Add a new test.

It covers some patterns that are relevant for upcoming changes to
`parse_literal_maybe_minus`.

Author: nnethercote

tests/ui/macros/parse-literal-maybe-minus.rs

@@ -0,0 +1,64 @@
+// Tests some cases relating to `parse_literal_maybe_minus`, which used to
+// accept more interpolated expressions than it should have.
+
+const A: u32 = 0;
+const B: u32 = 1;
+
+macro_rules! identity {
+    ($val:expr) => { $val }
+}
+
+macro_rules! range1 {
+    ($min:expr) => {
+        $min..
+    }
+}
+
+macro_rules! range2 {
+    ($min:expr, $max:expr) => {
+        $min ..= $max
+    }
+}
+
+macro_rules! range3 {
+    ($max:expr) => {
+        .. $max
+    }
+}
+
+macro_rules! m {
+    ($a:expr, $b:ident, $identity_mac_call:expr) => {
+        fn _f() {
+            match [1, 2] {
+                [$a, $b] => {}  // FIXME: doesn't compile, probably should
+                _ => {}
+            }
+
+            match (1, 2) {
+                ($a, $b) => {}  // FIXME: doesn't compile, probably should
+                _ => {}
+            }
+
+            match 3 {
+                $identity_mac_call => {}
+                0..$identity_mac_call => {}
+                _ => {}
+            }
+        }
+    };
+}
+
+m!(A, B, identity!(10));
+//~^ ERROR arbitrary expressions aren't allowed in patterns
+//~| ERROR arbitrary expressions aren't allowed in patterns
+
+fn main() {
+    match 3 {
+        identity!(A) => {}  // FIXME: doesn't compile, probably should
+        //~^ ERROR arbitrary expressions aren't allowed in patterns
+        range1!(A) => {}
+        range2!(A, B) => {}
+        range3!(B) => {}
+        _ => {}
+    }
+}

tests/ui/macros/parse-literal-maybe-minus.stderr

@@ -0,0 +1,27 @@
+error: arbitrary expressions aren't allowed in patterns
+  --> $DIR/parse-literal-maybe-minus.rs:57:19
+   |
+LL |         identity!(A) => {}  // FIXME: doesn't compile, probably should
+   |                   ^
+   |
+   = note: the `expr` fragment specifier forces the metavariable's content to be an expression
+
+error: arbitrary expressions aren't allowed in patterns
+  --> $DIR/parse-literal-maybe-minus.rs:51:4
+   |
+LL | m!(A, B, identity!(10));
+   |    ^
+   |
+   = note: the `expr` fragment specifier forces the metavariable's content to be an expression
+
+error: arbitrary expressions aren't allowed in patterns
+  --> $DIR/parse-literal-maybe-minus.rs:51:4
+   |
+LL | m!(A, B, identity!(10));
+   |    ^
+   |
+   = note: the `expr` fragment specifier forces the metavariable's content to be an expression
+   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
+
+error: aborting due to 3 previous errors
+