Rollup merge of #91981 - estebank:tweakaroo, r=lcnr

Recover suggestions and useful information lost in previous PR

Follow up to #91898.

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

@@ -333,6 +333,9 @@ pub fn same_type_modulo_infer<'tcx>(a: Ty<'tcx>, b: Ty<'tcx>) -> bool {
         )
         | (&ty::Infer(ty::InferTy::TyVar(_)), _)
         | (_, &ty::Infer(ty::InferTy::TyVar(_))) => true,
+        (&ty::Ref(reg_a, ty_a, mut_a), &ty::Ref(reg_b, ty_b, mut_b)) => {
+            reg_a == reg_b && mut_a == mut_b && same_type_modulo_infer(*ty_a, *ty_b)
+        }
         _ => a == b,
     }
 }
@@ -602,7 +605,8 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
         match *cause.code() {
             ObligationCauseCode::Pattern { origin_expr: true, span: Some(span), root_ty } => {
                 let ty = self.resolve_vars_if_possible(root_ty);
-                if ty.is_suggestable() {
+                if !matches!(ty.kind(), ty::Infer(ty::InferTy::TyVar(_) | ty::InferTy::FreshTy(_)))
+                {
                     // don't show type `_`
                     err.span_label(span, format!("this expression has type `{}`", ty));
                 }

compiler/rustc_infer/src/infer/mod.rs

@@ -1434,6 +1434,17 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
         value.fold_with(&mut r)
     }
 
+    pub fn resolve_numeric_literals_with_default<T>(&self, value: T) -> T
+    where
+        T: TypeFoldable<'tcx>,
+    {
+        if !value.needs_infer() {
+            return value; // Avoid duplicated subst-folding.
+        }
+        let mut r = InferenceLiteralEraser { tcx: self.tcx };
+        value.fold_with(&mut r)
+    }
+
     /// Returns the first unresolved variable contained in `T`. In the
     /// process of visiting `T`, this will resolve (where possible)
     /// type variables in `T`, but it never constructs the final,
@@ -1785,6 +1796,26 @@ impl<'tcx> TyOrConstInferVar<'tcx> {
     }
 }
 
+/// Replace `{integer}` with `i32` and `{float}` with `f64`.
+/// Used only for diagnostics.
+struct InferenceLiteralEraser<'tcx> {
+    tcx: TyCtxt<'tcx>,
+}
+
+impl<'tcx> TypeFolder<'tcx> for InferenceLiteralEraser<'tcx> {
+    fn tcx(&self) -> TyCtxt<'tcx> {
+        self.tcx
+    }
+
+    fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
+        match ty.kind() {
+            ty::Infer(ty::IntVar(_) | ty::FreshIntTy(_)) => self.tcx.types.i32,
+            ty::Infer(ty::FloatVar(_) | ty::FreshFloatTy(_)) => self.tcx.types.f64,
+            _ => ty.super_fold_with(self),
+        }
+    }
+}
+
 struct ShallowResolver<'a, 'tcx> {
     infcx: &'a InferCtxt<'a, 'tcx>,
 }

compiler/rustc_typeck/src/astconv/generics.rs

@@ -11,6 +11,7 @@ use rustc_hir as hir;
 use rustc_hir::def::{DefKind, Res};
 use rustc_hir::def_id::DefId;
 use rustc_hir::GenericArg;
+use rustc_infer::infer::TyCtxtInferExt;
 use rustc_middle::ty::{
     self, subst, subst::SubstsRef, GenericParamDef, GenericParamDefKind, Ty, TyCtxt,
 };
@@ -83,7 +84,9 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
                     if let Some(param_local_id) = param.def_id.as_local() {
                         let param_hir_id = tcx.hir().local_def_id_to_hir_id(param_local_id);
                         let param_name = tcx.hir().ty_param_name(param_hir_id);
-                        let param_type = tcx.type_of(param.def_id);
+                        let param_type = tcx.infer_ctxt().enter(|infcx| {
+                            infcx.resolve_numeric_literals_with_default(tcx.type_of(param.def_id))
+                        });
                         if param_type.is_suggestable() {
                             err.span_suggestion(
                                 tcx.def_span(src_def_id),

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

@@ -521,6 +521,8 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         can_suggest: bool,
         fn_id: hir::HirId,
     ) -> bool {
+        let found =
+            self.resolve_numeric_literals_with_default(self.resolve_vars_if_possible(found));
         // Only suggest changing the return type for methods that
         // haven't set a return type at all (and aren't `fn main()` or an impl).
         match (&fn_decl.output, found.is_suggestable(), can_suggest, expected.is_unit()) {
@@ -528,13 +530,20 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 err.span_suggestion(
                     span,
                     "try adding a return type",
-                    format!("-> {} ", self.resolve_vars_with_obligations(found)),
+                    format!("-> {} ", found),
                     Applicability::MachineApplicable,
                 );
                 true
             }
             (&hir::FnRetTy::DefaultReturn(span), false, true, true) => {
-                err.span_label(span, "possibly return type missing here?");
+                // FIXME: if `found` could be `impl Iterator` or `impl Fn*`, we should suggest
+                // that.
+                err.span_suggestion(
+                    span,
+                    "a return type might be missing here",
+                    "-> _ ".to_string(),
+                    Applicability::HasPlaceholders,
+                );
                 true
             }
             (&hir::FnRetTy::DefaultReturn(span), _, false, true) => {