make file and fn_name fields of SourceLocation also null-terminated

One of the main motivating use cases for this language feature is
tracing/profiling tools, which expect null-terminated strings for these
values. Since the data is statically allocated, making them
additionally null-terminated comes at no cost.

This prevents the requirement of compile-time code to convert to
null-termination, which could increase the compilation time of
code with tracing enabled.

See #2029

lib/std/builtin.zig

@@ -134,8 +134,8 @@ pub const CallingConvention = enum {
 /// This data structure is used by the Zig language code generation and
 /// therefore must be kept in sync with the compiler implementation.
 pub const SourceLocation = struct {
-    file: []const u8,
-    fn_name: []const u8,
+    file: [:0]const u8,
+    fn_name: [:0]const u8,
     line: u32,
     column: u32,
 };

src/ir.cpp

@@ -30881,10 +30881,10 @@ static IrInstGen *ir_analyze_instruction_src(IrAnalyze *ira, IrInstSrcSrc *instr
         return ira->codegen->invalid_inst_gen;
     }
 
-    ZigType *u8_ptr = get_pointer_to_type_extra(
+    ZigType *u8_ptr = get_pointer_to_type_extra2(
         ira->codegen, ira->codegen->builtin_types.entry_u8,
         true, false, PtrLenUnknown,
-        0, 0, 0, false);
+        0, 0, 0, false, VECTOR_INDEX_NONE, nullptr, ira->codegen->intern.for_zero_byte());
     ZigType *u8_slice = get_slice_type(ira->codegen, u8_ptr);
 
     ZigType *source_location_type = get_builtin_type(ira->codegen, "SourceLocation");
@@ -30899,23 +30899,23 @@ static IrInstGen *ir_analyze_instruction_src(IrAnalyze *ira, IrInstSrcSrc *instr
     ZigValue **fields = alloc_const_vals_ptrs(ira->codegen, 4);
     result->data.x_struct.fields = fields;
 
-    // file: []const u8
+    // file: [:0]const u8
     ensure_field_index(source_location_type, "file", 0);
     fields[0]->special = ConstValSpecialStatic;
-    fields[0]->type = u8_slice;
 
     ZigType *import = instruction->base.base.source_node->owner;
     Buf *path = import->data.structure.root_struct->path;
     ZigValue *file_name = create_const_str_lit(ira->codegen, path)->data.x_ptr.data.ref.pointee;
     init_const_slice(ira->codegen, fields[0], file_name, 0, buf_len(path), true);
+    fields[0]->type = u8_slice;
 
-    // fn_name: []const u8
+    // fn_name: [:0]const u8
     ensure_field_index(source_location_type, "fn_name", 1);
     fields[1]->special = ConstValSpecialStatic;
-    fields[1]->type = u8_slice;
 
     ZigValue *fn_name = create_const_str_lit(ira->codegen, &fn_entry->symbol_name)->data.x_ptr.data.ref.pointee;
     init_const_slice(ira->codegen, fields[1], fn_name, 0, buf_len(&fn_entry->symbol_name), true);
+    fields[1]->type = u8_slice;
 
     // line: u32
     ensure_field_index(source_location_type, "line", 2);

test/stage1/behavior/src.zig

@@ -12,4 +12,6 @@ fn doTheTest() void {
     expect(src.column == 17);
     expect(std.mem.endsWith(u8, src.fn_name, "doTheTest"));
     expect(std.mem.endsWith(u8, src.file, "src.zig"));
+    expect(src.fn_name[src.fn_name.len] == 0);
+    expect(src.file[src.file.len] == 0);
 }