langref: docs for invalid error set cast and incorrect pointer alignment

also add detection of incorrect pointer alignment at compile-time
of pointers that were constructed with `@intToPtr`.

doc/langref.html.in

@@ -6649,12 +6649,60 @@ pub fn main() void {
       {#header_close#}
 
       {#header_open|Invalid Error Set Cast#}
-      <p>TODO</p>
+      <p>At compile-time:</p>
+      {#code_begin|test_err|error.B not a member of error set 'Set2'#}
+const Set1 = error{
+    A,
+    B,
+};
+const Set2 = error{
+    A,
+    C,
+};
+comptime {
+    _ = @errSetCast(Set2, Set1.B);
+}
+      {#code_end#}
+      <p>At runtime:</p>
+      {#code_begin|exe_err#}
+const Set1 = error{
+    A,
+    B,
+};
+const Set2 = error{
+    A,
+    C,
+};
+pub fn main() void {
+    _ = foo(Set1.B);
+}
+fn foo(set1: Set1) Set2 {
+    return @errSetCast(Set2, set1);
+}
+      {#code_end#}
       {#header_close#}
 
       {#header_open|Incorrect Pointer Alignment#}
-      <p>TODO</p>
+      <p>At compile-time:</p>
-
+      {#code_begin|test_err|pointer address 0x1 is not aligned to 4 bytes#}
+comptime {
+    const ptr = @intToPtr(*i32, 0x1);
+    const aligned = @alignCast(4, ptr);
+}
+      {#code_end#}
+      <p>At runtime:</p>
+      {#code_begin|exe_err#}
+pub fn main() !void {
+    var array align(4) = []u32{ 0x11111111, 0x11111111 };
+    const bytes = @sliceToBytes(array[0..]);
+    if (foo(bytes) != 0x11111111) return error.Wrong;
+}
+fn foo(bytes: []u8) u32 {
+    const slice4 = bytes[1..5];
+    const int_slice = @bytesToSlice(u32, @alignCast(4, slice4));
+    return int_slice[0];
+}
+      {#code_end#}
       {#header_close#}
       {#header_open|Wrong Union Field Access#}
       <p>TODO</p>

src/ir.cpp

@@ -19370,6 +19370,15 @@ static IrInstruction *ir_align_cast(IrAnalyze *ira, IrInstruction *target, uint3
         if (!val)
             return ira->codegen->invalid_instruction;
 
+        if (val->data.x_ptr.special == ConstPtrSpecialHardCodedAddr &&
+            val->data.x_ptr.data.hard_coded_addr.addr % align_bytes != 0)
+        {
+            ir_add_error(ira, target,
+                    buf_sprintf("pointer address 0x%lx is not aligned to %" PRIu32 " bytes",
+                        val->data.x_ptr.data.hard_coded_addr.addr, align_bytes));
+            return ira->codegen->invalid_instruction;
+        }
+
         IrInstruction *result = ir_create_const(&ira->new_irb, target->scope, target->source_node, result_type);
         copy_const_val(&result->value, val, false);
         result->value.type = result_type;

test/compile_errors.zig

@@ -1,6 +1,16 @@
 const tests = @import("tests.zig");
 
 pub fn addCases(cases: *tests.CompileErrorContext) void {
+    cases.add(
+        "bad @alignCast at comptime",
+        \\comptime {
+        \\    const ptr = @intToPtr(*i32, 0x1);
+        \\    const aligned = @alignCast(4, ptr);
+        \\}
+    ,
+        ".tmp_source.zig:3:35: error: pointer address 0x1 is not aligned to 4 bytes",
+    );
+
     cases.add(
         "@ptrToInt on *void",
         \\export fn entry() bool {