stage1: make C++ switch fallthrough an error

Make fallthrough an error when compiler supports it. This requires a new
macro that is defined with such compilers to be used as a statement, at
all fallthrough sites:

    switch (...) {
        case 0:
            ...
            ZIG_FALLTHROUGH;
        case 1:
            ...
            break;
        default:
            ...
            break;
    }

If we ever move to C++17 as minimal requirement, then the macro can be
replaced with `[[fallthrough]];` at statement sites.

1 files changed, 4 insertions, 3 deletions

diff --git a/src/tokenizer.cpp b/src/tokenizer.cpp
index b4e9d910d9..11432ecac1 100644
--- a/src/tokenizer.cpp
+++ b/src/tokenizer.cpp
@@ -840,6 +840,7 @@ void tokenize(Buf *buf, Tokenization *out) {
                         t.state = TokenizeStateStart;
                         continue;
                 }
+                break;
             case TokenizeStateSawSlash:
                 switch (c) {
                     case '/':
@@ -1209,7 +1210,7 @@ void tokenize(Buf *buf, Tokenization *out) {
                     t.is_trailing_underscore = false;
                     t.state = TokenizeStateNumber;
                 }
-                // fall through
+                ZIG_FALLTHROUGH;
             case TokenizeStateNumber:
                 {
                     if (c == '_') {
@@ -1291,7 +1292,7 @@ void tokenize(Buf *buf, Tokenization *out) {
                     t.is_trailing_underscore = false;
                     t.state = TokenizeStateFloatFraction;
                 }
-                // fall through
+                ZIG_FALLTHROUGH;
             case TokenizeStateFloatFraction:
                 {
                     if (c == '_') {
@@ -1350,7 +1351,7 @@ void tokenize(Buf *buf, Tokenization *out) {
                     t.is_trailing_underscore = false;
                     t.state = TokenizeStateFloatExponentNumber;
                 }
-                // fall through
+                ZIG_FALLTHROUGH;
             case TokenizeStateFloatExponentNumber:
                 {
                     if (c == '_') {