add a test for byte string literal pattern mutability mismatches

Author: dianne

tests/ui/pattern/byte-string-mutability-mismatch.rs

@@ -0,0 +1,19 @@
+//! Byte string literal patterns use the mutability of the literal, rather than the mutability of
+//! the pattern's scrutinee. Since byte string literals are always shared references, it's a
+//! mismatch to use a byte string literal pattern to match on a mutable array or slice reference.
+
+fn main() {
+    let mut val = [97u8, 10u8];
+    match &mut val {
+        b"a\n" => {},
+        //~^ ERROR mismatched types
+        //~| types differ in mutability
+        _ => {},
+    }
+    match &mut val[..] {
+         b"a\n" => {},
+        //~^ ERROR mismatched types
+        //~| types differ in mutability
+         _ => {},
+    }
+}

tests/ui/pattern/byte-string-mutability-mismatch.stderr

@@ -0,0 +1,25 @@
+error[E0308]: mismatched types
+  --> $DIR/byte-string-mutability-mismatch.rs:8:9
+   |
+LL |     match &mut val {
+   |           -------- this expression has type `&mut [u8; 2]`
+LL |         b"a\n" => {},
+   |         ^^^^^^ types differ in mutability
+   |
+   = note: expected mutable reference `&mut _`
+                      found reference `&'static _`
+
+error[E0308]: mismatched types
+  --> $DIR/byte-string-mutability-mismatch.rs:14:10
+   |
+LL |     match &mut val[..] {
+   |           ------------ this expression has type `&mut [u8]`
+LL |          b"a\n" => {},
+   |          ^^^^^^ types differ in mutability
+   |
+   = note: expected mutable reference `&mut _`
+                      found reference `&'static _`
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0308`.