Suggest to swap a struct and a trait in trait impls

compiler/rustc_resolve/src/late.rs

@@ -496,6 +496,9 @@ struct DiagnosticMetadata<'ast> {
 
     /// The current impl items (used to suggest).
     current_impl_items: Option<&'ast [P<AssocItem>]>,
+
+    /// When processing impl trait
+    currently_processing_impl_trait: Option<(TraitRef, Ty)>,
 }
 
 struct LateResolutionVisitor<'a, 'b, 'ast> {
@@ -2066,18 +2069,22 @@ impl<'a: 'ast, 'b, 'ast> LateResolutionVisitor<'a, 'b, 'ast> {
     fn with_optional_trait_ref<T>(
         &mut self,
         opt_trait_ref: Option<&TraitRef>,
+        self_type: &'ast Ty,
         f: impl FnOnce(&mut Self, Option<DefId>) -> T,
     ) -> T {
         let mut new_val = None;
         let mut new_id = None;
         if let Some(trait_ref) = opt_trait_ref {
             let path: Vec<_> = Segment::from_path(&trait_ref.path);
+            self.diagnostic_metadata.currently_processing_impl_trait =
+                Some((trait_ref.clone(), self_type.clone()));
             let res = self.smart_resolve_path_fragment(
                 None,
                 &path,
                 PathSource::Trait(AliasPossibility::No),
                 Finalize::new(trait_ref.ref_id, trait_ref.path.span),
             );
+            self.diagnostic_metadata.currently_processing_impl_trait = None;
             if let Some(def_id) = res.base_res().opt_def_id() {
                 new_id = Some(def_id);
                 new_val = Some((self.r.expect_module(def_id), trait_ref.clone()));
@@ -2118,7 +2125,7 @@ impl<'a: 'ast, 'b, 'ast> LateResolutionVisitor<'a, 'b, 'ast> {
             this.with_self_rib(Res::SelfTy { trait_: None, alias_to: None }, |this| {
                 this.with_lifetime_rib(LifetimeRibKind::AnonymousCreateParameter(item_id), |this| {
                     // Resolve the trait reference, if necessary.
-                    this.with_optional_trait_ref(opt_trait_reference.as_ref(), |this, trait_id| {
+                    this.with_optional_trait_ref(opt_trait_reference.as_ref(), self_type, |this, trait_id| {
                         let item_def_id = this.r.local_def_id(item_id);
 
                         // Register the trait definitions from here.

compiler/rustc_resolve/src/late/diagnostics.rs

@@ -145,6 +145,8 @@ impl<'a: 'ast, 'ast> LateResolutionVisitor<'a, '_, 'ast> {
         let is_expected = &|res| source.is_expected(res);
         let is_enum_variant = &|res| matches!(res, Res::Def(DefKind::Variant, _));
 
+        debug!(?res, ?source);
+
         // Make the base error.
         struct BaseError<'a> {
             msg: String,
@@ -249,6 +251,8 @@ impl<'a: 'ast, 'ast> LateResolutionVisitor<'a, '_, 'ast> {
         let mut err =
             self.r.session.struct_span_err_with_code(base_error.span, &base_error.msg, code);
 
+        self.suggest_swapping_misplaced_self_ty_and_trait(&mut err, source, res, base_error.span);
+
         if let Some(sugg) = base_error.suggestion {
             err.span_suggestion_verbose(sugg.0, sugg.1, sugg.2, Applicability::MaybeIncorrect);
         }
@@ -683,6 +687,35 @@ impl<'a: 'ast, 'ast> LateResolutionVisitor<'a, '_, 'ast> {
         }
     }
 
+    fn suggest_swapping_misplaced_self_ty_and_trait(
+        &mut self,
+        err: &mut Diagnostic,
+        source: PathSource<'_>,
+        res: Option<Res>,
+        span: Span,
+    ) {
+        if let Some((trait_ref, self_ty)) =
+            self.diagnostic_metadata.currently_processing_impl_trait.clone()
+            && let TyKind::Path(_, self_ty_path) = &self_ty.kind
+            && let PathResult::Module(ModuleOrUniformRoot::Module(module)) =
+                self.resolve_path(&Segment::from_path(self_ty_path), Some(TypeNS), None)
+            && let ModuleKind::Def(DefKind::Trait, ..) = module.kind
+            && trait_ref.path.span == span
+            && let PathSource::Trait(_) = source
+            && let Some(Res::Def(DefKind::Struct | DefKind::Enum | DefKind::Union, _)) = res
+            && let Ok(self_ty_str) =
+                self.r.session.source_map().span_to_snippet(self_ty.span)
+            && let Ok(trait_ref_str) =
+                self.r.session.source_map().span_to_snippet(trait_ref.path.span)
+        {
+                err.multipart_suggestion(
+                    "`impl` items mention the trait being implemented first and the type it is being implemented for second",
+                    vec![(trait_ref.path.span, self_ty_str), (self_ty.span, trait_ref_str)],
+                    Applicability::MaybeIncorrect,
+                );
+        }
+    }
+
     fn get_single_associated_item(
         &mut self,
         path: &[Segment],

compiler/rustc_span/src/lib.rs

@@ -338,7 +338,7 @@ impl<'a> FileNameDisplay<'a> {
     pub fn to_string_lossy(&self) -> Cow<'a, str> {
         match self.inner {
             FileName::Real(ref inner) => inner.to_string_lossy(self.display_pref),
-            _ => Cow::from(format!("{}", self)),
+            _ => Cow::from(self.to_string()),
         }
     }
 }

src/test/ui/suggestions/suggest-swapping-self-ty-and-trait-edition-2021.rs

@@ -0,0 +1,22 @@
+// edition:2021
+
+pub trait Trait<'a, T> {}
+
+pub struct Struct<T>;
+pub enum Enum<T> {}
+
+pub union Union<T> {
+    f1: usize,
+}
+
+impl<'a, T> Struct<T> for Trait<'a, T> {}
+//~^ ERROR expected trait, found struct `Struct`
+//~| ERROR trait objects must include the `dyn` keyword
+
+impl<'a, T> Enum<T> for Trait<'a, T> {}
+//~^ ERROR expected trait, found enum `Enum`
+
+impl<'a, T> Union<T> for Trait<'a, T> {}
+//~^ ERROR expected trait, found union `Union`
+
+fn main() {}

src/test/ui/suggestions/suggest-swapping-self-ty-and-trait-edition-2021.stderr

@@ -0,0 +1,49 @@
+error[E0404]: expected trait, found struct `Struct`
+  --> $DIR/suggest-swapping-self-ty-and-trait-edition-2021.rs:12:13
+   |
+LL | impl<'a, T> Struct<T> for Trait<'a, T> {}
+   |             ^^^^^^^^^ not a trait
+   |
+help: `impl` items mention the trait being implemented first and the type it is being implemented for second
+   |
+LL | impl<'a, T> Trait<'a, T> for Struct<T> {}
+   |             ~~~~~~~~~~~~     ~~~~~~~~~
+
+error[E0404]: expected trait, found enum `Enum`
+  --> $DIR/suggest-swapping-self-ty-and-trait-edition-2021.rs:16:13
+   |
+LL | impl<'a, T> Enum<T> for Trait<'a, T> {}
+   |             ^^^^^^^ not a trait
+   |
+help: `impl` items mention the trait being implemented first and the type it is being implemented for second
+   |
+LL | impl<'a, T> Trait<'a, T> for Enum<T> {}
+   |             ~~~~~~~~~~~~     ~~~~~~~
+
+error[E0404]: expected trait, found union `Union`
+  --> $DIR/suggest-swapping-self-ty-and-trait-edition-2021.rs:19:13
+   |
+LL | impl<'a, T> Union<T> for Trait<'a, T> {}
+   |             ^^^^^^^^ not a trait
+   |
+help: `impl` items mention the trait being implemented first and the type it is being implemented for second
+   |
+LL | impl<'a, T> Trait<'a, T> for Union<T> {}
+   |             ~~~~~~~~~~~~     ~~~~~~~~
+
+error[E0782]: trait objects must include the `dyn` keyword
+  --> $DIR/suggest-swapping-self-ty-and-trait-edition-2021.rs:12:27
+   |
+LL | impl<'a, T> Struct<T> for Trait<'a, T> {}
+   |                           ^^^^^^^^^^^^
+   |
+help: add `dyn` keyword before this trait
+   |
+LL - impl<'a, T> Struct<T> for Trait<'a, T> {}
+LL + impl<'a, T> Struct<T> for dyn Trait<'a, T> {}
+   | 
+
+error: aborting due to 4 previous errors
+
+Some errors have detailed explanations: E0404, E0782.
+For more information about an error, try `rustc --explain E0404`.

src/test/ui/suggestions/suggest-swapping-self-ty-and-trait.rs

@@ -0,0 +1,21 @@
+pub trait Trait<'a, T> {}
+
+pub struct Struct<T>;
+pub enum Enum<T> {}
+
+pub union Union<T> {
+    f1: usize,
+}
+
+impl<'a, T> Struct<T> for Trait<'a, T> {}
+//~^ ERROR expected trait, found struct `Struct`
+//~| WARNING trait objects without an explicit `dyn` are deprecated
+//~| WARNING this is accepted in the current edition (Rust 2015) but is a hard error in Rust 2021!
+
+impl<'a, T> Enum<T> for Trait<'a, T> {}
+//~^ ERROR expected trait, found enum `Enum`
+
+impl<'a, T> Union<T> for Trait<'a, T> {}
+//~^ ERROR expected trait, found union `Union`
+
+fn main() {}

src/test/ui/suggestions/suggest-swapping-self-ty-and-trait.stderr

@@ -0,0 +1,51 @@
+error[E0404]: expected trait, found struct `Struct`
+  --> $DIR/suggest-swapping-self-ty-and-trait.rs:10:13
+   |
+LL | impl<'a, T> Struct<T> for Trait<'a, T> {}
+   |             ^^^^^^^^^ not a trait
+   |
+help: `impl` items mention the trait being implemented first and the type it is being implemented for second
+   |
+LL | impl<'a, T> Trait<'a, T> for Struct<T> {}
+   |             ~~~~~~~~~~~~     ~~~~~~~~~
+
+error[E0404]: expected trait, found enum `Enum`
+  --> $DIR/suggest-swapping-self-ty-and-trait.rs:15:13
+   |
+LL | impl<'a, T> Enum<T> for Trait<'a, T> {}
+   |             ^^^^^^^ not a trait
+   |
+help: `impl` items mention the trait being implemented first and the type it is being implemented for second
+   |
+LL | impl<'a, T> Trait<'a, T> for Enum<T> {}
+   |             ~~~~~~~~~~~~     ~~~~~~~
+
+error[E0404]: expected trait, found union `Union`
+  --> $DIR/suggest-swapping-self-ty-and-trait.rs:18:13
+   |
+LL | impl<'a, T> Union<T> for Trait<'a, T> {}
+   |             ^^^^^^^^ not a trait
+   |
+help: `impl` items mention the trait being implemented first and the type it is being implemented for second
+   |
+LL | impl<'a, T> Trait<'a, T> for Union<T> {}
+   |             ~~~~~~~~~~~~     ~~~~~~~~
+
+warning: trait objects without an explicit `dyn` are deprecated
+  --> $DIR/suggest-swapping-self-ty-and-trait.rs:10:27
+   |
+LL | impl<'a, T> Struct<T> for Trait<'a, T> {}
+   |                           ^^^^^^^^^^^^
+   |
+   = note: `#[warn(bare_trait_objects)]` on by default
+   = warning: this is accepted in the current edition (Rust 2015) but is a hard error in Rust 2021!
+   = note: for more information, see <https://doc.rust-lang.org/nightly/edition-guide/rust-2021/warnings-promoted-to-error.html>
+help: use `dyn`
+   |
+LL - impl<'a, T> Struct<T> for Trait<'a, T> {}
+LL + impl<'a, T> Struct<T> for dyn Trait<'a, T> {}
+   | 
+
+error: aborting due to 3 previous errors; 1 warning emitted
+
+For more information about this error, try `rustc --explain E0404`.