Enhance type inference errors involving the ? operator

compiler/rustc_infer/src/infer/error_reporting/mod.rs

@@ -69,7 +69,7 @@ use rustc_middle::ty::{
     subst::{Subst, SubstsRef},
     Region, Ty, TyCtxt, TypeFoldable,
 };
-use rustc_span::{BytePos, DesugaringKind, Pos, Span};
+use rustc_span::{sym, BytePos, DesugaringKind, Pos, Span};
 use rustc_target::spec::abi;
 use std::ops::ControlFlow;
 use std::{cmp, fmt};
@@ -2282,6 +2282,14 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
         self.note_region_origin(&mut err, &sub_origin);
         err.emit();
     }
+
+    /// Determine whether an error associated with the given span and definition
+    /// should be treated as being caused by the implicit `From` conversion
+    /// within `?` desugaring.
+    pub fn is_try_conversion(&self, span: Span, trait_def_id: DefId) -> bool {
+        span.is_desugaring(DesugaringKind::QuestionMark)
+            && self.tcx.is_diagnostic_item(sym::from_trait, trait_def_id)
+    }
 }
 
 impl<'a, 'tcx> InferCtxt<'a, 'tcx> {

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

@@ -280,18 +280,10 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
                         let OnUnimplementedNote { message, label, note, enclosing_scope } =
                             self.on_unimplemented_note(trait_ref, obligation);
                         let have_alt_message = message.is_some() || label.is_some();
-                        let is_try = self
-                            .tcx
-                            .sess
-                            .source_map()
-                            .span_to_snippet(span)
-                            .map(|s| &s == "?")
-                            .unwrap_or(false);
-                        let is_from = self.tcx.get_diagnostic_item(sym::from_trait)
-                            == Some(trait_ref.def_id());
+                        let is_try_conversion = self.is_try_conversion(span, trait_ref.def_id());
                         let is_unsize =
                             { Some(trait_ref.def_id()) == self.tcx.lang_items().unsize_trait() };
-                        let (message, note) = if is_try && is_from {
+                        let (message, note) = if is_try_conversion {
                             (
                                 Some(format!(
                                     "`?` couldn't convert the error to `{}`",
@@ -319,7 +311,7 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
                             ))
                         );
 
-                        if is_try && is_from {
+                        if is_try_conversion {
                             let none_error = self
                                 .tcx
                                 .get_diagnostic_item(sym::none_error)

src/test/ui/inference/cannot-infer-async-enabled-impl-trait-bindings.stderr

@@ -13,7 +13,9 @@ error[E0282]: type annotations needed for `impl Future`
 LL |     let fut = async {
    |         --- consider giving `fut` the explicit type `impl Future`, with the type parameters specified
 LL |         make_unit()?;
-   |                    ^ cannot infer type
+   |                    ^ cannot infer type of error for `?` operator
+   |
+   = note: `?` implicitly converts the error value into a type implementing `From<std::io::Error>`
 
 error: aborting due to previous error; 1 warning emitted
 

src/test/ui/inference/cannot-infer-async.stderr

@@ -4,7 +4,9 @@ error[E0282]: type annotations needed
 LL |     let fut = async {
    |         --- consider giving `fut` a type
 LL |         make_unit()?;
-   |                    ^ cannot infer type
+   |                    ^ cannot infer type of error for `?` operator
+   |
+   = note: `?` implicitly converts the error value into a type implementing `From<std::io::Error>`
 
 error: aborting due to previous error
 

src/test/ui/inference/cannot-infer-closure-circular.rs

@@ -0,0 +1,14 @@
+fn main() {
+    // Below we call the closure with its own return as the argument, unifying
+    // its inferred input and return types. We want to make sure that the generated
+    // error handles this gracefully, and in particular doesn't generate an extra
+    // note about the `?` operator in the closure body, which isn't relevant to
+    // the inference.
+    let x = |r| {
+        //~^ ERROR type annotations needed
+        let v = r?;
+        Ok(v)
+    };
+
+    let _ = x(x(Ok(())));
+}

src/test/ui/inference/cannot-infer-closure-circular.stderr

@@ -0,0 +1,9 @@
+error[E0282]: type annotations needed for `std::result::Result<(), E>`
+  --> $DIR/cannot-infer-closure-circular.rs:7:14
+   |
+LL |     let x = |r| {
+   |              ^ consider giving this closure parameter the explicit type `std::result::Result<(), E>`, with the type parameters specified
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0282`.

src/test/ui/inference/cannot-infer-closure.stderr

@@ -2,8 +2,9 @@ error[E0282]: type annotations needed for the closure `fn((), ()) -> std::result
   --> $DIR/cannot-infer-closure.rs:3:15
    |
 LL |         Err(a)?;
-   |               ^ cannot infer type
+   |               ^ cannot infer type of error for `?` operator
    |
+   = note: `?` implicitly converts the error value into a type implementing `From<()>`
 help: give this closure an explicit return type without `_` placeholders
    |
 LL |     let x = |a: (), b: ()| -> std::result::Result<(), _> {

src/test/ui/inference/cannot-infer-partial-try-return.rs

@@ -0,0 +1,22 @@
+struct QualifiedError<E>(E);
+
+impl<E, T> From<E> for QualifiedError<T>
+where
+    E: std::error::Error,
+    T: From<E>,
+{
+    fn from(e: E) -> QualifiedError<T> {
+        QualifiedError(e.into())
+    }
+}
+
+fn infallible() -> Result<(), std::convert::Infallible> {
+    Ok(())
+}
+
+fn main() {
+    let x = || -> Result<_, QualifiedError<_>> {
+        infallible()?; //~ERROR type annotations needed
+        Ok(())
+    };
+}

src/test/ui/inference/cannot-infer-partial-try-return.stderr

@@ -0,0 +1,15 @@
+error[E0282]: type annotations needed for the closure `fn() -> std::result::Result<(), QualifiedError<_>>`
+  --> $DIR/cannot-infer-partial-try-return.rs:19:9
+   |
+LL |         infallible()?;
+   |         ^^^^^^^^^^^^^ cannot infer type of error for `?` operator
+   |
+   = note: `?` implicitly converts the error value into `QualifiedError<_>` using its implementation of `From<Infallible>`
+help: give this closure an explicit return type without `_` placeholders
+   |
+LL |     let x = || -> std::result::Result<(), QualifiedError<_>> {
+   |                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0282`.