Fix span note for question mark expression

Author: chenyukang

compiler/rustc_trait_selection/src/error_reporting/traits/fulfillment_errors.rs

@@ -1229,7 +1229,9 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
         chain.push((expr.span, prev_ty));
 
         let mut prev = None;
-        for (span, err_ty) in chain.into_iter().rev() {
+        let mut iter = chain.into_iter().rev().peekable();
+        while let Some((span, err_ty)) = iter.next() {
+            let is_last = iter.peek().is_none();
             let err_ty = get_e_type(err_ty);
             let err_ty = match (err_ty, prev) {
                 (Some(err_ty), Some(prev)) if !self.can_eq(obligation.param_env, err_ty, prev) => {
@@ -1241,27 +1243,27 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
                     continue;
                 }
             };
-            if self
+
+            let implements_from = self
                 .infcx
                 .type_implements_trait(
                     self.tcx.get_diagnostic_item(sym::From).unwrap(),
                     [self_ty, err_ty],
                     obligation.param_env,
                 )
-                .must_apply_modulo_regions()
-            {
-                if !suggested {
-                    let err_ty = self.tcx.short_string(err_ty, err.long_ty_path());
-                    err.span_label(span, format!("this has type `Result<_, {err_ty}>`"));
-                }
+                .must_apply_modulo_regions();
+
+            let err_ty_str = self.tcx.short_string(err_ty, err.long_ty_path());
+            let label = if !implements_from && is_last {
+                format!(
+                    "this can't be annotated with `?` because it has type `Result<_, {err_ty_str}>`"
+                )
             } else {
-                let err_ty = self.tcx.short_string(err_ty, err.long_ty_path());
-                err.span_label(
-                    span,
-                    format!(
-                        "this can't be annotated with `?` because it has type `Result<_, {err_ty}>`",
-                    ),
-                );
+                format!("this has type `Result<_, {err_ty_str}>`")
+            };
+
+            if !suggested || !implements_from {
+                err.span_label(span, label);
             }
             prev = Some(err_ty);
         }

tests/ui/traits/question-mark-result-err-mismatch.rs

@@ -9,7 +9,7 @@ fn foo() -> Result<String, String> { //~ NOTE expected `String` because of this
         });
     let one = x
         .map(|s| ())
-        .map_err(|e| { //~ NOTE this can't be annotated with `?` because it has type `Result<_, ()>`
+        .map_err(|e| { //~ NOTE this has type `Result<_, ()>`
             e; //~ HELP remove this semicolon
         })
         .map(|()| "")?; //~ ERROR `?` couldn't convert the error to `String`

tests/ui/traits/question-mark-result-err-mismatch.stderr

@@ -9,7 +9,7 @@ LL |           .map_err(|e| {
 LL | |             e;
    | |              - help: remove this semicolon
 LL | |         })
-   | |__________- this can't be annotated with `?` because it has type `Result<_, ()>`
+   | |__________- this has type `Result<_, ()>`
 LL |           .map(|()| "")?;
    |                        ^ the trait `From<()>` is not implemented for `String`
    |

tests/ui/traits/question-mark-span-144304.rs

@@ -0,0 +1,7 @@
+fn f() -> Result<(), i32> {
+    Err("str").map_err(|e| e)?; //~ ERROR `?` couldn't convert the error to `i32`
+    Err("str").map_err(|e| e.to_string())?; //~ ERROR `?` couldn't convert the error to `i32`
+    Ok(())
+}
+
+fn main() {}

tests/ui/traits/question-mark-span-144304.stderr

@@ -0,0 +1,41 @@
+error[E0277]: `?` couldn't convert the error to `i32`
+  --> $DIR/question-mark-span-144304.rs:2:30
+   |
+LL | fn f() -> Result<(), i32> {
+   |           --------------- expected `i32` because of this
+LL |     Err("str").map_err(|e| e)?;
+   |     ----------               ^ the trait `From<&str>` is not implemented for `i32`
+   |     |
+   |     this has type `Result<_, &str>`
+   |
+   = note: the question mark operation (`?`) implicitly performs a conversion on the error value using the `From` trait
+   = help: the following other types implement trait `From<T>`:
+             `i32` implements `From<bool>`
+             `i32` implements `From<i16>`
+             `i32` implements `From<i8>`
+             `i32` implements `From<u16>`
+             `i32` implements `From<u8>`
+
+error[E0277]: `?` couldn't convert the error to `i32`
+  --> $DIR/question-mark-span-144304.rs:3:42
+   |
+LL | fn f() -> Result<(), i32> {
+   |           --------------- expected `i32` because of this
+LL |     Err("str").map_err(|e| e)?;
+LL |     Err("str").map_err(|e| e.to_string())?;
+   |     ---------- --------------------------^ the trait `From<String>` is not implemented for `i32`
+   |     |          |
+   |     |          this can't be annotated with `?` because it has type `Result<_, String>`
+   |     this has type `Result<_, &str>`
+   |
+   = note: the question mark operation (`?`) implicitly performs a conversion on the error value using the `From` trait
+   = help: the following other types implement trait `From<T>`:
+             `i32` implements `From<bool>`
+             `i32` implements `From<i16>`
+             `i32` implements `From<i8>`
+             `i32` implements `From<u16>`
+             `i32` implements `From<u8>`
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0277`.