Introduce special treatment for the blank identifier _

Mostly: Do not instantiate variables within for loops and allow
reusing differently typed blanks which previously wasn't possible.

This may be missing some corner-cases, but the added tests seem
to work as expected and nothing else fell apart it seems.

testing/btest/Baseline/language.blank-expr-errors-2/.stderr

@@ -0,0 +1,2 @@
+### BTest baseline data generated by btest-diff. Do not edit. Use "btest -U/-u" to update. Requires BTest >= 0.63.
+error in <...>/blank-expr-errors.zeek, line 4: blank identifier used in expression (_)

testing/btest/Baseline/language.blank-expr-errors-3/.stderr

@@ -0,0 +1,2 @@
+### BTest baseline data generated by btest-diff. Do not edit. Use "btest -U/-u" to update. Requires BTest >= 0.63.
+error in <...>/blank-expr-errors.zeek, line 6: blank identifier used in expression (MyModule::_)

testing/btest/Baseline/language.blank-expr-errors-4/.stderr

@@ -0,0 +1,2 @@
+### BTest baseline data generated by btest-diff. Do not edit. Use "btest -U/-u" to update. Requires BTest >= 0.63.
+error in <...>/blank-expr-errors.zeek, line 11: blank identifier used in expression (MyModule::_)

testing/btest/Baseline/language.blank-expr-errors-5/.stderr

@@ -0,0 +1,2 @@
+### BTest baseline data generated by btest-diff. Do not edit. Use "btest -U/-u" to update. Requires BTest >= 0.63.
+error in <...>/blank-expr-errors.zeek, line 6: blank identifier used in expression (MyModule::_)

testing/btest/Baseline/language.blank-expr-errors/.stderr

@@ -0,0 +1,2 @@
+### BTest baseline data generated by btest-diff. Do not edit. Use "btest -U/-u" to update. Requires BTest >= 0.63.
+error in <...>/blank-expr-errors.zeek, line 9: blank identifier used in expression (_)

testing/btest/Baseline/language.blank-for/output

@@ -0,0 +1,31 @@
+### BTest baseline data generated by btest-diff. Do not edit. Use "btest -U/-u" to update. Requires BTest >= 0.63.
+== vec 1
+a
+b
+c
+== vec 2
+idxsum(vec), 3
+== vec 3
+veclen(vec), 3
+== t1 1
+c
+b
+a
+== t1 2
+keyc
+keyb
+keya
+== t1 3
+t1len, 3
+== t2 1
+1, a1a
+3, c3c
+2, b2b
+== t2 2
+a, T
+c, T
+b, F
+== t2 3
+t2concat, a1ac3cb2b
+== s
+strlen(s), 10

testing/btest/Baseline/language.blank-local-2/.stderr

@@ -0,0 +1 @@
+### BTest baseline data generated by btest-diff. Do not edit. Use "btest -U/-u" to update. Requires BTest >= 0.63.

testing/btest/Baseline/language.blank-local-3/.stderr

@@ -0,0 +1 @@
+### BTest baseline data generated by btest-diff. Do not edit. Use "btest -U/-u" to update. Requires BTest >= 0.63.

testing/btest/Baseline/language.blank-local-4/.stderr

@@ -0,0 +1 @@
+### BTest baseline data generated by btest-diff. Do not edit. Use "btest -U/-u" to update. Requires BTest >= 0.63.

testing/btest/Baseline/language.blank-local/.stderr

@@ -0,0 +1 @@
+### BTest baseline data generated by btest-diff. Do not edit. Use "btest -U/-u" to update. Requires BTest >= 0.63.

testing/btest/Baseline/language.blank-option-error/.stderr

@@ -0,0 +1,2 @@
+### BTest baseline data generated by btest-diff. Do not edit. Use "btest -U/-u" to update. Requires BTest >= 0.63.
+error in <...>/blank-option-error.zeek, line 9: blank identifier used as option (MyModule::_)

testing/btest/language/blank-expr-errors.zeek

@@ -0,0 +1,54 @@
+# @TEST-DOC: Do not allow to reference the blank identifier.
+
+# @TEST-EXEC-FAIL: zeek -b %INPUT
+# @TEST-EXEC: TEST_DIFF_CANONIFIER=$SCRIPTS/diff-remove-abspath btest-diff .stderr
+event zeek_init()
+	{
+	local vec = vector( "1", "2", "3" );
+	for ( _, v in vec )
+		print _;
+	}
+
+@TEST-START-NEXT
+event zeek_init()
+	{
+	local _ = vector( "1", "2", "3" );
+	print _;
+	}
+
+@TEST-START-NEXT
+# Ensure it does not work in a module, either.
+module MyModule;
+event zeek_init()
+	{
+	local _ = vector( "1", "2", "3" );
+	print _;
+	}
+
+@TEST-START-NEXT
+# Ensure _ can not referenced when it's a const in an export section.
+# Adding the const _ isn't an error though.
+module MyModule;
+
+export {
+	const _: count = 1;
+}
+
+event zeek_init()
+	{
+	print MyModule::_;
+	}
+
+@TEST-START-NEXT
+# Ensure it does not work in a function.
+module MyModule;
+function helper()
+	{
+	local _ = vector( "1", "2", "3" );
+	print _;
+	}
+
+event zeek_init()
+	{
+	helper();
+	}

testing/btest/language/blank-for.zeek

@@ -0,0 +1,70 @@
+# @TEST-DOC: Some blank identifier tests iterating over vectors, tables and strings.
+# @TEST-EXEC: zeek -b %INPUT > output
+# @TEST-EXEC: btest-diff output
+event zeek_init()
+	{
+	local vec = vector("a", "b", "c");
+	local t1 = table(["keya"] = "a", ["keyb"] = "b", ["keyc"] = "c");
+	local t2 = table(["a",1,T] = "a1a", ["b",2,F] = "b2b", ["c",3,T] = "c3c");
+	local s = "the string";
+
+	# Ignore just the index.
+	print "== vec 1";
+	for ( _, v in vec )
+		print v;
+
+	# Ignore just the value.
+	print "== vec 2";
+	local idxsum = 0;
+	for ( idx, _ in vec )
+		idxsum += idx;
+	print "idxsum(vec)", idxsum;
+
+	# Ignore index and value
+	print "== vec 3";
+	local veclen = 0;
+	for ( _, _ in vec )
+		++veclen;
+	print "veclen(vec)", veclen;
+
+	# Ignore just the key
+	print "== t1 1";
+	for ( _, v in t1 )
+		print v;
+
+	# Ignore just the value
+	print "== t1 2";
+	for ( k, _ in t1 )
+		print k;
+
+	# Ignore index and value
+	local t1len = 0;
+	print "== t1 3";
+	for ( _, _ in t1 )
+		++t1len;
+	print "t1len", t1len;
+
+	# Ignore part of the index and the value.
+	print "== t2 1";
+	for ( [_,c,_], v in t2 )
+		print c, v;
+
+	# Ignore part of the index and the value.
+	print "== t2 2";
+	for ( [t2a,_,t2b], _ in t2 )
+		print t2a, t2b;
+
+	# Ignore the whole index with a single _
+	print "== t2 3";
+	local t2concat = "";
+	for ( _, v in t2 )
+		t2concat += v;
+	print "t2concat", t2concat;
+
+	# String iteration ignoring the value
+	print "== s";
+	local i = 0;
+	for ( _ in s )
+		++i;
+	print "strlen(s)", i;
+	}

testing/btest/language/blank-local.zeek

@@ -0,0 +1,29 @@
+# @TEST-DOC: Locals work with the blank identifier, but can not be referenced.
+
+# @TEST-EXEC: zeek -b %INPUT
+# @TEST-EXEC: TEST_DIFF_CANONIFIER=$SCRIPTS/diff-remove-abspath btest-diff .stderr
+event zeek_init()
+	{
+	local _ = "1";
+	}
+
+#@TEST-START-NEXT
+event zeek_init()
+	{
+	local _: string = "1";
+	local _: count = 1;
+	}
+
+#@TEST-START-NEXT
+event zeek_init()
+	{
+	local _: string = "1";
+	const _: count = 1;
+	}
+
+#@TEST-START-NEXT
+event zeek_init()
+	{
+	const _: string = "1";
+	const _: count = 1;
+	}

testing/btest/language/blank-option-error.zeek

@@ -0,0 +1,10 @@
+# @TEST-DOC: Do not allow blank options.
+
+# @TEST-EXEC-FAIL: zeek -b %INPUT
+# @TEST-EXEC: TEST_DIFF_CANONIFIER=$SCRIPTS/diff-remove-abspath btest-diff .stderr
+
+module MyModule;
+
+export {
+	option _: count = 42;
+}