create distinct interned string type

Interned strings are used pervasively, so it's a good idea to add a
layer of type safety to differentiate them from general cstrs and avoid
potential bugs from comparing non-interned and interned strings. Not
that that's happened so far that I can remember, but it could.

I'm 90% sure it's legal to alias `struct {char c;}` pointers with `char`
pointers. This specific typedef gives type safety but with a simple
one-way `internstr -> const char *` typecast (with `&istr->c`).
Converting the other way around is more intentional: a straight up cast
`(internstr)cstr` which sticks out as unchecked and probably wrong, or
calling the intern(cstr) function, which is the right way.

1 files changed, 4 insertions, 3 deletions

diff --git a/common.h b/common.h
index 061c379..3d2166b 100644
--- a/common.h
+++ b/common.h
@@ -179,7 +179,8 @@ void free(void *);
 #define xrealloc(p,n) xrealloc(p, n, __func__)
 
 /* string interning */
-const char *intern(const char *);
+typedef const struct internstr {char c;} *internstr;
+internstr intern(const char *);
 
 /* growable buffer that stores its capacity in the allocated memory */
 #define xbnew_(n) (void *)(1 + (size_t *)xcalloc(sizeof(size_t) + (n)))
@@ -404,8 +405,8 @@ struct memfile *getfile(int id);
 void addfileline(int id, uint off);
 void setfileline(int id, uint off, int line, const char *file);
 const char *getfilepos(int *line, int *col, int id, uint off);
-bool isoncefile(int id, const char **guard);
-void markfileonce(int id, const char *guard);
+bool isoncefile(int id, internstr *guard);
+void markfileonce(int id, internstr guard);
 void markfileseen(int id);
 bool isfileseen(int id);
 void closefile(int id);