Avoid &format("...") calls in error message code.

Error message all end up passing into a function as an `impl
Into<{D,Subd}iagnosticMessage>`. If an error message is creatd as
`&format("...")` that means we allocate a string (in the `format!`
call), then take a reference, and then clone (allocating again) the
reference to produce the `{D,Subd}iagnosticMessage`, which is silly.

This commit removes the leading `&` from a lot of these cases. This
means the original `String` is moved into the
`{D,Subd}iagnosticMessage`, avoiding the double allocations. This
requires changing some function argument types from `&str` to `String`
(when all arguments are `String`) or `impl
Into<{D,Subd}iagnosticMessage>` (when some arguments are `String` and
some are `&str`).

compiler/rustc_trait_selection/src/traits/project.rs

@@ -2260,7 +2260,7 @@ fn confirm_param_env_candidate<'cx, 'tcx>(
                 obligation, poly_cache_entry, e,
             );
             debug!("confirm_param_env_candidate: {}", msg);
-            let err = infcx.tcx.ty_error_with_message(obligation.cause.span, &msg);
+            let err = infcx.tcx.ty_error_with_message(obligation.cause.span, msg);
             Progress { term: err.into(), obligations: vec![] }
         }
     }