Provide context when ? can't be called because of Result<_, E>

When a method chain ending in `?` causes an E0277 because the
expression's `Result::Err` variant doesn't have a type that can be
converted to the `Result<_, E>` type parameter in the return type,
provide additional context of which parts of the chain can and can't
support the `?` operator.

```
error[E0277]: `?` couldn't convert the error to `String`
  --> $DIR/question-mark-result-err-mismatch.rs:28:25
   |
LL | fn bar() -> Result<(), String> {
   |             ------------------ expected `String` because of this
LL |     let x = foo();
   |             ----- this can be annotated with `?` because it has type `Result<String, String>`
LL |     let one = x
LL |         .map(|s| ())
   |          ----------- this can be annotated with `?` because it has type `Result<(), String>`
LL |         .map_err(|_| ())?;
   |          ---------------^ the trait `From<()>` is not implemented for `String`
   |          |
   |          this can't be annotated with `?` because it has type `Result<(), ()>`
   |
   = 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>`:
             <String as From<char>>
             <String as From<Box<str>>>
             <String as From<Cow<'a, str>>>
             <String as From<&str>>
             <String as From<&mut str>>
             <String as From<&String>>
   = note: required for `Result<(), String>` to implement `FromResidual<Result<Infallible, ()>>`
```

Fix #72124.

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

@@ -3,6 +3,7 @@ use super::suggestions::{get_explanation_based_on_obligation, TypeErrCtxtExt as
 use crate::errors::{ClosureFnMutLabel, ClosureFnOnceLabel, ClosureKindMismatch};
 use crate::infer::error_reporting::{TyCategory, TypeAnnotationNeeded as ErrorCode};
 use crate::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
+use crate::infer::InferCtxtExt as _;
 use crate::infer::{self, InferCtxt};
 use crate::traits::error_reporting::infer_ctxt_ext::InferCtxtExt;
 use crate::traits::error_reporting::{ambiguity, ambiguity::Ambiguity::*};
@@ -40,7 +41,7 @@ use rustc_session::config::{DumpSolverProofTree, TraitSolver};
 use rustc_session::Limit;
 use rustc_span::def_id::LOCAL_CRATE;
 use rustc_span::symbol::sym;
-use rustc_span::{ExpnKind, Span, DUMMY_SP};
+use rustc_span::{BytePos, ExpnKind, Span, DUMMY_SP};
 use std::borrow::Cow;
 use std::fmt;
 use std::iter;
@@ -106,6 +107,13 @@ pub trait TypeErrCtxtExt<'tcx> {
 
     fn fn_arg_obligation(&self, obligation: &PredicateObligation<'tcx>) -> bool;
 
+    fn try_conversion_context(
+        &self,
+        obligation: &PredicateObligation<'tcx>,
+        trait_ref: ty::TraitRef<'tcx>,
+        err: &mut Diagnostic,
+    );
+
     fn report_const_param_not_wf(
         &self,
         ty: Ty<'tcx>,
@@ -509,6 +517,10 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
 
                         let mut err = struct_span_err!(self.tcx.sess, span, E0277, "{}", err_msg);
 
+                        if is_try_conversion {
+                            self.try_conversion_context(&obligation, trait_ref.skip_binder(), &mut err);
+                        }
+
                         if is_try_conversion && let Some(ret_span) = self.return_type_span(&obligation) {
                             err.span_label(
                                 ret_span,
@@ -982,6 +994,136 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
         false
     }
 
+    /// When the `E` of the resulting `Result<T, E>` in an expression `foo().bar().baz()?`,
+    /// identify thoe method chain sub-expressions that could or could not have been annotated
+    /// with `?`.
+    fn try_conversion_context(
+        &self,
+        obligation: &PredicateObligation<'tcx>,
+        trait_ref: ty::TraitRef<'tcx>,
+        err: &mut Diagnostic,
+    ) {
+        let span = obligation.cause.span;
+        struct V<'v> {
+            search_span: Span,
+            found: Option<&'v hir::Expr<'v>>,
+        }
+        impl<'v> Visitor<'v> for V<'v> {
+            fn visit_expr(&mut self, ex: &'v hir::Expr<'v>) {
+                if let hir::ExprKind::Match(expr, _arms, hir::MatchSource::TryDesugar(_)) = ex.kind
+                {
+                    if ex.span.with_lo(ex.span.hi() - BytePos(1)).source_equal(self.search_span) {
+                        if let hir::ExprKind::Call(_, [expr, ..]) = expr.kind {
+                            self.found = Some(expr);
+                            return;
+                        }
+                    }
+                }
+                hir::intravisit::walk_expr(self, ex);
+            }
+        }
+        let hir_id = self.tcx.local_def_id_to_hir_id(obligation.cause.body_id);
+        let body_id = match self.tcx.hir().find(hir_id) {
+            Some(hir::Node::Item(hir::Item { kind: hir::ItemKind::Fn(_, _, body_id), .. })) => {
+                body_id
+            }
+            _ => return,
+        };
+        let mut v = V { search_span: span, found: None };
+        v.visit_body(self.tcx.hir().body(*body_id));
+        let Some(expr) = v.found else {
+            return;
+        };
+        let Some(typeck) = &self.typeck_results else {
+            return;
+        };
+        let Some((ObligationCauseCode::QuestionMark, Some(y))) = obligation.cause.code().parent()
+        else {
+            return;
+        };
+        if !self.tcx.is_diagnostic_item(sym::FromResidual, y.def_id()) {
+            return;
+        }
+        let self_ty = trait_ref.self_ty();
+
+        let mut prev_ty = self.resolve_vars_if_possible(
+            typeck.expr_ty_adjusted_opt(expr).unwrap_or(Ty::new_misc_error(self.tcx)),
+        );
+        let mut annotate_expr = |span: Span, prev_ty: Ty<'tcx>, self_ty: Ty<'tcx>| -> bool {
+            // We always look at the `E` type, because that's the only one affected by `?`. If the
+            // incorrect `Result<T, E>` is because of the `T`, we'll get an E0308 on the whole
+            // expression, after the `?` has "unwrapped" the `T`.
+            let ty::Adt(def, args) = prev_ty.kind() else {
+                return false;
+            };
+            let Some(arg) = args.get(1) else {
+                return false;
+            };
+            if !self.tcx.is_diagnostic_item(sym::Result, def.did()) {
+                return false;
+            }
+            let can = if self
+                .infcx
+                .type_implements_trait(
+                    self.tcx.get_diagnostic_item(sym::From).unwrap(),
+                    [self_ty.into(), *arg],
+                    obligation.param_env,
+                )
+                .must_apply_modulo_regions()
+            {
+                "can"
+            } else {
+                "can't"
+            };
+            err.span_label(
+                span,
+                format!("this {can} be annotated with `?` because it has type `{prev_ty}`"),
+            );
+            true
+        };
+
+        // The following logic is simlar to `point_at_chain`, but that's focused on associated types
+        let mut expr = expr;
+        while let hir::ExprKind::MethodCall(_path_segment, rcvr_expr, _args, span) = expr.kind {
+            // Point at every method call in the chain with the `Result` type.
+            // let foo = bar.iter().map(mapper)?;
+            //               ------ -----------
+            expr = rcvr_expr;
+            if !annotate_expr(span, prev_ty, self_ty) {
+                break;
+            }
+
+            prev_ty = self.resolve_vars_if_possible(
+                typeck.expr_ty_adjusted_opt(expr).unwrap_or(Ty::new_misc_error(self.tcx)),
+            );
+
+            if let hir::ExprKind::Path(hir::QPath::Resolved(None, path)) = expr.kind
+                && let hir::Path { res: hir::def::Res::Local(hir_id), .. } = path
+                && let Some(hir::Node::Pat(binding)) = self.tcx.hir().find(*hir_id)
+                && let Some(parent) = self.tcx.hir().find_parent(binding.hir_id)
+            {
+                // We've reached the root of the method call chain...
+                if let hir::Node::Local(local) = parent
+                    && let Some(binding_expr) = local.init
+                {
+                    // ...and it is a binding. Get the binding creation and continue the chain.
+                    expr = binding_expr;
+                }
+                if let hir::Node::Param(_param) = parent {
+                    // ...and it is a an fn argument.
+                    break;
+                }
+            }
+        }
+        // `expr` is now the "root" expression of the method call chain, which can be any
+        // expression kind, like a method call or a path. If this expression is `Result<T, E>` as
+        // well, then we also point at it.
+        prev_ty = self.resolve_vars_if_possible(
+            typeck.expr_ty_adjusted_opt(expr).unwrap_or(Ty::new_misc_error(self.tcx)),
+        );
+        annotate_expr(expr.span, prev_ty, self_ty);
+    }
+
     fn report_const_param_not_wf(
         &self,
         ty: Ty<'tcx>,

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

@@ -0,0 +1,59 @@
+fn foo() -> Result<String, String> { //~ NOTE expected `String` because of this
+    let test = String::from("one,two");
+    let x = test
+        .split_whitespace()
+        .next()
+        .ok_or_else(|| { //~ NOTE this can be annotated with `?` because it has type `Result<&str, &str>`
+            "Couldn't split the test string"
+        });
+    let one = x
+        .map(|s| ()) //~ NOTE this can be annotated with `?` because it has type `Result<(), &str>`
+        .map_err(|_| ()) //~ NOTE this can't be annotated with `?` because it has type `Result<(), ()>`
+        .map(|()| "")?; //~ ERROR `?` couldn't convert the error to `String`
+    //~^ NOTE in this expansion of desugaring of operator `?`
+    //~| NOTE in this expansion of desugaring of operator `?`
+    //~| NOTE in this expansion of desugaring of operator `?`
+    //~| NOTE in this expansion of desugaring of operator `?`
+    //~| NOTE this can't be annotated with `?` because it has type `Result<&str, ()>`
+    //~| NOTE the trait `From<()>` is not implemented for `String`
+    //~| NOTE the question mark operation (`?`) implicitly performs a conversion on the error value using the `From` trait
+    //~| NOTE required for `Result<String, String>` to implement `FromResidual<Result<Infallible, ()>>`
+    Ok(one.to_string())
+}
+
+fn bar() -> Result<(), String> { //~ NOTE expected `String` because of this
+    let x = foo(); //~ NOTE this can be annotated with `?` because it has type `Result<String, String>`
+    let one = x
+        .map(|s| ()) //~ NOTE this can be annotated with `?` because it has type `Result<(), String>`
+        .map_err(|_| ())?; //~ ERROR `?` couldn't convert the error to `String`
+    //~^ NOTE in this expansion of desugaring of operator `?`
+    //~| NOTE in this expansion of desugaring of operator `?`
+    //~| NOTE in this expansion of desugaring of operator `?`
+    //~| NOTE in this expansion of desugaring of operator `?`
+    //~| NOTE this can't be annotated with `?` because it has type `Result<(), ()>`
+    //~| NOTE the trait `From<()>` is not implemented for `String`
+    //~| NOTE the question mark operation (`?`) implicitly performs a conversion on the error value using the `From` trait
+    //~| NOTE required for `Result<(), String>` to implement `FromResidual<Result<Infallible, ()>>`
+    Ok(one)
+}
+
+fn baz() -> Result<String, String> { //~ NOTE expected `String` because of this
+    let test = String::from("one,two");
+    let one = test
+        .split_whitespace()
+        .next()
+        .ok_or_else(|| { //~ NOTE this can't be annotated with `?` because it has type `Result<&str, ()>`
+            "Couldn't split the test string";
+        })?;
+    //~^ ERROR `?` couldn't convert the error to `String`
+    //~| NOTE in this expansion of desugaring of operator `?`
+    //~| NOTE in this expansion of desugaring of operator `?`
+    //~| NOTE in this expansion of desugaring of operator `?`
+    //~| NOTE in this expansion of desugaring of operator `?`
+    //~| NOTE the trait `From<()>` is not implemented for `String`
+    //~| NOTE the question mark operation (`?`) implicitly performs a conversion on the error value using the `From` trait
+    //~| NOTE required for `Result<String, String>` to implement `FromResidual<Result<Infallible, ()>>`
+    Ok(one.to_string())
+}
+
+fn main() {}

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

@@ -0,0 +1,83 @@
+error[E0277]: `?` couldn't convert the error to `String`
+  --> $DIR/question-mark-result-err-mismatch.rs:12:22
+   |
+LL |   fn foo() -> Result<String, String> {
+   |               ---------------------- expected `String` because of this
+...
+LL |           .ok_or_else(|| {
+   |  __________-
+LL | |             "Couldn't split the test string"
+LL | |         });
+   | |__________- this can be annotated with `?` because it has type `Result<&str, &str>`
+LL |       let one = x
+LL |           .map(|s| ())
+   |            ----------- this can be annotated with `?` because it has type `Result<(), &str>`
+LL |           .map_err(|_| ())
+   |            --------------- this can't be annotated with `?` because it has type `Result<(), ()>`
+LL |           .map(|()| "")?;
+   |            ------------^ the trait `From<()>` is not implemented for `String`
+   |            |
+   |            this can't be annotated with `?` because it 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>`:
+             <String as From<char>>
+             <String as From<Box<str>>>
+             <String as From<Cow<'a, str>>>
+             <String as From<&str>>
+             <String as From<&mut str>>
+             <String as From<&String>>
+   = note: required for `Result<String, String>` to implement `FromResidual<Result<Infallible, ()>>`
+
+error[E0277]: `?` couldn't convert the error to `String`
+  --> $DIR/question-mark-result-err-mismatch.rs:28:25
+   |
+LL | fn bar() -> Result<(), String> {
+   |             ------------------ expected `String` because of this
+LL |     let x = foo();
+   |             ----- this can be annotated with `?` because it has type `Result<String, String>`
+LL |     let one = x
+LL |         .map(|s| ())
+   |          ----------- this can be annotated with `?` because it has type `Result<(), String>`
+LL |         .map_err(|_| ())?;
+   |          ---------------^ the trait `From<()>` is not implemented for `String`
+   |          |
+   |          this can't be annotated with `?` because it has type `Result<(), ()>`
+   |
+   = 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>`:
+             <String as From<char>>
+             <String as From<Box<str>>>
+             <String as From<Cow<'a, str>>>
+             <String as From<&str>>
+             <String as From<&mut str>>
+             <String as From<&String>>
+   = note: required for `Result<(), String>` to implement `FromResidual<Result<Infallible, ()>>`
+
+error[E0277]: `?` couldn't convert the error to `String`
+  --> $DIR/question-mark-result-err-mismatch.rs:47:11
+   |
+LL |   fn baz() -> Result<String, String> {
+   |               ---------------------- expected `String` because of this
+...
+LL |           .ok_or_else(|| {
+   |  __________-
+LL | |             "Couldn't split the test string";
+LL | |         })?;
+   | |          -^ the trait `From<()>` is not implemented for `String`
+   | |__________|
+   |            this can't be annotated with `?` because it 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>`:
+             <String as From<char>>
+             <String as From<Box<str>>>
+             <String as From<Cow<'a, str>>>
+             <String as From<&str>>
+             <String as From<&mut str>>
+             <String as From<&String>>
+   = note: required for `Result<String, String>` to implement `FromResidual<Result<Infallible, ()>>`
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0277`.

tests/ui/try-block/try-block-bad-type.stderr

@@ -2,7 +2,9 @@ error[E0277]: `?` couldn't convert the error to `TryFromSliceError`
   --> $DIR/try-block-bad-type.rs:7:16
    |
 LL |         Err("")?;
-   |                ^ the trait `From<&str>` is not implemented for `TryFromSliceError`
+   |         -------^ the trait `From<&str>` is not implemented for `TryFromSliceError`
+   |         |
+   |         this can't be annotated with `?` because it has type `Result<_, &str>`
    |
    = note: the question mark operation (`?`) implicitly performs a conversion on the error value using the `From` trait
    = help: the trait `From<Infallible>` is implemented for `TryFromSliceError`

tests/ui/try-trait/bad-interconversion.stderr

@@ -4,7 +4,9 @@ error[E0277]: `?` couldn't convert the error to `u8`
 LL | fn result_to_result() -> Result<u64, u8> {
    |                          --------------- expected `u8` because of this
 LL |     Ok(Err(123_i32)?)
-   |                    ^ the trait `From<i32>` is not implemented for `u8`
+   |        ------------^ the trait `From<i32>` is not implemented for `u8`
+   |        |
+   |        this can't be annotated with `?` because it has type `Result<_, i32>`
    |
    = 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>`:

tests/ui/try-trait/issue-32709.stderr

@@ -4,7 +4,9 @@ error[E0277]: `?` couldn't convert the error to `()`
 LL | fn a() -> Result<i32, ()> {
    |           --------------- expected `()` because of this
 LL |     Err(5)?;
-   |           ^ the trait `From<{integer}>` is not implemented for `()`
+   |     ------^ the trait `From<{integer}>` is not implemented for `()`
+   |     |
+   |     this can't be annotated with `?` because it has type `Result<_, {integer}>`
    |
    = 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>`: