For E0277 suggest adding Result return type for function which usin…

…g QuesionMark `?` in the body.

fixes rust-lang#125997

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

@@ -4586,6 +4586,58 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
             _ => "/* value */".to_string(),
         })
     }
+
+    fn suggest_add_result_as_return_type(
+        &self,
+        obligation: &PredicateObligation<'tcx>,
+        err: &mut Diag<'_>,
+        trait_ref: ty::PolyTraitRef<'tcx>,
+    ) {
+        if ObligationCauseCode::QuestionMark != *obligation.cause.code().peel_derives() {
+            return;
+        }
+
+        let node = self.tcx.hir_node_by_def_id(obligation.cause.body_id);
+        if let hir::Node::Item(item) = node
+            && let hir::ItemKind::Fn(sig, _, body_id) = item.kind
+            && let hir::FnRetTy::DefaultReturn(ret_span) = sig.decl.output
+            && trait_ref.skip_binder().args.len() == 2
+            && let ty::Tuple(l) = trait_ref.skip_binder().args.type_at(0).kind()
+            && l.len() == 0
+            && let ty::Adt(def, _) = trait_ref.skip_binder().args.type_at(1).kind()
+            && self.tcx.is_diagnostic_item(sym::Result, def.did())
+        {
+            let body = self.tcx.hir().body(body_id);
+            let mut sugg_spans =
+                vec![(ret_span, " -> Result<(), Box<dyn std::error::Error>>".to_string())];
+
+            if let hir::ExprKind::Block(b, _) = body.value.kind
+                && b.expr.is_none()
+                && let Some(s) = b.stmts.last()
+                && !is_ret_expr(s)
+            {
+                sugg_spans.push((
+                    s.span.shrink_to_hi().until(body.value.span.shrink_to_hi()),
+                    "\n\n    return Ok(());\n}".to_string(),
+                ));
+            }
+            err.multipart_suggestion_verbose(
+                format!("consider adding return type"),
+                sugg_spans,
+                Applicability::MaybeIncorrect,
+            );
+        }
+
+        fn is_ret_expr<'hir>(stmt: &hir::Stmt<'hir>) -> bool {
+            if let hir::StmtKind::Semi(expr) = stmt.kind
+                && let hir::ExprKind::Ret(Some(_)) = expr.kind
+            {
+                true
+            } else {
+                false
+            }
+        }
+    }
 }
 
 /// Add a hint to add a missing borrow or remove an unnecessary one.

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

@@ -612,6 +612,10 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
                             &mut err,
                             trait_predicate,
                         );
+                        self.suggest_add_result_as_return_type(&obligation,
+                            &mut err,
+                            trait_ref);
+
                         if self.suggest_add_reference_to_arg(
                             &obligation,
                             &mut err,

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

@@ -0,0 +1,19 @@
+//@ run-rustfix
+
+#![allow(unused_imports)]
+#![allow(dead_code)]
+
+use std::fs::File;
+use std::io::prelude::*;
+
+fn test1() -> Result<(), Box<dyn std::error::Error>> {
+    let mut _file = File::create("foo.txt")?;
+
+    return Ok(());
+}
+
+fn main() -> Result<(), Box<dyn std::error::Error>> {
+    let mut _file = File::create("foo.txt")?;
+
+    return Ok(());
+}

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

@@ -0,0 +1,17 @@
+//@ run-rustfix
+
+#![allow(unused_imports)]
+#![allow(dead_code)]
+
+use std::fs::File;
+use std::io::prelude::*;
+
+fn test1() {
+    let mut _file = File::create("foo.txt")?;
+    //~^ ERROR the `?` operator can only be used in a function
+}
+
+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

@@ -0,0 +1,39 @@
+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:10:44
+   |
+LL | fn test1() {
+   | ---------- this function should return `Result` or `Option` to accept `?`
+LL |     let mut _file = File::create("foo.txt")?;
+   |                                            ^ cannot use the `?` operator in a function that returns `()`
+   |
+   = help: the trait `FromResidual<Result<Infallible, std::io::Error>>` is not implemented for `()`
+help: consider adding return type
+   |
+LL ~ fn test1() -> Result<(), Box<dyn std::error::Error>> {
+LL ~     let mut _file = File::create("foo.txt")?;
+LL + 
+LL +     return 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:15:44
+   |
+LL | fn main() {
+   | --------- this function should return `Result` or `Option` to accept `?`
+LL |     let mut _file = File::create("foo.txt")?;
+   |                                            ^ cannot use the `?` operator in a function that returns `()`
+   |
+   = help: the trait `FromResidual<Result<Infallible, std::io::Error>>` is not implemented for `()`
+help: consider adding return type
+   |
+LL ~ fn main() -> Result<(), Box<dyn std::error::Error>> {
+LL ~     let mut _file = File::create("foo.txt")?;
+LL + 
+LL +     return Ok(());
+LL + }
+   |
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0277`.

tests/ui/try-trait/try-operator-on-main.stderr

@@ -8,6 +8,17 @@ LL |     std::fs::File::open("foo")?;
    |                               ^ cannot use the `?` operator in a function that returns `()`
    |
    = help: the trait `FromResidual<Result<Infallible, std::io::Error>>` is not implemented for `()`
+help: consider adding return type
+   |
+LL ~ fn main() -> Result<(), Box<dyn std::error::Error>> {
+LL |     // error for a `Try` type on a non-`Try` fn
+ ...
+LL |     // an unrelated use of `Try`
+LL ~     try_trait_generic::<()>();
+LL + 
+LL +     return Ok(());
+LL + }
+   |
 
 error[E0277]: the `?` operator can only be applied to values that implement `Try`
   --> $DIR/try-operator-on-main.rs:10:5