Rollup merge of #128084 - surechen:fix_125997_v1, r=cjgillot

Suggest adding Result return type for associated method in E0277.

Recommit #126515 because I messed up during rebase,

Suggest adding Result return type for associated method in E0277.

For following:

```rust
struct A;
impl A {
    fn test4(&self) {
        let mut _file = File::create("foo.txt")?;
        //~^ ERROR the `?` operator can only be used in a method
    }
```

Suggest:

```rust
impl A {
    fn test4(&self) -> Result<(), Box<dyn std::error::Error>> {
        let mut _file = File::create("foo.txt")?;
        //~^ ERROR the `?` operator can only be used in a method

    Ok(())
    }
}
```

For #125997

r? `@cjgillot`

tests/ui/return/return-from-residual-sugg-issue-125997.fixed

@@ -33,6 +33,25 @@ macro_rules! mac {
     };
 }
 
+struct A;
+
+impl A {
+    fn test4(&self) -> Result<(), Box<dyn std::error::Error>> {
+        let mut _file = File::create("foo.txt")?;
+        //~^ ERROR the `?` operator can only be used in a method
+    
+    Ok(())
+}
+
+    fn test5(&self) -> Result<(), Box<dyn std::error::Error>> {
+        let mut _file = File::create("foo.txt")?;
+        //~^ ERROR the `?` operator can only be used in a method
+        println!();
+    
+    Ok(())
+}
+}
+
 fn main() -> Result<(), Box<dyn std::error::Error>> {
     let mut _file = File::create("foo.txt")?;
     //~^ ERROR the `?` operator can only be used in a function

tests/ui/return/return-from-residual-sugg-issue-125997.rs

@@ -27,6 +27,21 @@ macro_rules! mac {
     };
 }
 
+struct A;
+
+impl A {
+    fn test4(&self) {
+        let mut _file = File::create("foo.txt")?;
+        //~^ ERROR the `?` operator can only be used in a method
+    }
+
+    fn test5(&self) {
+        let mut _file = File::create("foo.txt")?;
+        //~^ ERROR the `?` operator can only be used in a method
+        println!();
+    }
+}
+
 fn main() {
     let mut _file = File::create("foo.txt")?;
     //~^ ERROR the `?` operator can only be used in a function

tests/ui/return/return-from-residual-sugg-issue-125997.stderr

@@ -37,8 +37,47 @@ LL +     Ok(())
 LL + }
    |
 
+error[E0277]: the `?` operator can only be used in a method that returns `Result` or `Option` (or another type that implements `FromResidual`)
+  --> $DIR/return-from-residual-sugg-issue-125997.rs:34:48
+   |
+LL |     fn test4(&self) {
+   |     --------------- this function should return `Result` or `Option` to accept `?`
+LL |         let mut _file = File::create("foo.txt")?;
+   |                                                ^ cannot use the `?` operator in a method that returns `()`
+   |
+   = help: the trait `FromResidual<Result<Infallible, std::io::Error>>` is not implemented for `()`
+help: consider adding return type
+   |
+LL ~     fn test4(&self) -> Result<(), Box<dyn std::error::Error>> {
+LL |         let mut _file = File::create("foo.txt")?;
+LL |
+LL ~     
+LL +     Ok(())
+LL + }
+   |
+
+error[E0277]: the `?` operator can only be used in a method that returns `Result` or `Option` (or another type that implements `FromResidual`)
+  --> $DIR/return-from-residual-sugg-issue-125997.rs:39:48
+   |
+LL |     fn test5(&self) {
+   |     --------------- this function should return `Result` or `Option` to accept `?`
+LL |         let mut _file = File::create("foo.txt")?;
+   |                                                ^ cannot use the `?` operator in a method that returns `()`
+   |
+   = help: the trait `FromResidual<Result<Infallible, std::io::Error>>` is not implemented for `()`
+help: consider adding return type
+   |
+LL ~     fn test5(&self) -> Result<(), Box<dyn std::error::Error>> {
+LL |         let mut _file = File::create("foo.txt")?;
+LL |
+LL |         println!();
+LL ~     
+LL +     Ok(())
+LL + }
+   |
+
 error[E0277]: the `?` operator can only be used in a function that returns `Result` or `Option` (or another type that implements `FromResidual`)
-  --> $DIR/return-from-residual-sugg-issue-125997.rs:31:44
+  --> $DIR/return-from-residual-sugg-issue-125997.rs:46:44
    |
 LL | fn main() {
    | --------- this function should return `Result` or `Option` to accept `?`
@@ -81,6 +120,6 @@ LL +     Ok(())
 LL + }
    |
 
-error: aborting due to 4 previous errors
+error: aborting due to 6 previous errors
 
 For more information about this error, try `rustc --explain E0277`.