_func.decl,
-        .func => |func| func.owner_decl,
+        // TODO: these 3 cases are weird; these aren't pointer values!
+        .variable => |v| v.owner_nav,
+        .@"extern" => |e| e.owner_nav,
+        .func => |func| func.owner_nav,
         .ptr => |ptr| if (ptr.byte_offset == 0) switch (ptr.base_addr) {
-            .decl => |decl| decl,
+            .nav => |nav| nav,
             else => null,
         } else null,
         else => null,
@@ -1341,10 +1326,14 @@ pub fn isLazySize(val: Value, mod: *Module) bool {
     };
 }
 
-pub fn isPtrToThreadLocal(val: Value, mod: *Module) bool {
-    const backing_decl = mod.intern_pool.getBackingDecl(val.toIntern()).unwrap() orelse return false;
-    const variable = mod.declPtr(backing_decl).getOwnedVariable(mod) orelse return false;
-    return variable.is_threadlocal;
+pub fn isPtrToThreadLocal(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,
+        .variable => |v| v.is_threadlocal,
+        else => false,
+    };
 }
 
 // Asserts that the provided start/end are in-bounds.
@@ -4031,8 +4020,8 @@ pub const PointerDeriveStep = union(enum) {
         addr: u64,
         ptr_ty: Type,
     },
-    decl_ptr: InternPool.DeclIndex,
-    anon_decl_ptr: InternPool.Key.Ptr.BaseAddr.AnonDecl,
+    nav_ptr: InternPool.Nav.Index,
+    uav_ptr: InternPool.Key.Ptr.BaseAddr.Uav,
     comptime_alloc_ptr: struct {
         val: Value,
         ptr_ty: Type,
@@ -4069,8 +4058,8 @@ pub const PointerDeriveStep = union(enum) {
     pub fn ptrType(step: PointerDeriveStep, pt: Zcu.PerThread) !Type {
         return switch (step) {
             .int => |int| int.ptr_ty,
-            .decl_ptr => |decl| try pt.zcu.declPtr(decl).declPtrType(pt),
-            .anon_decl_ptr => |ad| Type.fromInterned(ad.orig_ty),
+            .nav_ptr => |nav| try pt.navPtrType(nav),
+            .uav_ptr => |uav| Type.fromInterned(uav.orig_ty),
             .comptime_alloc_ptr => |info| info.ptr_ty,
             .comptime_field_ptr => |val| try pt.singleConstPtrType(val.typeOf(pt.zcu)),
             .offset_and_cast => |oac| oac.new_ptr_ty,
@@ -4098,17 +4087,17 @@ pub fn pointerDerivationAdvanced(ptr_val: Value, arena: Allocator, pt: Zcu.PerTh
             .addr = ptr.byte_offset,
             .ptr_ty = Type.fromInterned(ptr.ty),
         } },
-        .decl => |decl| .{ .decl_ptr = decl },
-        .anon_decl => |ad| base: {
+        .nav => |nav| .{ .nav_ptr = nav },
+        .uav => |uav| base: {
             // A slight tweak: `orig_ty` here is sometimes not `const`, but it ought to be.
             // TODO: fix this in the sites interning anon decls!
             const const_ty = try pt.ptrType(info: {
-                var info = Type.fromInterned(ad.orig_ty).ptrInfo(zcu);
+                var info = Type.fromInterned(uav.orig_ty).ptrInfo(zcu);
                 info.flags.is_const = true;
                 break :info info;
             });
-            break :base .{ .anon_decl_ptr = .{
-                .val = ad.val,
+            break :base .{ .uav_ptr = .{
+                .val = uav.val,
                 .orig_ty = const_ty.toIntern(),
             } };
         },
@@ -4357,7 +4346,7 @@ pub fn resolveLazy(val: Value, arena: Allocator, pt: Zcu.PerThread) Zcu.SemaErro
         },
         .ptr => |ptr| {
             switch (ptr.base_addr) {
-                .decl, .comptime_alloc, .anon_decl, .int => return val,
+                .nav, .comptime_alloc, .uav, .int => return val,
                 .comptime_field => |field_val| {
                     const resolved_field_val = (try Value.fromInterned(field_val).resolveLazy(arena, pt)).toIntern();
                     return if (resolved_field_val == field_val)