Suggest calling method if fn does not exist

compiler/rustc_errors/src/lib.rs

@@ -460,6 +460,7 @@ pub enum StashKey {
     ItemNoType,
     UnderscoreForArrayLengths,
     EarlySyntaxWarning,
+    CallIntoMethod,
 }
 
 fn default_track_diagnostic(_: &Diagnostic) {}

compiler/rustc_hir_analysis/src/check/callee.rs

@@ -1,8 +1,10 @@
+use super::method::probe::{IsSuggestion, Mode, ProbeScope};
 use super::method::MethodCallee;
 use super::{DefIdOrName, Expectation, FnCtxt, TupleArgumentsFlag};
 use crate::type_error_struct;
 
-use rustc_errors::{struct_span_err, Applicability, Diagnostic};
+use rustc_ast::util::parser::PREC_POSTFIX;
+use rustc_errors::{struct_span_err, Applicability, Diagnostic, StashKey};
 use rustc_hir as hir;
 use rustc_hir::def::{self, Namespace, Res};
 use rustc_hir::def_id::DefId;
@@ -60,6 +62,7 @@ pub fn check_legal_trait_for_method_call(
     }
 }
 
+#[derive(Debug)]
 enum CallStep<'tcx> {
     Builtin(Ty<'tcx>),
     DeferredClosure(LocalDefId, ty::FnSig<'tcx>),
@@ -188,6 +191,10 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 return None;
             }
 
+            ty::Error(_) => {
+                return None;
+            }
+
             _ => {}
         }
 
@@ -394,6 +401,31 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             }
             ty::FnPtr(sig) => (sig, None),
             _ => {
+                if let hir::ExprKind::Path(hir::QPath::Resolved(_, path)) = &callee_expr.kind
+                    && let [segment] = path.segments
+                    && let Some(mut diag) = self
+                        .tcx
+                        .sess
+                        .diagnostic()
+                        .steal_diagnostic(segment.ident.span, StashKey::CallIntoMethod)
+                {
+                    // Try suggesting `foo(a)` -> `a.foo()` if possible.
+                    if let Some(ty) =
+                        self.suggest_call_as_method(
+                            &mut diag,
+                            segment,
+                            arg_exprs,
+                            call_expr,
+                            expected
+                        )
+                    {
+                        diag.emit();
+                        return ty;
+                    } else {
+                        diag.emit();
+                    }
+                }
+
                 self.report_invalid_callee(call_expr, callee_expr, callee_ty, arg_exprs);
 
                 // This is the "default" function signature, used in case of error.
@@ -441,6 +473,105 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         fn_sig.output()
     }
 
+    /// Attempts to reinterpret `method(rcvr, args...)` as `rcvr.method(args...)`
+    /// and suggesting the fix if the method probe is successful.
+    fn suggest_call_as_method(
+        &self,
+        diag: &mut Diagnostic,
+        segment: &'tcx hir::PathSegment<'tcx>,
+        arg_exprs: &'tcx [hir::Expr<'tcx>],
+        call_expr: &'tcx hir::Expr<'tcx>,
+        expected: Expectation<'tcx>,
+    ) -> Option<Ty<'tcx>> {
+        if let [callee_expr, rest @ ..] = arg_exprs {
+            let callee_ty = self.check_expr(callee_expr);
+            // First, do a probe with `IsSuggestion(true)` to avoid emitting
+            // any strange errors. If it's successful, then we'll do a true
+            // method lookup.
+            let Ok(pick) = self
+            .probe_for_name(
+                call_expr.span,
+                Mode::MethodCall,
+                segment.ident,
+                IsSuggestion(true),
+                callee_ty,
+                call_expr.hir_id,
+                // We didn't record the in scope traits during late resolution
+                // so we need to probe AllTraits unfortunately
+                ProbeScope::AllTraits,
+            ) else {
+                return None;
+            };
+
+            let pick = self.confirm_method(
+                call_expr.span,
+                callee_expr,
+                call_expr,
+                callee_ty,
+                pick,
+                segment,
+            );
+            if pick.illegal_sized_bound.is_some() {
+                return None;
+            }
+
+            let up_to_rcvr_span = segment.ident.span.until(callee_expr.span);
+            let rest_span = callee_expr.span.shrink_to_hi().to(call_expr.span.shrink_to_hi());
+            let rest_snippet = if let Some(first) = rest.first() {
+                self.tcx
+                    .sess
+                    .source_map()
+                    .span_to_snippet(first.span.to(call_expr.span.shrink_to_hi()))
+            } else {
+                Ok(")".to_string())
+            };
+
+            if let Ok(rest_snippet) = rest_snippet {
+                let sugg = if callee_expr.precedence().order() >= PREC_POSTFIX {
+                    vec![
+                        (up_to_rcvr_span, "".to_string()),
+                        (rest_span, format!(".{}({rest_snippet}", segment.ident)),
+                    ]
+                } else {
+                    vec![
+                        (up_to_rcvr_span, "(".to_string()),
+                        (rest_span, format!(").{}({rest_snippet}", segment.ident)),
+                    ]
+                };
+                let self_ty = self.resolve_vars_if_possible(pick.callee.sig.inputs()[0]);
+                diag.multipart_suggestion(
+                    format!(
+                        "use the `.` operator to call the method `{}{}` on `{self_ty}`",
+                        self.tcx
+                            .associated_item(pick.callee.def_id)
+                            .trait_container(self.tcx)
+                            .map_or_else(
+                                || String::new(),
+                                |trait_def_id| self.tcx.def_path_str(trait_def_id) + "::"
+                            ),
+                        segment.ident
+                    ),
+                    sugg,
+                    Applicability::MaybeIncorrect,
+                );
+
+                // Let's check the method fully now
+                let return_ty = self.check_method_argument_types(
+                    segment.ident.span,
+                    call_expr,
+                    Ok(pick.callee),
+                    rest,
+                    TupleArgumentsFlag::DontTupleArguments,
+                    expected,
+                );
+
+                return Some(return_ty);
+            }
+        }
+
+        None
+    }
+
     fn report_invalid_callee(
         &self,
         call_expr: &'tcx hir::Expr<'tcx>,
@@ -459,10 +590,8 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 def::CtorOf::Struct => "struct",
                 def::CtorOf::Variant => "enum variant",
             };
-            let removal_span =
-                callee_expr.span.shrink_to_hi().to(call_expr.span.shrink_to_hi());
-            unit_variant =
-                Some((removal_span, descr, rustc_hir_pretty::qpath_to_string(qpath)));
+            let removal_span = callee_expr.span.shrink_to_hi().to(call_expr.span.shrink_to_hi());
+            unit_variant = Some((removal_span, descr, rustc_hir_pretty::qpath_to_string(qpath)));
         }
 
         let callee_ty = self.resolve_vars_if_possible(callee_ty);
@@ -525,7 +654,8 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         };
 
         if !self.maybe_suggest_bad_array_definition(&mut err, call_expr, callee_expr) {
-            if let Some((maybe_def, output_ty, _)) = self.extract_callable_info(callee_expr, callee_ty)
+            if let Some((maybe_def, output_ty, _)) =
+                self.extract_callable_info(callee_expr, callee_ty)
                 && !self.type_is_sized_modulo_regions(self.param_env, output_ty, callee_expr.span)
             {
                 let descr = match maybe_def {

compiler/rustc_resolve/src/diagnostics.rs

@@ -120,14 +120,15 @@ impl<'a> Resolver<'a> {
     }
 
     fn report_with_use_injections(&mut self, krate: &Crate) {
-        for UseError { mut err, candidates, def_id, instead, suggestion, path } in
+        for UseError { mut err, candidates, def_id, instead, suggestion, path, is_call } in
             self.use_injections.drain(..)
         {
             let (span, found_use) = if let Some(def_id) = def_id.as_local() {
                 UsePlacementFinder::check(krate, self.def_id_to_node_id[def_id])
             } else {
                 (None, FoundUse::No)
             };
+
             if !candidates.is_empty() {
                 show_candidates(
                     &self.session,
@@ -140,10 +141,15 @@ impl<'a> Resolver<'a> {
                     IsPattern::No,
                     path,
                 );
+                err.emit();
             } else if let Some((span, msg, sugg, appl)) = suggestion {
                 err.span_suggestion(span, msg, sugg, appl);
+                err.emit();
+            } else if let [segment] = path.as_slice() && is_call {
+                err.stash(segment.ident.span, rustc_errors::StashKey::CallIntoMethod);
+            } else {
+                err.emit();
             }
-            err.emit();
         }
     }
 

compiler/rustc_resolve/src/late.rs

@@ -3263,6 +3263,7 @@ impl<'a: 'ast, 'b, 'ast> LateResolutionVisitor<'a, 'b, 'ast> {
                     instead,
                     suggestion,
                     path: path.into(),
+                    is_call: source.is_call(),
                 });
             }
 
@@ -3327,6 +3328,7 @@ impl<'a: 'ast, 'b, 'ast> LateResolutionVisitor<'a, 'b, 'ast> {
                     instead: false,
                     suggestion: None,
                     path: path.into(),
+                    is_call: source.is_call(),
                 });
             } else {
                 err.cancel();

compiler/rustc_resolve/src/lib.rs

@@ -674,6 +674,8 @@ struct UseError<'a> {
     /// Path `Segment`s at the place of use that failed. Used for accurate suggestion after telling
     /// the user to import the item directly.
     path: Vec<Segment>,
+    /// Whether the expected source is a call
+    is_call: bool,
 }
 
 #[derive(Clone, Copy, PartialEq, Debug)]

src/test/ui/did_you_mean/issue-43871-enum-instead-of-variant.stderr

@@ -1,11 +1,3 @@
-error[E0423]: expected function, tuple struct or tuple variant, found enum `Option`
-  --> $DIR/issue-43871-enum-instead-of-variant.rs:19:13
-   |
-LL |     let x = Option(1);
-   |             ^^^^^^ help: try to construct one of the enum's variants: `std::option::Option::Some`
-   |
-   = help: you might have meant to construct the enum's non-tuple variant
-
 error[E0532]: expected tuple struct or tuple variant, found enum `Option`
   --> $DIR/issue-43871-enum-instead-of-variant.rs:21:12
    |
@@ -27,6 +19,14 @@ note: the enum is defined here
 LL | enum Example { Ex(String), NotEx }
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
+error[E0423]: expected function, tuple struct or tuple variant, found enum `Option`
+  --> $DIR/issue-43871-enum-instead-of-variant.rs:19:13
+   |
+LL |     let x = Option(1);
+   |             ^^^^^^ help: try to construct one of the enum's variants: `std::option::Option::Some`
+   |
+   = help: you might have meant to construct the enum's non-tuple variant
+
 error[E0423]: expected function, tuple struct or tuple variant, found enum `Void`
   --> $DIR/issue-43871-enum-instead-of-variant.rs:31:13
    |

src/test/ui/empty/empty-struct-braces-expr.stderr

@@ -21,29 +21,6 @@ help: a unit struct with a similar name exists
 LL |     let e1 = XEmpty2;
    |              ~~~~~~~
 
-error[E0423]: expected function, tuple struct or tuple variant, found struct `Empty1`
-  --> $DIR/empty-struct-braces-expr.rs:16:14
-   |
-LL | struct Empty1 {}
-   | ---------------- `Empty1` defined here
-...
-LL |     let e1 = Empty1();
-   |              ^^^^^^^^
-   |
-  ::: $DIR/auxiliary/empty-struct.rs:2:1
-   |
-LL | pub struct XEmpty2;
-   | ------------------ similarly named unit struct `XEmpty2` defined here
-   |
-help: use struct literal syntax instead
-   |
-LL |     let e1 = Empty1 {};
-   |              ~~~~~~~~~
-help: a unit struct with a similar name exists
-   |
-LL |     let e1 = XEmpty2();
-   |              ~~~~~~~
-
 error[E0423]: expected value, found struct variant `E::Empty3`
   --> $DIR/empty-struct-braces-expr.rs:18:14
    |
@@ -84,6 +61,29 @@ help: a unit struct with a similar name exists
 LL |     let xe1 = XEmpty2;
    |               ~~~~~~~
 
+error[E0423]: expected function, tuple struct or tuple variant, found struct `Empty1`
+  --> $DIR/empty-struct-braces-expr.rs:16:14
+   |
+LL | struct Empty1 {}
+   | ---------------- `Empty1` defined here
+...
+LL |     let e1 = Empty1();
+   |              ^^^^^^^^
+   |
+  ::: $DIR/auxiliary/empty-struct.rs:2:1
+   |
+LL | pub struct XEmpty2;
+   | ------------------ similarly named unit struct `XEmpty2` defined here
+   |
+help: use struct literal syntax instead
+   |
+LL |     let e1 = Empty1 {};
+   |              ~~~~~~~~~
+help: a unit struct with a similar name exists
+   |
+LL |     let e1 = XEmpty2();
+   |              ~~~~~~~
+
 error[E0423]: expected function, tuple struct or tuple variant, found struct `XEmpty1`
   --> $DIR/empty-struct-braces-expr.rs:23:15
    |

src/test/ui/error-codes/E0423.stderr

@@ -26,6 +26,17 @@ help: surround the struct literal with parentheses
 LL |     for _ in (std::ops::Range { start: 0, end: 10 }) {}
    |              +                                     +
 
+error[E0423]: expected value, found struct `T`
+  --> $DIR/E0423.rs:14:8
+   |
+LL |     if T {} == T {} { println!("Ok"); }
+   |        ^ not a value
+   |
+help: surround the struct literal with parentheses
+   |
+LL |     if (T {}) == T {} { println!("Ok"); }
+   |        +    +
+
 error[E0423]: expected function, tuple struct or tuple variant, found struct `Foo`
   --> $DIR/E0423.rs:4:13
    |
@@ -47,17 +58,6 @@ help: a function with a similar name exists
 LL |     let f = foo();
    |             ~~~
 
-error[E0423]: expected value, found struct `T`
-  --> $DIR/E0423.rs:14:8
-   |
-LL |     if T {} == T {} { println!("Ok"); }
-   |        ^ not a value
-   |
-help: surround the struct literal with parentheses
-   |
-LL |     if (T {}) == T {} { println!("Ok"); }
-   |        +    +
-
 error: aborting due to 5 previous errors
 
 For more information about this error, try `rustc --explain E0423`.