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.

compiler/rustc_lint/src/lints.rs

@@ -1555,8 +1555,29 @@ pub struct UnusedOp<'a> {
     pub op: &'a str,
     #[label]
     pub label: Span,
-    #[suggestion(style = "verbose", code = "let _ = ", applicability = "maybe-incorrect")]
-    pub suggestion: Span,
+    #[subdiagnostic]
+    pub suggestion: UnusedOpSuggestion,
+}
+
+#[derive(Subdiagnostic)]
+pub enum UnusedOpSuggestion {
+    #[suggestion(
+        lint_suggestion,
+        style = "verbose",
+        code = "let _ = ",
+        applicability = "maybe-incorrect"
+    )]
+    NormalExpr {
+        #[primary_span]
+        span: Span,
+    },
+    #[multipart_suggestion(lint_suggestion, style = "verbose", applicability = "maybe-incorrect")]
+    BlockTailExpr {
+        #[suggestion_part(code = "let _ = ")]
+        before_span: Span,
+        #[suggestion_part(code = ";")]
+        after_span: Span,
+    },
 }
 
 #[derive(LintDiagnostic)]
@@ -1599,15 +1620,25 @@ pub struct UnusedDef<'a, 'b> {
 }
 
 #[derive(Subdiagnostic)]
-#[suggestion(
-    lint_suggestion,
-    style = "verbose",
-    code = "let _ = ",
-    applicability = "maybe-incorrect"
-)]
-pub struct UnusedDefSuggestion {
-    #[primary_span]
-    pub span: Span,
+
+pub enum UnusedDefSuggestion {
+    #[suggestion(
+        lint_suggestion,
+        style = "verbose",
+        code = "let _ = ",
+        applicability = "maybe-incorrect"
+    )]
+    NormalExpr {
+        #[primary_span]
+        span: Span,
+    },
+    #[multipart_suggestion(lint_suggestion, style = "verbose", applicability = "maybe-incorrect")]
+    BlockTailExpr {
+        #[suggestion_part(code = "let _ = ")]
+        before_span: Span,
+        #[suggestion_part(code = ";")]
+        after_span: Span,
+    },
 }
 
 // Needed because of def_path_str