stage1: Error out when trying to execute unreachable

Closes #6802

Author: LemonBoy

doc/langref.html.in

@@ -8675,7 +8675,7 @@ test "safety check" {
       {#code_end#}
       {#header_open|Reaching Unreachable Code#}
       <p>At compile-time:</p>
-      {#code_begin|test_err|unable to evaluate constant expression#}
+      {#code_begin|test_err|reached unreachable code#}
 comptime {
     assert(false);
 }

src/stage1/ir.cpp

@@ -21592,6 +21592,11 @@ static IrInstGen *ir_analyze_instruction_cond_br(IrAnalyze *ira, IrInstSrcCondBr
 static IrInstGen *ir_analyze_instruction_unreachable(IrAnalyze *ira,
         IrInstSrcUnreachable *unreachable_instruction)
 {
+    if (ir_should_inline(ira->old_irb.exec, unreachable_instruction->base.base.scope)) {
+        ir_add_error(ira, &unreachable_instruction->base.base, buf_sprintf("reached unreachable code"));
+        return ir_unreach_error(ira);
+    }
+
     IrInstGen *result = ir_build_unreachable_gen(ira, &unreachable_instruction->base.base);
     return ir_finish_anal(ira, result);
 }

test/compile_errors.zig

@@ -2,6 +2,21 @@ const tests = @import("tests.zig");
 const std = @import("std");
 
 pub fn addCases(cases: *tests.CompileErrorContext) void {
+    cases.add("unreachable executed at comptime",
+        \\fn foo(comptime x: i32) i32 {
+        \\    comptime {
+        \\        if (x >= 0) return -x;
+        \\        unreachable;
+        \\    }
+        \\}
+        \\export fn entry() void {
+        \\    _ = foo(-42);
+        \\}
+    , &[_][]const u8{
+        "tmp.zig:4:9: error: reached unreachable code",
+        "tmp.zig:8:12: note: called from here",
+    });
+
     cases.add("indexing a undefined slice at comptime",
         \\comptime {
         \\    var slice: []u8 = undefined;
@@ -6208,7 +6223,7 @@ pub fn addCases(cases: *tests.CompileErrorContext) void {
         \\    if (!ok) unreachable;
         \\}
     , &[_][]const u8{
-        "tmp.zig:10:14: error: unable to evaluate constant expression",
+        "tmp.zig:10:14: error: reached unreachable code",
         "tmp.zig:6:20: note: referenced here",
     });