rust-lang · rust-bors · Mar 16, 2026 · Mar 15, 2026
diff --git a/compiler/rustc_trait_selection/src/error_reporting/infer/need_type_info.rs b/compiler/rustc_trait_selection/src/error_reporting/infer/need_type_info.rs
@@ -555,6 +555,7 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
                 def_id: _,
                 generic_args,
                 have_turbofish,
+                hir_id,
             } => {
                 let generics = self.tcx.generics_of(generics_def_id);
                 let is_type = term.as_type().is_some();
@@ -577,7 +578,32 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
                 let args = if self.tcx.get_diagnostic_item(sym::iterator_collect_fn)
                     == Some(generics_def_id)
                 {
-                    "Vec<_>".to_string()
+                    if let hir::Node::Expr(expr) = self.tcx.parent_hir_node(hir_id)
+                        && let hir::ExprKind::Call(expr, _args) = expr.kind
+                        && let hir::ExprKind::Path(hir::QPath::Resolved(_, path)) = expr.kind
+                        && let Res::Def(DefKind::AssocFn, def_id) = path.res
+                        && let Some(try_trait) = self.tcx.lang_items().try_trait()
+                        && try_trait == self.tcx.parent(def_id)
+                        && let DefKind::Fn | DefKind::AssocFn =
+                            self.tcx.def_kind(body_def_id.to_def_id())
+                        && let ret = self
+                            .tcx
+                            .fn_sig(body_def_id.to_def_id())
+                            .instantiate_identity()
+                            .skip_binder()
+                            .output()
+                        && let ty::Adt(adt, _args) = ret.kind()
+                        && let Some(sym::Option | sym::Result) =
+                            self.tcx.get_diagnostic_name(adt.did())
+                    {
+                        if let Some(sym::Option) = self.tcx.get_diagnostic_name(adt.did()) {
+                            "Option<_>".to_string()
+                        } else {
+                            "Result<_, _>".to_string()
+                        }
+                    } else {
+                        "Vec<_>".to_string()
+                    }
                 } else {
                     let mut p = fmt_printer(self, Namespace::TypeNS);
                     p.comma_sep(generic_args.iter().copied().map(|arg| {
@@ -710,6 +736,7 @@ enum InferSourceKind<'tcx> {
         def_id: DefId,
         generic_args: &'tcx [GenericArg<'tcx>],
         have_turbofish: bool,
+        hir_id: HirId,
     },
     FullyQualifiedMethodCall {
         receiver: &'tcx Expr<'tcx>,
@@ -1188,19 +1215,45 @@ impl<'a, 'tcx> Visitor<'tcx> for FindInferSourceVisitor<'a, 'tcx> {
 
         if let Some(ty) = self.opt_node_type(local.hir_id) {
             if self.generic_arg_contains_target(ty.into()) {
-                match local.source {
-                    LocalSource::Normal if local.ty.is_none() => {
-                        self.update_infer_source(InferSource {
-                            span: local.pat.span,
-                            kind: InferSourceKind::LetBinding {
-                                insert_span: local.pat.span.shrink_to_hi(),
-                                pattern_name: local.pat.simple_ident(),
-                                ty,
-                                def_id: None,
-                            },
-                        })
+                fn get_did(
+                    typeck_results: &TypeckResults<'_>,
+                    expr: &hir::Expr<'_>,
+                ) -> Option<DefId> {
+                    match expr.kind {
+                        hir::ExprKind::Match(expr, _, hir::MatchSource::TryDesugar(_))
+                            if let hir::ExprKind::Call(_, [expr]) = expr.kind =>
+                        {
+                            get_did(typeck_results, expr)
+                        }
+                        hir::ExprKind::Call(base, _args)
+                            if let hir::ExprKind::Path(path) = base.kind
+                                && let hir::QPath::Resolved(_, path) = path
+                                && let Res::Def(_, did) = path.res =>
+                        {
+                            Some(did)
+                        }
+                        hir::ExprKind::MethodCall(..)
+                            if let Some(did) =
+                                typeck_results.type_dependent_def_id(expr.hir_id) =>
+                        {
+                            Some(did)
+                        }
+                        _ => None,
                     }
-                    _ => {}
+                }
+
+                if let LocalSource::Normal = local.source
+                    && local.ty.is_none()
+                {
+                    self.update_infer_source(InferSource {
+                        span: local.pat.span,
+                        kind: InferSourceKind::LetBinding {
+                            insert_span: local.pat.span.shrink_to_hi(),
+                            pattern_name: local.pat.simple_ident(),
+                            ty,
+                            def_id: local.init.and_then(|expr| get_did(self.typeck_results, expr)),
+                        },
+                    });
                 }
             }
         }
@@ -1285,6 +1338,7 @@ impl<'a, 'tcx> Visitor<'tcx> for FindInferSourceVisitor<'a, 'tcx> {
                         def_id,
                         generic_args,
                         have_turbofish,
+                        hir_id: expr.hir_id,
                     },
                 });
             }

diff --git a/tests/ui/inference/question-mark-type-infer.rs b/tests/ui/inference/question-mark-type-infer.rs
@@ -10,6 +10,20 @@ fn g() -> Result<Vec<i32>, ()> {
     l.iter().map(f).collect()?
     //~^ ERROR type annotations needed
 }
+fn h() -> Result<(), ()> {
+    let l = [1, 2, 3, 4];
+    // The resulting binding doesn't have a type, so we need to guess the `Ok` type too.
+    let x = l.iter().map(f).collect()?;
+    //~^ ERROR type annotations needed
+    Ok(())
+}
+fn i() -> Result<(), ()> {
+    let l = [1, 2, 3, 4];
+    // The resulting binding already has a type, so we don't need to specify the `Ok` type.
+    let x: Vec<i32> = l.iter().map(f).collect()?;
+    //~^ ERROR type annotations needed
+    Ok(())
+}
 
 fn main() {
     g();

diff --git a/tests/ui/inference/question-mark-type-infer.stderr b/tests/ui/inference/question-mark-type-infer.stderr
@@ -9,9 +9,37 @@ note: required by a bound in `collect`
   --> $SRC_DIR/core/src/iter/traits/iterator.rs:LL:COL
 help: consider specifying the generic argument
    |
-LL |     l.iter().map(f).collect::<Vec<_>>()?
-   |                            ++++++++++
+LL |     l.iter().map(f).collect::<Result<_, _>>()?
+   |                            ++++++++++++++++
 
-error: aborting due to 1 previous error
+error[E0283]: type annotations needed
+  --> $DIR/question-mark-type-infer.rs:16:29
+   |
+LL |     let x = l.iter().map(f).collect()?;
+   |                             ^^^^^^^ cannot infer type of the type parameter `B` declared on the method `collect`
+   |
+   = note: cannot satisfy `_: FromIterator<Result<i32, ()>>`
+note: required by a bound in `collect`
+  --> $SRC_DIR/core/src/iter/traits/iterator.rs:LL:COL
+help: consider specifying the generic argument
+   |
+LL |     let x = l.iter().map(f).collect::<Result<_, _>>()?;
+   |                                    ++++++++++++++++
+
+error[E0283]: type annotations needed
+  --> $DIR/question-mark-type-infer.rs:23:39
+   |
+LL |     let x: Vec<i32> = l.iter().map(f).collect()?;
+   |                                       ^^^^^^^ cannot infer type of the type parameter `B` declared on the method `collect`
+   |
+   = note: cannot satisfy `_: FromIterator<Result<i32, ()>>`
+note: required by a bound in `collect`
+  --> $SRC_DIR/core/src/iter/traits/iterator.rs:LL:COL
+help: consider specifying the generic argument
+   |
+LL |     let x: Vec<i32> = l.iter().map(f).collect::<Result<_, _>>()?;
+   |                                              ++++++++++++++++
+
+error: aborting due to 3 previous errors
 
 For more information about this error, try `rustc --explain E0283`.
diff --git a/tests/ui/inference/question-mark-type-inference-in-chain.rs b/tests/ui/inference/question-mark-type-inference-in-chain.rs
@@ -0,0 +1,84 @@
+// #129269
+use std::fmt::Display;
+
+#[derive(Debug)]
+struct AnotherError;
+type Result<T, E = AnotherError> = core::result::Result<T, E>;
+
+#[derive(Debug)]
+pub struct Error;
+
+impl From<AnotherError> for Error {
+    fn from(_: AnotherError) -> Self { Error }
+}
+
+impl std::error::Error for Error {}
+
+impl Display for Error {
+    fn fmt(&self, _f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        todo!()
+    }
+}
+
+#[derive(Ord, PartialEq, PartialOrd, Eq)]
+pub struct Version {}
+
+fn parse(_s: &str) -> std::result::Result<Version, Error> {
+    todo!()
+}
+
+pub fn error1(lines: &[&str]) -> Result<Vec<Version>> {
+    let mut tags = lines.iter().map(|e| parse(e)).collect()?;
+    //~^ ERROR: type annotations needed
+    //~| HELP: consider giving `tags` an explicit type
+
+    tags.sort(); //~ NOTE: type must be known at this point
+
+    Ok(tags)
+}
+
+pub fn error2(lines: &[&str]) -> Result<Vec<Version>> {
+    let mut tags: Vec<Version> = lines.iter().map(|e| parse(e)).collect()?;
+    //~^ ERROR: type annotations needed
+    //~| NOTE: cannot infer type of the type parameter `B`
+    //~| NOTE: cannot satisfy `_: FromIterator<std::result::Result<Version, Error>>`
+    //~| NOTE: required by a bound in `collect`
+    //~| HELP: consider specifying the generic argument
+    tags.sort();
+
+    Ok(tags)
+}
+
+pub fn error3(lines: &[&str]) -> Result<Vec<Version>> {
+    let mut tags = lines.iter().map(|e| parse(e)).collect::<Vec<_>>()?;
+    //~^ ERROR: the `?` operator can only be applied to values that implement `Try`
+    //~| NOTE: the `?` operator cannot be applied to type `Vec<std::result::Result<Version, Error>>`
+    //~| HELP: the nightly-only, unstable trait `Try` is not implemented
+    //~| NOTE: in this expansion of desugaring of operator `?`
+    //~| NOTE: in this expansion of desugaring of operator `?`
+    tags.sort();
+
+    Ok(tags)
+}
+
+pub fn error4(lines: &[&str]) -> Result<Vec<Version>> {
+    let mut tags = lines
+        //~^ NOTE: this expression has type `&[&str]`
+        .iter()
+        //~^ NOTE: `Iterator::Item` is `&&str` here
+        .map(|e| parse(e))
+        //~^ NOTE: the method call chain might not have had the expected associated types
+        //~| NOTE: `Iterator::Item` changed to `Result<Version, Error>` here
+        .collect::<Result<Vec<Version>>>()?;
+        //~^ ERROR: a value of type `std::result::Result<Vec<Version>, AnotherError>` cannot be built from an iterator over elements of type `std::result::Result<Version, Error>`
+        //~| NOTE: value of type `std::result::Result<Vec<Version>, AnotherError>` cannot be built from `std::iter::Iterator<Item=std::result::Result<Version, Error>>`
+        //~| NOTE: required by a bound introduced by this call
+        //~| HELP: the trait
+        //~| HELP: for that trait implementation, expected `AnotherError`, found `Error`
+        //~| NOTE: required by a bound in `collect`
+    tags.sort();
+
+    Ok(tags)
+}
+
+fn main() {}
diff --git a/tests/ui/inference/question-mark-type-inference-in-chain.stderr b/tests/ui/inference/question-mark-type-inference-in-chain.stderr
@@ -0,0 +1,66 @@
+error[E0282]: type annotations needed
+  --> $DIR/question-mark-type-inference-in-chain.rs:31:9
+   |
+LL |     let mut tags = lines.iter().map(|e| parse(e)).collect()?;
+   |         ^^^^^^^^
+...
+LL |     tags.sort();
+   |     ---- type must be known at this point
+   |
+help: consider giving `tags` an explicit type
+   |
+LL |     let mut tags: Vec<_> = lines.iter().map(|e| parse(e)).collect()?;
+   |                 ++++++++
+
+error[E0283]: type annotations needed
+  --> $DIR/question-mark-type-inference-in-chain.rs:41:65
+   |
+LL |     let mut tags: Vec<Version> = lines.iter().map(|e| parse(e)).collect()?;
+   |                                                                 ^^^^^^^ cannot infer type of the type parameter `B` declared on the method `collect`
+   |
+   = note: cannot satisfy `_: FromIterator<std::result::Result<Version, Error>>`
+note: required by a bound in `collect`
+  --> $SRC_DIR/core/src/iter/traits/iterator.rs:LL:COL
+help: consider specifying the generic argument
+   |
+LL |     let mut tags: Vec<Version> = lines.iter().map(|e| parse(e)).collect::<Result<_, _>>()?;
+   |                                                                        ++++++++++++++++
+
+error[E0277]: the `?` operator can only be applied to values that implement `Try`
+  --> $DIR/question-mark-type-inference-in-chain.rs:53:20
+   |
+LL |     let mut tags = lines.iter().map(|e| parse(e)).collect::<Vec<_>>()?;
+   |                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the `?` operator cannot be applied to type `Vec<std::result::Result<Version, Error>>`
+   |
+   = help: the nightly-only, unstable trait `Try` is not implemented for `Vec<std::result::Result<Version, Error>>`
+
+error[E0277]: a value of type `std::result::Result<Vec<Version>, AnotherError>` cannot be built from an iterator over elements of type `std::result::Result<Version, Error>`
+  --> $DIR/question-mark-type-inference-in-chain.rs:72:20
+   |
+LL |         .collect::<Result<Vec<Version>>>()?;
+   |          -------   ^^^^^^^^^^^^^^^^^^^^ value of type `std::result::Result<Vec<Version>, AnotherError>` cannot be built from `std::iter::Iterator<Item=std::result::Result<Version, Error>>`
+   |          |
+   |          required by a bound introduced by this call
+   |
+help: the trait `FromIterator<std::result::Result<_, Error>>` is not implemented for `std::result::Result<Vec<Version>, AnotherError>`
+      but trait `FromIterator<std::result::Result<_, AnotherError>>` is implemented for it
+  --> $SRC_DIR/core/src/result.rs:LL:COL
+   = help: for that trait implementation, expected `AnotherError`, found `Error`
+note: the method call chain might not have had the expected associated types
+  --> $DIR/question-mark-type-inference-in-chain.rs:69:10
+   |
+LL |     let mut tags = lines
+   |                    ----- this expression has type `&[&str]`
+LL |
+LL |         .iter()
+   |          ------ `Iterator::Item` is `&&str` here
+LL |
+LL |         .map(|e| parse(e))
+   |          ^^^^^^^^^^^^^^^^^ `Iterator::Item` changed to `Result<Version, Error>` here
+note: required by a bound in `collect`
+  --> $SRC_DIR/core/src/iter/traits/iterator.rs:LL:COL
+
+error: aborting due to 4 previous errors
+
+Some errors have detailed explanations: E0277, E0282, E0283.
+For more information about an error, try `rustc --explain E0277`.