Address edition 2024 UI test failures

The (necessary) use of function signature inference for the read
function, in the absence of an actually valid function to infer
from, results in the compiler generating warnings about `!` to `()`
decay behaviour changing in Rust 2024, even though this code is
already doomed to fail. We will see once edition 2024 is released
whether the compiler emits a diagnostic about `BinRead` not being
implemented for `!` or not.

(cherry picked from commit 4ca8521)

binrw/tests/ui/invalid_map_fn.rs

@@ -1,3 +1,4 @@
+#![allow(dependency_on_unit_never_type_fallback)]
 use binrw::BinRead;
 
 #[derive(BinRead)]

binrw/tests/ui/invalid_map_fn.stderr

@@ -1,5 +1,5 @@
 error[E0425]: cannot find value `does_not_exist` in this scope
- --> $DIR/invalid_map_fn.rs:5:16
+ --> tests/ui/invalid_map_fn.rs:6:16
   |
-5 |     #[br(map = does_not_exist)]
+6 |     #[br(map = does_not_exist)]
   |                ^^^^^^^^^^^^^^ not found in this scope

binrw/tests/ui/invalid_map_return_type.rs

@@ -2,7 +2,7 @@ use binrw::BinRead;
 
 #[derive(BinRead)]
 struct Foo {
-    #[br(map = |_| 0u8)]
+    #[br(map = |_: ()| 0u8)]
     a: i32,
 }
 

binrw/tests/ui/invalid_map_return_type.stderr

@@ -1,10 +1,10 @@
 error[E0308]: mismatched types
- --> $DIR/invalid_map_return_type.rs:5:20
+ --> tests/ui/invalid_map_return_type.rs:5:24
   |
-5 |     #[br(map = |_| 0u8)]
-  |                    ^^^ expected `i32`, found `u8`
+5 |     #[br(map = |_: ()| 0u8)]
+  |                        ^^^ expected `i32`, found `u8`
   |
 help: change the type of the numeric literal from `u8` to `i32`
   |
-5 |     #[br(map = |_| 0i32)]
-  |                     ~~~
+5 |     #[br(map = |_: ()| 0i32)]
+  |                         ~~~

binrw/tests/ui/invalid_try_map_return_type.rs

@@ -2,7 +2,7 @@ use binrw::BinRead;
 
 #[derive(BinRead)]
 struct Foo {
-    #[br(try_map = |_| 0)]
+    #[br(try_map = |_: ()| 0)]
     a: i32,
 }
 

binrw/tests/ui/invalid_try_map_return_type.stderr

@@ -1,14 +1,14 @@
 error[E0308]: mismatched types
- --> tests/ui/invalid_try_map_return_type.rs:5:24
+ --> tests/ui/invalid_try_map_return_type.rs:5:28
   |
-5 |     #[br(try_map = |_| 0)]
-  |                        ^ expected `Result<i32, _>`, found integer
+5 |     #[br(try_map = |_: ()| 0)]
+  |                            ^ expected `Result<i32, _>`, found integer
   |
   = note: expected enum `Result<i32, _>`
              found type `{integer}`
 help: try wrapping the expression in a variant of `Result`
   |
-5 |     #[br(try_map = |_| Ok(0))]
-  |                        +++ +
-5 |     #[br(try_map = |_| Err(0))]
-  |                        ++++ +
+5 |     #[br(try_map = |_: ()| Ok(0))]
+  |                            +++ +
+5 |     #[br(try_map = |_: ()| Err(0))]
+  |                            ++++ +