Introduce rust_at_least helper proc

This adds a 'rust_at_least' helper proc, for checking the version of
the Rust compiler in use.  It then changes various tests to use this
with 'require'.

gdb/testsuite/gdb.rust/rawids.exp

@@ -18,12 +18,7 @@
 load_lib rust-support.exp
 require allow_rust_tests
 require {can_compile rust}
-
-set v [split [rust_compiler_version] .]
-if {[lindex $v 0] == 1 && [lindex $v 1] < 30} {
-    untested "raw identifiers require rust 1.30 or greater"
-    return -1
-}
+require {rust_at_least 1.30}
 
 standard_testfile .rs
 if {[prepare_for_testing "failed to prepare" $testfile $srcfile {debug rust}]} {

gdb/testsuite/gdb.rust/unicode.exp

@@ -20,11 +20,7 @@ require allow_rust_tests
 require {can_compile rust}
 
 # Non-ASCII identifiers were allowed starting in 1.53.
-set v [split [rust_compiler_version] .]
-if {[lindex $v 0] == 1 && [lindex $v 1] < 53} {
-    untested "this test requires rust 1.53 or greater"
-    return -1
-}
+require {rust_at_least 1.53}
 
 # Enable basic use of UTF-8.  LC_ALL gets reset for each testfile.
 setenv LC_ALL C.UTF-8

gdb/testsuite/gdb.rust/unsized.exp

@@ -32,10 +32,7 @@ if {![runto ${srcfile}:$line]} {
 
 gdb_test "ptype us" " = .*V<\\\[u8\\\]>.*"
 
-set v [split [rust_compiler_version] .]
-# The necessary debuginfo generation landed in 1.60, but had a bug
-# that was fixed in 1.61.
-if {[lindex $v 0] > 1 || [lindex $v 1] >= 61} {
+if {[rust_at_least 1.61]} {
     gdb_test "print us2" " = .*Box<.*> \\\[1, 2, 3\\\]"
     gdb_test "ptype us2" "type = .*"
 }

gdb/testsuite/lib/rust-support.exp

@@ -112,3 +112,28 @@ gdb_caching_proc rust_compiler_version {} {
     }
     return 0.0
 }
+
+# A helper that checks that the rust compiler is at least the given
+# version.  This is handy for use with 'require'.
+proc rust_at_least {atleast} {
+    foreach n1 [split [rust_compiler_version] .] n2 [split $atleast .] {
+	if {$n1 == ""} {
+	    # Have 1.2, wanted 1.2.1.
+	    return 0
+	}
+	if {$n2 == ""} {
+	    # Have 1.2.1, wanted 1.2.
+	    return 1
+	}
+	if {$n1 > $n2} {
+	    # Have 1.3, wanted 1.2.
+	    return 1
+	}
+	if {$n1 < $n2} {
+	    # Have 1.1, wanted 1.2.
+	    return 0
+	}
+    }
+    # Completely equal.
+    return 1
+}