Module: improve source spans for initializers and var types

```zig
const U = union { foo: u32, bar: u32 };
test {
    var a = U{ .foo = 1213, .bar = 1123 };
    _ = a;
}
test {
    var a: (123 + 5238094) = 0;
    _ = a;
}
```

before:
```
:30: note: additional initializer here
    var a = U{ .foo = 1213, .bar = 1123 };
                             ^~~
:12: error: expected type 'type', found 'comptime_int'
    var a: (123 + 5238094) = 0;
           ^
```
after:
```
:30: note: additional initializer here
    var a = U{ .foo = 1213, .bar = 1123 };
                            ~^~~~~~~~~~
:12: error: expected type 'type', found 'comptime_int'
    var a: (123 + 5238094) = 0;
           ^~~~~~~~~~~~~~~
```

2 files changed, 38 insertions, 33 deletions

diff --git a/src/Module.zig b/src/Module.zig
index 35b48406cb..4a5257a4f3 100644
--- a/src/Module.zig
+++ b/src/Module.zig
@@ -2132,13 +2132,15 @@ pub const SrcLoc = struct {
                 assert(src_loc.file_scope.tree_loaded);
                 return nodeToSpan(tree, node);
             },
-            .node_offset_back2tok => |node_off| {
+            .node_offset_initializer => |node_off| {
                 const tree = try src_loc.file_scope.getTree(gpa);
                 const node = src_loc.declRelativeToNodeIndex(node_off);
-                const tok_index = tree.firstToken(node) - 2;
-                const start = tree.tokens.items(.start)[tok_index];
-                const end = start + @intCast(u32, tree.tokenSlice(tok_index).len);
-                return Span{ .start = start, .end = end, .main = start };
+                return tokensToSpan(
+                    tree,
+                    tree.firstToken(node) - 3,
+                    tree.lastToken(node),
+                    tree.nodes.items(.main_token)[node] - 2,
+                );
             },
             .node_offset_var_decl_ty => |node_off| {
                 const tree = try src_loc.file_scope.getTree(gpa);
@@ -2151,12 +2153,10 @@ pub const SrcLoc = struct {
                     .aligned_var_decl => tree.alignedVarDecl(node),
                     else => unreachable,
                 };
-                const tok_index = if (full.ast.type_node != 0) blk: {
-                    const main_tokens = tree.nodes.items(.main_token);
-                    break :blk main_tokens[full.ast.type_node];
-                } else blk: {
-                    break :blk full.ast.mut_token + 1; // the name token
-                };
+                if (full.ast.type_node != 0) {
+                    return nodeToSpan(tree, full.ast.type_node);
+                }
+                const tok_index = full.ast.mut_token + 1; // the name token
                 const start = tree.tokens.items(.start)[tok_index];
                 const end = start + @intCast(u32, tree.tokenSlice(tok_index).len);
                 return Span{ .start = start, .end = end, .main = start };
@@ -2492,26 +2492,32 @@ pub const SrcLoc = struct {
     }
 
     pub fn nodeToSpan(tree: *const Ast, node: u32) Span {
+        return tokensToSpan(
+            tree,
+            tree.firstToken(node),
+            tree.lastToken(node),
+            tree.nodes.items(.main_token)[node],
+        );
+    }
+
+    fn tokensToSpan(tree: *const Ast, start: Ast.TokenIndex, end: Ast.TokenIndex, main: Ast.TokenIndex) Span {
         const token_starts = tree.tokens.items(.start);
-        const main_token = tree.nodes.items(.main_token)[node];
-        const start = tree.firstToken(node);
-        const end = tree.lastToken(node);
         var start_tok = start;
         var end_tok = end;
 
         if (tree.tokensOnSameLine(start, end)) {
             // do nothing
-        } else if (tree.tokensOnSameLine(start, main_token)) {
-            end_tok = main_token;
-        } else if (tree.tokensOnSameLine(main_token, end)) {
-            start_tok = main_token;
+        } else if (tree.tokensOnSameLine(start, main)) {
+            end_tok = main;
+        } else if (tree.tokensOnSameLine(main, end)) {
+            start_tok = main;
         } else {
-            start_tok = main_token;
-            end_tok = main_token;
+            start_tok = main;
+            end_tok = main;
         }
         const start_off = token_starts[start_tok];
         const end_off = token_starts[end_tok] + @intCast(u32, tree.tokenSlice(end_tok).len);
-        return Span{ .start = start_off, .end = end_off, .main = token_starts[main_token] };
+        return Span{ .start = start_off, .end = end_off, .main = token_starts[main] };
     }
 };
 
@@ -2565,10 +2571,9 @@ pub const LazySrcLoc = union(enum) {
     /// from its containing Decl node AST index.
     /// The Decl is determined contextually.
     node_offset: TracedOffset,
-    /// The source location points to two tokens left of the first token of an AST node,
-    /// which is this value offset from its containing Decl node AST index.
+    /// The source location points to the beginning of a struct initializer.
     /// The Decl is determined contextually.
-    node_offset_back2tok: i32,
+    node_offset_initializer: i32,
     /// The source location points to a variable declaration type expression,
     /// found by taking this AST node index offset from the containing
     /// Decl AST node, which points to a variable declaration AST node. Next, navigate
@@ -2764,7 +2769,7 @@ pub const LazySrcLoc = union(enum) {
             .byte_offset,
             .token_offset,
             .node_offset,
-            .node_offset_back2tok,
+            .node_offset_initializer,
             .node_offset_var_decl_ty,
             .node_offset_for_cond,
             .node_offset_builtin_call_arg0,
diff --git a/src/Sema.zig b/src/Sema.zig
index 29840820d0..305210517a 100644
--- a/src/Sema.zig
+++ b/src/Sema.zig
@@ -3403,7 +3403,7 @@ fn validateUnionInit(
 
             for (instrs[1..]) |inst| {
                 const inst_data = sema.code.instructions.items(.data)[inst].pl_node;
-                const inst_src: LazySrcLoc = .{ .node_offset_back2tok = inst_data.src_node };
+                const inst_src: LazySrcLoc = .{ .node_offset_initializer = inst_data.src_node };
                 try sema.errNote(block, inst_src, msg, "additional initializer here", .{});
             }
             try sema.addDeclaredHereNote(msg, union_ty);
@@ -3421,7 +3421,7 @@ fn validateUnionInit(
 
     const field_ptr = instrs[0];
     const field_ptr_data = sema.code.instructions.items(.data)[field_ptr].pl_node;
-    const field_src: LazySrcLoc = .{ .node_offset_back2tok = field_ptr_data.src_node };
+    const field_src: LazySrcLoc = .{ .node_offset_initializer = field_ptr_data.src_node };
     const field_ptr_extra = sema.code.extraData(Zir.Inst.Field, field_ptr_data.payload_index).data;
     const field_name = sema.code.nullTerminatedString(field_ptr_extra.field_name_start);
     const field_index = try sema.unionFieldIndex(block, union_ty, field_name, field_src);
@@ -3523,7 +3523,7 @@ fn validateStructInit(
 
     for (instrs) |field_ptr| {
         const field_ptr_data = sema.code.instructions.items(.data)[field_ptr].pl_node;
-        const field_src: LazySrcLoc = .{ .node_offset_back2tok = field_ptr_data.src_node };
+        const field_src: LazySrcLoc = .{ .node_offset_initializer = field_ptr_data.src_node };
         const field_ptr_extra = sema.code.extraData(Zir.Inst.Field, field_ptr_data.payload_index).data;
         struct_ptr_zir_ref = field_ptr_extra.lhs;
         const field_name = sema.code.nullTerminatedString(field_ptr_extra.field_name_start);
@@ -3531,7 +3531,7 @@ fn validateStructInit(
         if (found_fields[field_index] != 0) {
             const other_field_ptr = found_fields[field_index];
             const other_field_ptr_data = sema.code.instructions.items(.data)[other_field_ptr].pl_node;
-            const other_field_src: LazySrcLoc = .{ .node_offset_back2tok = other_field_ptr_data.src_node };
+            const other_field_src: LazySrcLoc = .{ .node_offset_initializer = other_field_ptr_data.src_node };
             const msg = msg: {
                 const msg = try sema.errMsg(block, field_src, "duplicate field", .{});
                 errdefer msg.destroy(gpa);
@@ -3606,7 +3606,7 @@ fn validateStructInit(
     field: for (found_fields) |field_ptr, i| {
         if (field_ptr != 0) {
             const field_ptr_data = sema.code.instructions.items(.data)[field_ptr].pl_node;
-            const field_src: LazySrcLoc = .{ .node_offset_back2tok = field_ptr_data.src_node };
+            const field_src: LazySrcLoc = .{ .node_offset_initializer = field_ptr_data.src_node };
 
             // Determine whether the value stored to this pointer is comptime-known.
             const field_ty = struct_ty.structFieldType(i);
@@ -13999,14 +13999,14 @@ fn zirStructInit(
             extra_index = item.end;
 
             const field_type_data = zir_datas[item.data.field_type].pl_node;
-            const field_src: LazySrcLoc = .{ .node_offset_back2tok = field_type_data.src_node };
+            const field_src: LazySrcLoc = .{ .node_offset_initializer = field_type_data.src_node };
             const field_type_extra = sema.code.extraData(Zir.Inst.FieldType, field_type_data.payload_index).data;
             const field_name = sema.code.nullTerminatedString(field_type_extra.name_start);
             const field_index = try sema.structFieldIndex(block, resolved_ty, field_name, field_src);
             if (field_inits[field_index] != .none) {
                 const other_field_type = found_fields[field_index];
                 const other_field_type_data = zir_datas[other_field_type].pl_node;
-                const other_field_src: LazySrcLoc = .{ .node_offset_back2tok = other_field_type_data.src_node };
+                const other_field_src: LazySrcLoc = .{ .node_offset_initializer = other_field_type_data.src_node };
                 const msg = msg: {
                     const msg = try sema.errMsg(block, field_src, "duplicate field", .{});
                     errdefer msg.destroy(gpa);
@@ -14028,7 +14028,7 @@ fn zirStructInit(
         const item = sema.code.extraData(Zir.Inst.StructInit.Item, extra.end);
 
         const field_type_data = zir_datas[item.data.field_type].pl_node;
-        const field_src: LazySrcLoc = .{ .node_offset_back2tok = field_type_data.src_node };
+        const field_src: LazySrcLoc = .{ .node_offset_initializer = field_type_data.src_node };
         const field_type_extra = sema.code.extraData(Zir.Inst.FieldType, field_type_data.payload_index).data;
         const field_name = sema.code.nullTerminatedString(field_type_extra.name_start);
         const field_index = try sema.unionFieldIndex(block, resolved_ty, field_name, field_src);