Rollup merge of #33406 - Manishearth:diag-improve-const-let, r=GuillaumeGomez

Improve diagnostics for constants being used in irrefutable patterns

It's pretty confusing and this error triggers in resolve only when "shadowing" a const, so let's make that clearer.

r? @steveklabnik

src/test/compile-fail/const-pattern-irrefutable.rs

@@ -19,10 +19,10 @@ use foo::d; //~ NOTE constant imported here
 const a: u8 = 2; //~ NOTE constant defined here
 
 fn main() {
-    let a = 4; //~ ERROR only irrefutable
+    let a = 4; //~ ERROR variable bindings cannot
                //~^ NOTE there already is a constant in scope
-    let c = 4; //~ ERROR only irrefutable
+    let c = 4; //~ ERROR variable bindings cannot
                //~^ NOTE there already is a constant in scope
-    let d = 4; //~ ERROR only irrefutable
+    let d = 4; //~ ERROR variable bindings cannot
                //~^ NOTE there already is a constant in scope
 }

src/test/compile-fail/issue-27033.rs

@@ -14,7 +14,7 @@ fn main() {
     };
     const C: u8 = 1;
     match 1 {
-        C @ 2 => { //~ ERROR only irrefutable patterns allowed here
+        C @ 2 => { //~ ERROR variable bindings cannot shadow constants
             println!("{}", C);
         }
         _ => {}