Point at expressions where inference refines an unexpected type

compiler/rustc_hir_typeck/src/demand.rs

@@ -1,21 +1,26 @@
 use crate::FnCtxt;
 use rustc_ast::util::parser::PREC_POSTFIX;
+use rustc_data_structures::fx::FxHashMap;
 use rustc_errors::MultiSpan;
 use rustc_errors::{Applicability, Diagnostic, DiagnosticBuilder, ErrorGuaranteed};
 use rustc_hir as hir;
 use rustc_hir::def::CtorKind;
+use rustc_hir::intravisit::Visitor;
 use rustc_hir::lang_items::LangItem;
 use rustc_hir::{is_range_literal, Node};
 use rustc_infer::infer::InferOk;
 use rustc_middle::lint::in_external_macro;
 use rustc_middle::middle::stability::EvalResult;
 use rustc_middle::ty::adjustment::AllowTwoPhase;
 use rustc_middle::ty::error::{ExpectedFound, TypeError};
-use rustc_middle::ty::print::with_no_trimmed_paths;
-use rustc_middle::ty::{self, Article, AssocItem, Ty, TypeAndMut};
+use rustc_middle::ty::fold::{BottomUpFolder, TypeFolder};
+use rustc_middle::ty::print::{with_forced_trimmed_paths, with_no_trimmed_paths};
+use rustc_middle::ty::relate::TypeRelation;
+use rustc_middle::ty::{self, Article, AssocItem, Ty, TypeAndMut, TypeVisitable};
 use rustc_span::symbol::{sym, Symbol};
 use rustc_span::{BytePos, Span};
 use rustc_trait_selection::infer::InferCtxtExt as _;
+use rustc_trait_selection::traits::error_reporting::method_chain::CollectAllMismatches;
 use rustc_trait_selection::traits::ObligationCause;
 
 use super::method::probe;
@@ -40,7 +45,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         self.annotate_alternative_method_deref(err, expr, error);
 
         // Use `||` to give these suggestions a precedence
-        let _ = self.suggest_missing_parentheses(err, expr)
+        let suggested = self.suggest_missing_parentheses(err, expr)
             || self.suggest_remove_last_method_call(err, expr, expected)
             || self.suggest_associated_const(err, expr, expected)
             || self.suggest_deref_ref_or_into(err, expr, expected, expr_ty, expected_ty_expr)
@@ -54,6 +59,9 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             || self.suggest_copied_or_cloned(err, expr, expr_ty, expected)
             || self.suggest_into(err, expr, expr_ty, expected)
             || self.suggest_floating_point_literal(err, expr, expected);
+        if !suggested {
+            self.point_at_expr_source_of_inferred_type(err, expr, expr_ty, expected);
+        }
     }
 
     pub fn emit_coerce_suggestions(
@@ -205,6 +213,215 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         (expected, Some(err))
     }
 
+    pub fn point_at_expr_source_of_inferred_type(
+        &self,
+        err: &mut Diagnostic,
+        expr: &hir::Expr<'_>,
+        found: Ty<'tcx>,
+        expected: Ty<'tcx>,
+    ) -> bool {
+        let map = self.tcx.hir();
+
+        let hir::ExprKind::Path(hir::QPath::Resolved(None, p)) = expr.kind else { return false; };
+        let [hir::PathSegment { ident, args: None, .. }] = p.segments else { return false; };
+        let hir::def::Res::Local(hir_id) = p.res else { return false; };
+        let Some(hir::Node::Pat(pat)) = map.find(hir_id) else { return false; };
+        let Some(hir::Node::Local(hir::Local {
+            ty: None,
+            init: Some(init),
+            ..
+        })) = map.find_parent(pat.hir_id) else { return false; };
+        let Some(ty) = self.node_ty_opt(init.hir_id) else { return false; };
+        if ty.is_closure() || init.span.overlaps(expr.span) || pat.span.from_expansion() {
+            return false;
+        }
+
+        // Locate all the usages of the relevant binding.
+        struct FindExprs<'hir> {
+            hir_id: hir::HirId,
+            uses: Vec<&'hir hir::Expr<'hir>>,
+        }
+        impl<'v> Visitor<'v> for FindExprs<'v> {
+            fn visit_expr(&mut self, ex: &'v hir::Expr<'v>) {
+                if let hir::ExprKind::Path(hir::QPath::Resolved(None, path)) = ex.kind
+                    && let hir::def::Res::Local(hir_id) = path.res
+                    && hir_id == self.hir_id
+                {
+                    self.uses.push(ex);
+                }
+                hir::intravisit::walk_expr(self, ex);
+            }
+        }
+
+        let mut expr_finder = FindExprs { hir_id, uses: vec![] };
+        let id = map.get_parent_item(hir_id);
+        let hir_id: hir::HirId = id.into();
+
+        let Some(node) = map.find(hir_id) else { return false; };
+        let Some(body_id) = node.body_id() else { return false; };
+        let body = map.body(body_id);
+        expr_finder.visit_expr(body.value);
+        // Hack to make equality checks on types with inference variables and regions useful.
+        let mut eraser = BottomUpFolder {
+            tcx: self.tcx,
+            lt_op: |_| self.tcx.lifetimes.re_erased,
+            ct_op: |c| c,
+            ty_op: |t| match *t.kind() {
+                ty::Infer(ty::TyVar(vid)) => self.tcx.mk_ty_infer(ty::TyVar(self.root_var(vid))),
+                ty::Infer(ty::IntVar(_)) => {
+                    self.tcx.mk_ty_infer(ty::IntVar(ty::IntVid { index: 0 }))
+                }
+                ty::Infer(ty::FloatVar(_)) => {
+                    self.tcx.mk_ty_infer(ty::FloatVar(ty::FloatVid { index: 0 }))
+                }
+                _ => t,
+            },
+        };
+        let mut prev = eraser.fold_ty(ty);
+        let mut prev_span = None;
+
+        for binding in expr_finder.uses {
+            // In every expression where the binding is referenced, we will look at that
+            // expression's type and see if it is where the incorrect found type was fully
+            // "materialized" and point at it. We will also try to provide a suggestion there.
+            if let Some(hir::Node::Expr(expr)
+            | hir::Node::Stmt(hir::Stmt {
+                kind: hir::StmtKind::Expr(expr) | hir::StmtKind::Semi(expr),
+                ..
+            })) = &map.find_parent(binding.hir_id)
+                && let hir::ExprKind::MethodCall(segment, rcvr, args, _span) = expr.kind
+                && rcvr.hir_id == binding.hir_id
+                && let Some(def_id) = self.typeck_results.borrow().type_dependent_def_id(expr.hir_id)
+            {
+                // We special case methods, because they can influence inference through the
+                // call's arguments and we can provide a more explicit span.
+                let sig = self.tcx.fn_sig(def_id);
+                let def_self_ty = sig.input(0).skip_binder();
+                let rcvr_ty = self.node_ty(rcvr.hir_id);
+                // Get the evaluated type *after* calling the method call, so that the influence
+                // of the arguments can be reflected in the receiver type. The receiver
+                // expression has the type *before* theis analysis is done.
+                let ty = match self.lookup_probe(
+                    segment.ident,
+                    rcvr_ty,
+                    expr,
+                    probe::ProbeScope::TraitsInScope,
+                ) {
+                    Ok(pick) => pick.self_ty,
+                    Err(_) => rcvr_ty,
+                };
+                // Remove one layer of references to account for `&mut self` and
+                // `&self`, so that we can compare it against the binding.
+                let (ty, def_self_ty) = match (ty.kind(), def_self_ty.kind()) {
+                    (ty::Ref(_, ty, a), ty::Ref(_, self_ty, b)) if a == b => (*ty, *self_ty),
+                    _ => (ty, def_self_ty),
+                };
+                let mut param_args = FxHashMap::default();
+                let mut param_expected = FxHashMap::default();
+                let mut param_found = FxHashMap::default();
+                if self.can_eq(self.param_env, ty, found).is_ok() {
+                    // We only point at the first place where the found type was inferred.
+                    for (i, param_ty) in sig.inputs().skip_binder().iter().skip(1).enumerate() {
+                        if def_self_ty.contains(*param_ty) && let ty::Param(_) = param_ty.kind() {
+                            // We found an argument that references a type parameter in `Self`,
+                            // so we assume that this is the argument that caused the found
+                            // type, which we know already because of `can_eq` above was first
+                            // inferred in this method call.
+                            let arg = &args[i];
+                            let arg_ty = self.node_ty(arg.hir_id);
+                            err.span_label(
+                                arg.span,
+                                &format!(
+                                    "this is of type `{arg_ty}`, which causes `{ident}` to be \
+                                     inferred as `{ty}`",
+                                ),
+                            );
+                            param_args.insert(param_ty, (arg, arg_ty));
+                        }
+                    }
+                }
+
+                // Here we find, for a type param `T`, the type that `T` is in the current
+                // method call *and* in the original expected type. That way, we can see if we
+                // can give any structured suggestion for the function argument.
+                let mut c = CollectAllMismatches {
+                    infcx: &self.infcx,
+                    param_env: self.param_env,
+                    errors: vec![],
+                };
+                let _ = c.relate(def_self_ty, ty);
+                for error in c.errors {
+                    if let TypeError::Sorts(error) = error {
+                        param_found.insert(error.expected, error.found);
+                    }
+                }
+                c.errors = vec![];
+                let _ = c.relate(def_self_ty, expected);
+                for error in c.errors {
+                    if let TypeError::Sorts(error) = error {
+                        param_expected.insert(error.expected, error.found);
+                    }
+                }
+                for (param, (arg, arg_ty)) in param_args.iter() {
+                    let Some(expected) = param_expected.get(param) else { continue; };
+                    let Some(found) = param_found.get(param) else { continue; };
+                    if self.can_eq(self.param_env, *arg_ty, *found).is_err() { continue; }
+                    self.emit_coerce_suggestions(err, arg, *found, *expected, None, None);
+                }
+
+                let ty = eraser.fold_ty(ty);
+                if ty.references_error() {
+                    break;
+                }
+                if ty != prev
+                    && param_args.is_empty()
+                    && self.can_eq(self.param_env, ty, found).is_ok()
+                {
+                    // We only point at the first place where the found type was inferred.
+                    err.span_label(
+                        segment.ident.span,
+                        with_forced_trimmed_paths!(format!(
+                            "here the type of `{ident}` is inferred to be `{ty}`",
+                        )),
+                    );
+                    break;
+                } else if !param_args.is_empty() {
+                    break;
+                }
+                prev = ty;
+            } else {
+                let ty = eraser.fold_ty(self.node_ty(binding.hir_id));
+                if ty.references_error() {
+                    break;
+                }
+                if ty != prev
+                    && let Some(span) = prev_span
+                    && self.can_eq(self.param_env, ty, found).is_ok()
+                {
+                    // We only point at the first place where the found type was inferred.
+                    // We use the *previous* span because if the type is known *here* it means
+                    // it was *evaluated earlier*. We don't do this for method calls because we
+                    // evaluate the method's self type eagerly, but not in any other case.
+                    err.span_label(
+                        span,
+                        with_forced_trimmed_paths!(format!(
+                            "here the type of `{ident}` is inferred to be `{ty}`",
+                        )),
+                    );
+                    break;
+                }
+                prev = ty;
+            }
+            if binding.hir_id == expr.hir_id {
+                // Do not look at expressions that come after the expression we were originally
+                // evaluating and had a type error.
+                break;
+            }
+            prev_span = Some(binding.span);
+        }
+        true
+    }
+
     fn annotate_expected_due_to_let_ty(
         &self,
         err: &mut Diagnostic,

compiler/rustc_hir_typeck/src/expr.rs

@@ -234,6 +234,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             ) => self.check_expr_path(qpath, expr, args),
             _ => self.check_expr_kind(expr, expected),
         });
+        let ty = self.resolve_vars_if_possible(ty);
 
         // Warn for non-block expressions with diverging children.
         match expr.kind {

compiler/rustc_hir_typeck/src/fn_ctxt/checks.rs

@@ -798,6 +798,18 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 full_call_span,
                 format!("arguments to this {} are incorrect", call_name),
             );
+            if let (Some(callee_ty), hir::ExprKind::MethodCall(_, rcvr, _, _)) =
+                (callee_ty, &call_expr.kind)
+            {
+                // Type that would have accepted this argument if it hadn't been inferred earlier.
+                // FIXME: We leave an inference variable for now, but it'd be nice to get a more
+                // specific type to increase the accuracy of the diagnostic.
+                let expected = self.infcx.next_ty_var(TypeVariableOrigin {
+                    kind: TypeVariableOriginKind::MiscVariable,
+                    span: full_call_span,
+                });
+                self.point_at_expr_source_of_inferred_type(&mut err, rcvr, expected, callee_ty);
+            }
             // Call out where the function is defined
             self.label_fn_like(
                 &mut err,

compiler/rustc_infer/src/infer/opaque_types.rs

@@ -61,7 +61,7 @@ impl<'tcx> InferCtxt<'tcx> {
                 .as_local()
                 .map_or(false, |def_id| self.opaque_type_origin(def_id, span).is_some())
         };
-        let value = value.fold_with(&mut ty::fold::BottomUpFolder {
+        let value = value.fold_with(&mut BottomUpFolder {
             tcx: self.tcx,
             lt_op: |lt| lt,
             ct_op: |ct| ct,

src/test/ui/type/type-check/assignment-in-if.stderr

@@ -67,6 +67,9 @@ LL |             x == 5
 error[E0308]: mismatched types
   --> $DIR/assignment-in-if.rs:44:18
    |
+LL |     if y = (Foo { foo: x }) {
+   |                        - here the type of `x` is inferred to be `usize`
+...
 LL |     if x == x && x = x && x == x {
    |        ------    ^ expected `bool`, found `usize`
    |        |
@@ -75,6 +78,9 @@ LL |     if x == x && x = x && x == x {
 error[E0308]: mismatched types
   --> $DIR/assignment-in-if.rs:44:22
    |
+LL |     if y = (Foo { foo: x }) {
+   |                        - here the type of `x` is inferred to be `usize`
+...
 LL |     if x == x && x = x && x == x {
    |                      ^ expected `bool`, found `usize`
 
@@ -92,6 +98,9 @@ LL |     if x == x && x == x && x == x {
 error[E0308]: mismatched types
   --> $DIR/assignment-in-if.rs:51:28
    |
+LL |     if y = (Foo { foo: x }) {
+   |                        - here the type of `x` is inferred to be `usize`
+...
 LL |     if x == x && x == x && x = x {
    |        ----------------    ^ expected `bool`, found `usize`
    |        |

src/test/ui/type/type-check/point-at-inference-2.rs

@@ -0,0 +1,13 @@
+fn bar(_: Vec<i32>) {}
+fn baz(_: &Vec<&i32>) {}
+fn main() {
+    let v = vec![&1];
+    bar(v); //~ ERROR E0308
+    let v = vec![];
+    baz(&v);
+    baz(&v);
+    bar(v); //~ ERROR E0308
+    let v = vec![];
+    baz(&v);
+    bar(v); //~ ERROR E0308
+}