Rollup merge of #112529 - jieyouxu:block-expr-unused-must-use, r=oli-obk

Extend `unused_must_use` to cover block exprs

Given code like

```rust
#[must_use]
fn foo() -> i32 {
    42
}

fn warns() {
    {
        foo();
    }
}

fn does_not_warn() {
    {
        foo()
    };
}

fn main() {
    warns();
    does_not_warn();
}
```

### Before This PR

```
warning: unused return value of `foo` that must be used
 --> test.rs:8:9
  |
8 |         foo();
  |         ^^^^^
  |
  = note: `#[warn(unused_must_use)]` on by default
help: use `let _ = ...` to ignore the resulting value
  |
8 |         let _ = foo();
  |         +++++++

warning: 1 warning emitted
```

### After This PR

```
warning: unused return value of `foo` that must be used
 --> test.rs:8:9
  |
8 |         foo();
  |         ^^^^^
  |
  = note: `#[warn(unused_must_use)]` on by default
help: use `let _ = ...` to ignore the resulting value
  |
8 |         let _ = foo();
  |         +++++++

warning: unused return value of `foo` that must be used
  --> test.rs:14:9
   |
14 |         foo()
   |         ^^^^^
   |
help: use `let _ = ...` to ignore the resulting value
   |
14 |         let _ = foo();
   |         +++++++      +

warning: 2 warnings emitted
```

Fixes #104253.

Author: GuillaumeGomez

tests/fail/dangling_pointers/storage_dead_dangling.rs

@@ -10,7 +10,7 @@ fn fill(v: &mut i32) {
 }
 
 fn evil() {
-    unsafe { &mut *(LEAK as *mut i32) }; //~ ERROR: is a dangling pointer
+    let _ = unsafe { &mut *(LEAK as *mut i32) }; //~ ERROR: is a dangling pointer
 }
 
 fn main() {

tests/fail/dangling_pointers/storage_dead_dangling.stderr

@@ -1,8 +1,8 @@
 error: Undefined Behavior: dereferencing pointer failed: $HEX[noalloc] is a dangling pointer (it has no provenance)
   --> $DIR/storage_dead_dangling.rs:LL:CC
    |
-LL |     unsafe { &mut *(LEAK as *mut i32) };
-   |              ^^^^^^^^^^^^^^^^^^^^^^^^ dereferencing pointer failed: $HEX[noalloc] is a dangling pointer (it has no provenance)
+LL |     let _ = unsafe { &mut *(LEAK as *mut i32) };
+   |                      ^^^^^^^^^^^^^^^^^^^^^^^^ dereferencing pointer failed: $HEX[noalloc] is a dangling pointer (it has no provenance)
    |
    = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
    = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information

tests/fail/intrinsics/uninit_uninhabited_type.rs

@@ -2,6 +2,6 @@
 
 #[allow(deprecated, invalid_value)]
 fn main() {
-    unsafe { std::mem::uninitialized::<!>() };
+    let _ = unsafe { std::mem::uninitialized::<!>() };
     //~^ ERROR: attempted to instantiate uninhabited type `!`
 }

tests/fail/intrinsics/uninit_uninhabited_type.stderr

@@ -1,8 +1,8 @@
 error: abnormal termination: aborted execution: attempted to instantiate uninhabited type `!`
   --> $DIR/uninit_uninhabited_type.rs:LL:CC
    |
-LL |     unsafe { std::mem::uninitialized::<!>() };
-   |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ aborted execution: attempted to instantiate uninhabited type `!`
+LL |     let _ = unsafe { std::mem::uninitialized::<!>() };
+   |                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ aborted execution: attempted to instantiate uninhabited type `!`
    |
    = note: inside `main` at $DIR/uninit_uninhabited_type.rs:LL:CC
 

tests/fail/intrinsics/zero_fn_ptr.rs

@@ -1,5 +1,5 @@
 #[allow(deprecated, invalid_value)]
 fn main() {
-    unsafe { std::mem::zeroed::<fn()>() };
+    let _ = unsafe { std::mem::zeroed::<fn()>() };
     //~^ ERROR: attempted to zero-initialize type `fn()`, which is invalid
 }

tests/fail/intrinsics/zero_fn_ptr.stderr

@@ -1,8 +1,8 @@
 error: abnormal termination: aborted execution: attempted to zero-initialize type `fn()`, which is invalid
   --> $DIR/zero_fn_ptr.rs:LL:CC
    |
-LL |     unsafe { std::mem::zeroed::<fn()>() };
-   |              ^^^^^^^^^^^^^^^^^^^^^^^^^^ aborted execution: attempted to zero-initialize type `fn()`, which is invalid
+LL |     let _ = unsafe { std::mem::zeroed::<fn()>() };
+   |                      ^^^^^^^^^^^^^^^^^^^^^^^^^^ aborted execution: attempted to zero-initialize type `fn()`, which is invalid
    |
    = note: inside `main` at $DIR/zero_fn_ptr.rs:LL:CC