Auto merge of #106399 - estebank:type-err-span-label, r=nagisa

Modify primary span label for E0308

Looking at the reactions to https://hachyderm.io/`@ekuber/109622160673605438,` a lot of people seem to have trouble understanding the current output, where the primary span label on type errors talks about the specific types that diverged, but these can be deeply nested type parameters. Because of that we could see "expected i32, found u32" in the label while the note said "expected Vec<i32>, found Vec<u32>". This understandably confuses people. I believe that once people learn to read these errors it starts to make more sense, but this PR changes the output to be more in line with what people might expect, without sacrificing terseness.

Fix #68220.

Author: bors

compiler/rustc_hir_analysis/src/check/check.rs

@@ -442,6 +442,7 @@ fn check_opaque_meets_bounds<'tcx>(
     match ocx.eq(&misc_cause, param_env, opaque_ty, hidden_ty) {
         Ok(()) => {}
         Err(ty_err) => {
+            let ty_err = ty_err.to_string(tcx);
             tcx.sess.delay_span_bug(
                 span,
                 &format!("could not unify `{hidden_ty}` with revealed type:\n{ty_err}"),

compiler/rustc_hir_typeck/src/method/suggest.rs

@@ -259,7 +259,8 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         let mode = no_match_data.mode;
         let tcx = self.tcx;
         let rcvr_ty = self.resolve_vars_if_possible(rcvr_ty);
-        let ty_str = with_forced_trimmed_paths!(self.ty_to_string(rcvr_ty));
+        let (ty_str, ty_file) = tcx.short_ty_string(rcvr_ty);
+        let short_ty_str = with_forced_trimmed_paths!(rcvr_ty.to_string());
         let is_method = mode == Mode::MethodCall;
         let unsatisfied_predicates = &no_match_data.unsatisfied_predicates;
         let similar_candidate = no_match_data.similar_candidate;
@@ -276,11 +277,24 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             }
         };
 
-        if self.suggest_wrapping_range_with_parens(tcx, rcvr_ty, source, span, item_name, &ty_str)
-            || self.suggest_constraining_numerical_ty(
-                tcx, rcvr_ty, source, span, item_kind, item_name, &ty_str,
-            )
-        {
+        // We could pass the file for long types into these two, but it isn't strictly necessary
+        // given how targetted they are.
+        if self.suggest_wrapping_range_with_parens(
+            tcx,
+            rcvr_ty,
+            source,
+            span,
+            item_name,
+            &short_ty_str,
+        ) || self.suggest_constraining_numerical_ty(
+            tcx,
+            rcvr_ty,
+            source,
+            span,
+            item_kind,
+            item_name,
+            &short_ty_str,
+        ) {
             return None;
         }
         span = item_name.span;
@@ -319,6 +333,14 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             rcvr_ty.prefix_string(self.tcx),
             ty_str_reported,
         );
+        let ty_str = if short_ty_str.len() < ty_str.len() && ty_str.len() > 10 {
+            short_ty_str
+        } else {
+            ty_str
+        };
+        if let Some(file) = ty_file {
+            err.note(&format!("the full type name has been written to '{}'", file.display(),));
+        }
         if rcvr_ty.references_error() {
             err.downgrade_to_delayed_bug();
         }
@@ -826,7 +848,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 let primary_message = primary_message.unwrap_or_else(|| {
                     format!(
                         "the {item_kind} `{item_name}` exists for {actual_prefix} `{ty_str}`, \
-                    but its trait bounds were not satisfied"
+                         but its trait bounds were not satisfied"
                     )
                 });
                 err.set_primary_message(&primary_message);

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

@@ -67,6 +67,7 @@ use rustc_hir::def_id::{DefId, LocalDefId};
 use rustc_hir::lang_items::LangItem;
 use rustc_hir::Node;
 use rustc_middle::dep_graph::DepContext;
+use rustc_middle::ty::print::with_forced_trimmed_paths;
 use rustc_middle::ty::relate::{self, RelateResult, TypeRelation};
 use rustc_middle::ty::{
     self, error::TypeError, List, Region, Ty, TyCtxt, TypeFoldable, TypeSuperVisitable,
@@ -1612,16 +1613,31 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
                 {
                     format!("expected this to be `{}`", expected)
                 } else {
-                    terr.to_string()
+                    terr.to_string(self.tcx).to_string()
                 };
                 label_or_note(sp, &terr);
                 label_or_note(span, &msg);
             } else {
-                label_or_note(span, &terr.to_string());
+                label_or_note(span, &terr.to_string(self.tcx));
                 label_or_note(sp, &msg);
             }
         } else {
-            label_or_note(span, &terr.to_string());
+            if let Some(values) = values
+                && let Some((e, f)) = values.ty()
+                && let TypeError::ArgumentSorts(..) | TypeError::Sorts(_) = terr
+            {
+                let e = self.tcx.erase_regions(e);
+                let f = self.tcx.erase_regions(f);
+                let expected = with_forced_trimmed_paths!(e.sort_string(self.tcx));
+                let found = with_forced_trimmed_paths!(f.sort_string(self.tcx));
+                if expected == found {
+                    label_or_note(span, &terr.to_string(self.tcx));
+                } else {
+                    label_or_note(span, &format!("expected {expected}, found {found}"));
+                }
+            } else {
+                label_or_note(span, &terr.to_string(self.tcx));
+            }
         }
 
         if let Some((expected, found, exp_p, found_p)) = expected_found {
@@ -1849,6 +1865,18 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
         self.check_and_note_conflicting_crates(diag, terr);
 
         self.note_and_explain_type_err(diag, terr, cause, span, cause.body_id.to_def_id());
+        if let Some(exp_found) = exp_found
+            && let exp_found = TypeError::Sorts(exp_found)
+            && exp_found != terr
+        {
+            self.note_and_explain_type_err(
+                diag,
+                exp_found,
+                cause,
+                span,
+                cause.body_id.to_def_id(),
+            );
+        }
 
         if let Some(ValuePairs::PolyTraitRefs(exp_found)) = values
             && let ty::Closure(def_id, _) = exp_found.expected.skip_binder().self_ty().kind()

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

@@ -137,25 +137,25 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
                             diag.help(
                                 "given a type parameter `T` and a method `foo`:
 ```
-trait Trait<T> { fn foo(&tcx) -> T; }
+trait Trait<T> { fn foo(&self) -> T; }
 ```
 the only ways to implement method `foo` are:
 - constrain `T` with an explicit type:
 ```
 impl Trait<String> for X {
-    fn foo(&tcx) -> String { String::new() }
+    fn foo(&self) -> String { String::new() }
 }
 ```
 - add a trait bound to `T` and call a method on that trait that returns `Self`:
 ```
 impl<T: std::default::Default> Trait<T> for X {
-    fn foo(&tcx) -> T { <T as std::default::Default>::default() }
+    fn foo(&self) -> T { <T as std::default::Default>::default() }
 }
 ```
 - change `foo` to return an argument of type `T`:
 ```
 impl<T> Trait<T> for X {
-    fn foo(&tcx, x: T) -> T { x }
+    fn foo(&self, x: T) -> T { x }
 }
 ```",
                             );
@@ -218,6 +218,13 @@ impl<T> Trait<T> for X {
                             );
                         }
                     }
+                    (ty::FnPtr(_), ty::FnDef(def, _))
+                    if let hir::def::DefKind::Fn = tcx.def_kind(def) => {
+                        diag.note(
+                            "when the arguments and return types match, functions can be coerced \
+                             to function pointers",
+                        );
+                    }
                     _ => {}
                 }
                 debug!(
@@ -389,14 +396,14 @@ impl<T> Trait<T> for X {
 ```
 trait Trait {
 type T;
-fn foo(&tcx) -> Self::T;
+fn foo(&self) -> Self::T;
 }
 ```
 the only way of implementing method `foo` is to constrain `T` with an explicit associated type:
 ```
 impl Trait for X {
 type T = String;
-fn foo(&tcx) -> Self::T { String::new() }
+fn foo(&self) -> Self::T { String::new() }
 }
 ```",
             );

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

@@ -404,7 +404,7 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
                         (msg, sug)
                     }
                 };
-                diag.span_suggestion(span, msg, sug, Applicability::MaybeIncorrect);
+                diag.span_suggestion_verbose(span, msg, sug, Applicability::MaybeIncorrect);
             }
             (ty::FnDef(did1, substs1), ty::FnDef(did2, substs2)) => {
                 let expected_sig =