[PATCH 3/3] C: hints for missing stdlib includes for macros and types

[David Malcolm <dmalcolm at redhat dot com>]

The C frontend already "knows" about many common functions in
the C standard library:

  test.c: In function 'test':
  test.c:3:3: warning: implicit declaration of function 'printf' [-Wimplicit-function-declaration]
     printf ("hello world\n");
     ^~~~~~
  test.c:3:3: warning: incompatible implicit declaration of built-in function 'printf'
  test.c:3:3: note: include '<stdio.h>' or provide a declaration of 'printf'

and which header file they are in.

However it doesn't know about various types and macros:

test.c:1:13: error: 'NULL' undeclared here (not in a function)
 void *ptr = NULL;
             ^~~~

This patch uses the name_hint/deferred_diagnostic machinery to
add hints for missing C standard library headers for some of the
most common type and macro names.

For example, the above becomes:
test.c:1:13: error: 'NULL' undeclared here (not in a function)
 void *ptr = NULL;
             ^~~~
test.c:1:13: note: 'NULL' is defined in header '<stddef.h>'; did you forget to '#include <stddef.h>'?

If the patch to add fix-it hints for missing #includes is approved:
  https://gcc.gnu.org/ml/gcc-patches/2017-05/msg00321.html
then it's trivial to add a fix-it hint to the note.

gcc/c/ChangeLog:
	* c-decl.c (get_c_name_hint): New function.
	(class suggest_missing_header): New class.
	(lookup_name_fuzzy): Call get_c_name_hint and use it to
	suggest missing headers to the user.

gcc/testsuite/ChangeLog:
	* gcc.dg/spellcheck-stdlib.c: New test case.
---
 gcc/c/c-decl.c                           | 87 +++++++++++++++++++++++++++++++-
 gcc/testsuite/gcc.dg/spellcheck-stdlib.c | 55 ++++++++++++++++++++
 2 files changed, 140 insertions(+), 2 deletions(-)
 create mode 100644 gcc/testsuite/gcc.dg/spellcheck-stdlib.c

diff --git a/gcc/c/c-decl.c b/gcc/c/c-decl.c
index 64075f9..d3c2bc5 100644
--- a/gcc/c/c-decl.c
+++ b/gcc/c/c-decl.c
@@ -4000,6 +4000,78 @@ lookup_name_in_scope (tree name, struct c_scope *scope)
   return NULL_TREE;
 }
 
+/* Subroutine of suggest_missing_header::emit for handling unrecognized names
+   for some of the most common names within the C standard library.
+   Given non-NULL NAME, return the header name defining it within the C
+   standard library (with '<' and '>'), or NULL.  */
+
+static const char *
+get_c_name_hint (const char *name)
+{
+  struct std_name_hint
+  {
+    const char *name;
+    const char *header;
+  };
+  static const std_name_hint hints[] = {
+    /* <errno.h>.  */
+    {"errno", "<errno.h>"},
+
+    /* <stdarg.h>.  */
+    {"va_list", "<stdarg.h>"},
+
+    /* <stddef.h>.  */
+    {"NULL", "<stddef.h>"},
+    {"ptrdiff_t", "<stddef.h>"},
+    {"wchar_t", "<stddef.h>"},
+    {"size_t", "<stddef.h>"},
+
+    /* <stdio.h>.  */
+    {"BUFSIZ", "<stdio.h>"},
+    {"EOF", "<stdio.h>"},
+    {"FILE", "<stdio.h>"},
+    {"FILENAME_MAX", "<stdio.h>"},
+    {"fpos_t", "<stdio.h>"},
+    {"stderr", "<stdio.h>"},
+    {"stdin", "<stdio.h>"},
+    {"stdout", "<stdio.h>"}
+  };
+  const size_t num_hints = sizeof (hints) / sizeof (hints[0]);
+  for (size_t i = 0; i < num_hints; i++)
+    {
+      if (0 == strcmp (name, hints[i].name))
+	return hints[i].header;
+    }
+  return NULL;
+}
+
+/* Subclass of deferred_diagnostic for suggesting to the user
+   that they have missed a #include.  */
+
+class suggest_missing_header : public deferred_diagnostic
+{
+ public:
+  suggest_missing_header (location_t loc, const char *name,
+			  const char *header_hint)
+  : deferred_diagnostic (loc), m_name_str (name), m_header_hint (header_hint)
+  {
+    gcc_assert (name);
+    gcc_assert (header_hint);
+  }
+
+  void emit ()
+  {
+    inform (get_location (),
+	    "%qs is defined in header %qs;"
+	    " did you forget to %<#include %s%>?",
+	    m_name_str, m_header_hint, m_header_hint);
+  }
+
+ private:
+  const char *m_name_str;
+  const char *m_header_hint;
+};
+
 /* Look for the closest match for NAME within the currently valid
    scopes.
 
@@ -4014,13 +4086,24 @@ lookup_name_in_scope (tree name, struct c_scope *scope)
    identifier to the C frontend.
 
    It also looks for start_typename keywords, to detect "singed" vs "signed"
-   typos.  */
+   typos.
+
+   Use LOC for any deferred diagnostics.  */
 
 name_hint
-lookup_name_fuzzy (tree name, enum lookup_name_fuzzy_kind kind, location_t)
+lookup_name_fuzzy (tree name, enum lookup_name_fuzzy_kind kind, location_t loc)
 {
   gcc_assert (TREE_CODE (name) == IDENTIFIER_NODE);
 
+  /* First, try some well-known names in the C standard library, in case
+     the user forgot a #include.  */
+  const char *header_hint = get_c_name_hint (IDENTIFIER_POINTER (name));
+  if (header_hint)
+    return name_hint (NULL,
+		      new suggest_missing_header (loc,
+						  IDENTIFIER_POINTER (name),
+						  header_hint));
+
   best_match<tree, tree> bm (name);
 
   /* Look within currently valid scopes.  */
diff --git a/gcc/testsuite/gcc.dg/spellcheck-stdlib.c b/gcc/testsuite/gcc.dg/spellcheck-stdlib.c
new file mode 100644
index 0000000..85a21c3
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/spellcheck-stdlib.c
@@ -0,0 +1,55 @@
+/* Missing <stddef.h>.  */
+
+void *ptr = NULL; /* { dg-error "'NULL' undeclared here" } */
+/* { dg-message "'NULL' is defined in header '<stddef.h>'; did you forget to '#include <stddef.h>'?" "" { target *-*-* } .-1 } */
+
+ptrdiff_t pd; /* { dg-error "unknown type name 'ptrdiff_t'" } */
+/* { dg-message "'ptrdiff_t' is defined in header '<stddef.h>'; did you forget to '#include <stddef.h>'?" "" { target *-*-* } .-1 } */
+
+wchar_t wc; /* { dg-error "unknown type name 'wchar_t'" } */
+/* { dg-message "'wchar_t' is defined in header '<stddef.h>'; did you forget to '#include <stddef.h>'?" "" { target *-*-* } .-1 } */
+
+size_t sz; /* { dg-error "unknown type name 'size_t'" } */
+/* { dg-message "'size_t' is defined in header '<stddef.h>'; did you forget to '#include <stddef.h>'?" "" { target *-*-* } .-1 } */
+
+/* Missing <stdio.h>.  */
+
+void test_stdio_h (void)
+{
+  FILE *f; /* { dg-error "unknown type name 'FILE'" } */
+  /* { dg-message "'FILE' is defined in header '<stdio.h>'; did you forget to '#include <stdio.h>'?" "" { target *-*-* } .-1 } */
+
+  char buf[BUFSIZ]; /* { dg-error "'BUFSIZ' undeclared" } */
+  /* { dg-message "'BUFSIZ' is defined in header '<stdio.h>'; did you forget to '#include <stdio.h>'?" "" { target *-*-* } .-1 } */
+
+  char buf2[FILENAME_MAX]; /* { dg-error "'FILENAME_MAX' undeclared" } */
+  /* { dg-message "'FILENAME_MAX' is defined in header '<stdio.h>'; did you forget to '#include <stdio.h>'?" "" { target *-*-* } .-1 } */
+
+  stderr; /* { dg-error "'stderr' undeclared" } */
+  /* { dg-message "'stderr' is defined in header '<stdio.h>'; did you forget to '#include <stdio.h>'?" "" { target *-*-* } .-1 } */
+
+  stdin; /* { dg-error "'stdin' undeclared" } */
+  /* { dg-message "'stdin' is defined in header '<stdio.h>'; did you forget to '#include <stdio.h>'?" "" { target *-*-* } .-1 } */
+
+  stdout; /* { dg-error "'stdout' undeclared" } */
+  /* { dg-message "'stdout' is defined in header '<stdio.h>'; did you forget to '#include <stdio.h>'?" "" { target *-*-* } .-1 } */
+
+  EOF; /* { dg-error "'EOF' undeclared" } */
+  /* { dg-message "'EOF' is defined in header '<stdio.h>'; did you forget to '#include <stdio.h>'?" "" { target *-*-* } .-1 } */
+}
+
+/* Missing <errno.h>.  */
+
+int test_errno_h (void)
+{
+  return errno; /* { dg-error "'errno' undeclared" } */
+  /* { dg-message "'errno' is defined in header '<errno.h>'; did you forget to '#include <errno.h>'?" "" { target *-*-* } .-1 } */
+}
+
+/* Missing <stdarg.h>.  */
+
+void test_stdarg_h (void)
+{
+  va_list ap; /* { dg-error "unknown type name 'va_list'" } */
+  /* { dg-message "'va_list' is defined in header '<stdarg.h>'; did you forget to '#include <stdarg.h>'?" "" { target *-*-* } .-1 } */
+}
-- 
1.8.5.3