emit diagnostic suggestion for error when if let used with enum variant without being initialized

compare the span base id to get the correct expression and add suggestion to it
	modified:   compiler/rustc_infer/src/infer/error_reporting/mod.rs
	modified:   compiler/rustc_span/src/span_encoding.rs
	modified:   compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs
	modified:   compiler/rustc_typeck/src/check/compare_method.rs
	modified:   compiler/rustc_typeck/src/check/fn_ctxt/checks.rs
	new file:   src/test/ui/type/issue-101208.rs
	new file:   src/test/ui/type/issue-101208.stderr

Author: Yiming Lei

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

@@ -64,8 +64,12 @@ use rustc_errors::{Applicability, DiagnosticBuilder, DiagnosticStyledString, Mul
 use rustc_hir as hir;
 use rustc_hir::def::DefKind;
 use rustc_hir::def_id::{DefId, LocalDefId};
+use rustc_hir::intravisit::walk_block;
+use rustc_hir::intravisit::walk_expr;
+use rustc_hir::intravisit::Visitor;
 use rustc_hir::lang_items::LangItem;
-use rustc_hir::Node;
+use rustc_hir::HirId;
+use rustc_hir::{Expr, Node};
 use rustc_middle::dep_graph::DepContext;
 use rustc_middle::ty::print::with_no_trimmed_paths;
 use rustc_middle::ty::relate::{self, RelateResult, TypeRelation};
@@ -564,12 +568,56 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
         }
     }
 
+    fn note_enum_suggestion(
+        &self,
+        err: &mut Diagnostic,
+        span: Span,
+        body_id: HirId,
+        arg_size: usize,
+    ) {
+        let body_node = self.tcx.hir().get(body_id);
+        let hir::Node::Expr(&hir::Expr{kind:hir::ExprKind::Block(body_expr, ..), ..}) = body_node else {return ()};
+        struct FindExprVisitor<'tcx> {
+            target_id: u32,
+            size: usize,
+            terr: &'tcx mut Diagnostic,
+        }
+        impl<'tcx> Visitor<'tcx> for FindExprVisitor<'tcx> {
+            fn visit_expr(&mut self, expr: &'tcx Expr<'tcx>) {
+                if expr.span.get_base_or_index() == self.target_id {
+                    let mut suggest_vec = vec![];
+                    let mut i = 0;
+                    suggest_vec.push((expr.span.shrink_to_hi(), "(".to_string()));
+                    while i < self.size {
+                        suggest_vec.push((expr.span.shrink_to_hi(), "_".to_string()));
+                        if i != self.size - 1 {
+                            suggest_vec.push((expr.span.shrink_to_hi(), ",".to_string()));
+                        }
+                        i = i + 1;
+                    }
+                    suggest_vec.push((expr.span.shrink_to_hi(), ")".to_string()));
+
+                    self.terr.multipart_suggestion(
+                        "use parentheses to instantiate this tuple variant",
+                        suggest_vec,
+                        Applicability::MachineApplicable,
+                    );
+                }
+                walk_expr(self, expr);
+            }
+        }
+        let mut visitor =
+            FindExprVisitor { target_id: span.get_base_or_index(), size: arg_size, terr: err };
+        walk_block(&mut visitor, body_expr);
+    }
+
     fn note_error_origin(
         &self,
         err: &mut Diagnostic,
         cause: &ObligationCause<'tcx>,
         exp_found: Option<ty::error::ExpectedFound<Ty<'tcx>>>,
         terr: TypeError<'tcx>,
+        detect_enum_noparm: bool,
     ) {
         match *cause.code() {
             ObligationCauseCode::Pattern { origin_expr: true, span: Some(span), root_ty } => {
@@ -584,6 +632,12 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
                         err.span_label(span, format!("this is an iterator with items of type `{}`", substs.type_at(0)));
                     } else {
                         err.span_label(span, format!("this expression has type `{}`", ty));
+                        if detect_enum_noparm &&
+                        let ty::FnDef(def_id, substs) = ty.kind(){
+                            let sig = self.tcx.bound_fn_sig(*def_id).subst(self.tcx, substs);
+                            let sig = self.tcx.erase_late_bound_regions(sig);
+                            self.note_enum_suggestion(err, span, cause.body_id, sig.inputs().len());
+                        }
                     }
                 }
                 if let Some(ty::error::ExpectedFound { found, .. }) = exp_found
@@ -1432,6 +1486,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
         terr: TypeError<'tcx>,
         swap_secondary_and_primary: bool,
         prefer_label: bool,
+        detect_enum_noparm: bool,
     ) {
         let span = cause.span();
 
@@ -1882,7 +1937,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
 
         // It reads better to have the error origin as the final
         // thing.
-        self.note_error_origin(diag, cause, exp_found, terr);
+        self.note_error_origin(diag, cause, exp_found, terr, detect_enum_noparm);
 
         debug!(?diag);
     }
@@ -2225,6 +2280,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
 
         let span = trace.cause.span();
         let failure_code = trace.cause.as_failure_code(terr);
+        let mut detect_enum_noparm = false;
         let mut diag = match failure_code {
             FailureCode::Error0038(did) => {
                 let violations = self.tcx.object_safety_violations(did);
@@ -2279,6 +2335,9 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
                                 }
                             }
                         }
+                        (ty::FnDef(_, _), ty::Adt(adt_id, _)) if adt_id.is_enum()  => {
+                            detect_enum_noparm = true;
+                        }
                         _ => {}
                     }
                 }
@@ -2299,7 +2358,16 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
                 struct_span_err!(self.tcx.sess, span, E0644, "{}", failure_str)
             }
         };
-        self.note_type_err(&mut diag, &trace.cause, None, Some(trace.values), terr, false, false);
+        self.note_type_err(
+            &mut diag,
+            &trace.cause,
+            None,
+            Some(trace.values),
+            terr,
+            false,
+            false,
+            detect_enum_noparm,
+        );
         diag
     }
 

compiler/rustc_span/src/span_encoding.rs

@@ -149,6 +149,10 @@ impl Span {
             with_span_interner(|interner| interner.spans[index as usize].ctxt)
         }
     }
+
+    pub fn get_base_or_index(self) -> u32 {
+        self.base_or_index
+    }
 }
 
 #[derive(Default)]

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

@@ -1661,6 +1661,7 @@ impl<'a, 'tcx> InferCtxtPrivExt<'a, 'tcx> for InferCtxt<'a, 'tcx> {
                 err,
                 true,
                 false,
+                false,
             );
             self.note_obligation_cause(&mut diag, obligation);
             diag.emit();

compiler/rustc_typeck/src/check/compare_method.rs

@@ -405,6 +405,7 @@ fn compare_predicate_entailment<'tcx>(
                 terr,
                 false,
                 false,
+                false,
             );
 
             return Err(diag.emit());
@@ -520,6 +521,7 @@ pub fn collect_trait_impl_trait_tys<'tcx>(
                     terr,
                     false,
                     false,
+                    false,
                 );
                 return Err(diag.emit());
             }
@@ -1389,6 +1391,7 @@ pub(crate) fn compare_const_impl<'tcx>(
                 terr,
                 false,
                 false,
+                false,
             );
             diag.emit();
         }

compiler/rustc_typeck/src/check/fn_ctxt/checks.rs

@@ -848,6 +848,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                                 e,
                                 false,
                                 true,
+                                false,
                             );
                         }
                     }

src/test/ui/type/issue-101208.rs

@@ -0,0 +1,10 @@
+enum E {
+    One(i32, i32)
+}
+fn main() {
+    let var = E::One;
+    if let E::One(var1, var2) = var {
+    //~^ ERROR 0308
+        println!("{var1} {var2}");
+    }
+}

src/test/ui/type/issue-101208.stderr

@@ -0,0 +1,18 @@
+error[E0308]: mismatched types
+  --> $DIR/issue-101208.rs:6:12
+   |
+LL |     if let E::One(var1, var2) = var {
+   |            ^^^^^^^^^^^^^^^^^^   --- this expression has type `fn(i32, i32) -> E {E::One}`
+   |            |
+   |            expected fn item, found enum `E`
+   |
+   = note: expected fn item `fn(i32, i32) -> E {E::One}`
+                 found enum `E`
+help: use parentheses to instantiate this tuple variant
+   |
+LL |     if let E::One(var1, var2) = var(_,_) {
+   |                                    +
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0308`.