Do not suggest <_ as Into<_>> blanket impl

We will usually have a better, more specific, alternative suggestion.

Author: estebank

compiler/rustc_trait_selection/src/error_reporting/infer/need_type_info.rs

@@ -843,7 +843,7 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
                     assoc
                 } else {
                     // The method isn't in this `impl` or `trait`? Not useful to us then.
-                return;
+                    return;
                 }
             };
             let Some(trait_assoc_item) = assoc.trait_item_def_id else {
@@ -863,7 +863,15 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
                     });
                 }
                 InferenceSuggestionFormat::FullyQualifiedMethodCall => {
-                    paths.push(self.tcx.value_path_str_with_args(def_id, args));
+                    if let Some(did) = tcx.get_diagnostic_item(sym::Into)
+                        && did == trait_def_id
+                        && let Some(arg) = impl_args.types().skip(1).next()
+                        && let ty::Infer(_) | ty::Param(_) = arg.kind()
+                    {
+                        // Skip `<T as Into<_>>` blanket
+                    } else {
+                        paths.push(self.tcx.value_path_str_with_args(def_id, args));
+                    }
                 }
             }
         });

tests/ui/error-codes/E0283.stderr

@@ -30,10 +30,6 @@ LL | impl Into<u32> for Impl {
              where U: From<T>;
 help: try using a fully qualified path to specify the expected types
    |
-LL |     let bar = <_ as Into<_>>::into(foo_impl) * 1u32;
-   |               +++++++++++++++++++++        ~
-help: try using a fully qualified path to specify the expected types
-   |
 LL |     let bar = <Impl as Into<u32>>::into(foo_impl) * 1u32;
    |               ++++++++++++++++++++++++++        ~
 

tests/ui/pattern/slice-pattern-refutable.stderr

@@ -23,10 +23,6 @@ LL |     if let [a, b, c] = Zeroes.into() {
    |
 help: try using a fully qualified path to specify the expected types
    |
-LL |     if let [a, b, c] = <_ as Into<_>>::into(Zeroes) {
-   |                        +++++++++++++++++++++      ~
-help: try using a fully qualified path to specify the expected types
-   |
 LL |     if let [a, b, c] = <Zeroes as Into<[usize; 3]>>::into(Zeroes) {
    |                        +++++++++++++++++++++++++++++++++++      ~
 
@@ -40,10 +36,6 @@ LL |     if let [a, b, c] = Zeroes.into() {
    |
 help: try using a fully qualified path to specify the expected types
    |
-LL |     if let [a, b, c] = <_ as Into<_>>::into(Zeroes) {
-   |                        +++++++++++++++++++++      ~
-help: try using a fully qualified path to specify the expected types
-   |
 LL |     if let [a, b, c] = <Zeroes as Into<[usize; 3]>>::into(Zeroes) {
    |                        +++++++++++++++++++++++++++++++++++      ~
 

tests/ui/pattern/slice-patterns-ambiguity.stderr

@@ -19,10 +19,6 @@ LL |     if let &[a, b] = Zeroes.into() {
    |
 help: try using a fully qualified path to specify the expected types
    |
-LL |     if let &[a, b] = <_ as Into<_>>::into(Zeroes) {
-   |                      +++++++++++++++++++++      ~
-help: try using a fully qualified path to specify the expected types
-   |
 LL |     if let &[a, b] = <Zeroes as Into<&'static [usize; 2]>>::into(Zeroes) {
    |                      ++++++++++++++++++++++++++++++++++++++++++++      ~
 help: try using a fully qualified path to specify the expected types
@@ -40,10 +36,6 @@ LL |     if let &[a, b] = Zeroes.into() {
    |
 help: try using a fully qualified path to specify the expected types
    |
-LL |     if let &[a, b] = <_ as Into<_>>::into(Zeroes) {
-   |                      +++++++++++++++++++++      ~
-help: try using a fully qualified path to specify the expected types
-   |
 LL |     if let &[a, b] = <Zeroes as Into<&'static [usize; 2]>>::into(Zeroes) {
    |                      ++++++++++++++++++++++++++++++++++++++++++++      ~
 help: try using a fully qualified path to specify the expected types

tests/ui/suggestions/types/into-inference-needs-type.stderr

@@ -8,7 +8,7 @@ LL |         .into()?;
    = note: required for `FilterMap<Map<std::slice::Iter<'_, &str>, {closure@$DIR/into-inference-needs-type.rs:10:14: 10:17}>, fn(Option<&str>) -> Option<Option<&str>> {Option::<Option<&str>>::Some}>` to implement `Into<_>`
 help: try using a fully qualified path to specify the expected types
    |
-LL ~     let list = <_ as Into<_>>::into(vec
+LL ~     let list = <FilterMap<Map<std::slice::Iter<'_, &str>, _>, _> as Into<T>>::into(vec
 LL |         .iter()
 LL |         .map(|s| s.strip_prefix("t"))
 LL ~         .filter_map(Option::Some))?;