Auto merge of #75536 - estebank:e0255-suggestion, r=varkor

Tweak output of E0225 When encountering multiple non-auto trait bounds suggest creating a new trait and explain what auto-traits are. _Inspired by https://fasterthanli.me/articles/frustrated-its-not-you-its-rust_
rust-lang · Aug 16, 2020 · 97ba0c7 · 97ba0c7
2 parents de32266 + 0afb9c2
commit 97ba0c7
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diff --git a/src/librustc_trait_selection/traits/util.rs b/src/librustc_trait_selection/traits/util.rs
@@ -53,10 +53,14 @@ impl<'tcx> TraitAliasExpansionInfo<'tcx> {
                 diag.span_label(*sp, format!("referenced here ({})", use_desc));
             }
         }
-        diag.span_label(
-            self.bottom().1,
-            format!("trait alias used in trait object type ({})", use_desc),
-        );
+        if self.top().1 != self.bottom().1 {
+            // When the trait object is in a return type these two spans match, we don't want
+            // redundant labels.
+            diag.span_label(
+                self.bottom().1,
+                format!("trait alias used in trait object type ({})", use_desc),
+            );
+        }
     }
 
     pub fn trait_ref(&self) -> ty::PolyTraitRef<'tcx> {

diff --git a/src/librustc_typeck/astconv.rs b/src/librustc_typeck/astconv.rs
@@ -1666,6 +1666,20 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
                 "additional use",
             );
             first_trait.label_with_exp_info(&mut err, "first non-auto trait", "first use");
+            err.help(&format!(
+                "consider creating a new trait with all of these as super-traits and using that \
+                 trait here instead: `trait NewTrait: {} {{}}`",
+                regular_traits
+                    .iter()
+                    .map(|t| t.trait_ref().print_only_trait_path().to_string())
+                    .collect::<Vec<_>>()
+                    .join(" + "),
+            ));
+            err.note(
+                "auto-traits like `Send` and `Sync` are traits that have special properties; \
+                 for more information on them, visit \
+                 <https://doc.rust-lang.org/reference/special-types-and-traits.html#auto-traits>",
+            );
             err.emit();
         }
 

diff --git a/src/test/ui/associated-types/missing-associated-types.stderr b/src/test/ui/associated-types/missing-associated-types.stderr
@@ -2,12 +2,12 @@ error[E0225]: only auto traits can be used as additional traits in a trait objec
   --> $DIR/missing-associated-types.rs:12:32
    |
 LL | type Foo<Rhs> = dyn Add<Rhs> + Sub<Rhs> + X<Rhs> + Y<Rhs>;
-   |                     --------   ^^^^^^^^
-   |                     |          |
-   |                     |          additional non-auto trait
-   |                     |          trait alias used in trait object type (additional use)
+   |                     --------   ^^^^^^^^ additional non-auto trait
+   |                     |
    |                     first non-auto trait
-   |                     trait alias used in trait object type (first use)
+   |
+   = help: consider creating a new trait with all of these as super-traits and using that trait here instead: `trait NewTrait: std::ops::Add<Rhs> + std::ops::Sub<Rhs> + X<Rhs> + Y<Rhs> {}`
+   = note: auto-traits like `Send` and `Sync` are traits that have special properties; for more information on them, visit <https://doc.rust-lang.org/reference/special-types-and-traits.html#auto-traits>
 
 error[E0191]: the value of the associated types `A` (from trait `Y`), `Output` (from trait `std::ops::Add`), `Output` (from trait `std::ops::Mul`), `Output` (from trait `std::ops::Sub`) must be specified
   --> $DIR/missing-associated-types.rs:12:21
@@ -31,12 +31,12 @@ error[E0225]: only auto traits can be used as additional traits in a trait objec
   --> $DIR/missing-associated-types.rs:15:32
    |
 LL | type Bar<Rhs> = dyn Add<Rhs> + Sub<Rhs> + X<Rhs> + Z<Rhs>;
-   |                     --------   ^^^^^^^^
-   |                     |          |
-   |                     |          additional non-auto trait
-   |                     |          trait alias used in trait object type (additional use)
+   |                     --------   ^^^^^^^^ additional non-auto trait
+   |                     |
    |                     first non-auto trait
-   |                     trait alias used in trait object type (first use)
+   |
+   = help: consider creating a new trait with all of these as super-traits and using that trait here instead: `trait NewTrait: std::ops::Add<Rhs> + std::ops::Sub<Rhs> + X<Rhs> + Z<Rhs> {}`
+   = note: auto-traits like `Send` and `Sync` are traits that have special properties; for more information on them, visit <https://doc.rust-lang.org/reference/special-types-and-traits.html#auto-traits>
 
 error[E0191]: the value of the associated types `A` (from trait `Z`), `B` (from trait `Z`), `Output` (from trait `std::ops::Add`), `Output` (from trait `std::ops::Div`), `Output` (from trait `std::ops::Div`), `Output` (from trait `std::ops::Mul`), `Output` (from trait `std::ops::Sub`) must be specified
   --> $DIR/missing-associated-types.rs:15:21
@@ -67,12 +67,12 @@ error[E0225]: only auto traits can be used as additional traits in a trait objec
   --> $DIR/missing-associated-types.rs:18:32
    |
 LL | type Baz<Rhs> = dyn Add<Rhs> + Sub<Rhs> + Y<Rhs>;
-   |                     --------   ^^^^^^^^
-   |                     |          |
-   |                     |          additional non-auto trait
-   |                     |          trait alias used in trait object type (additional use)
+   |                     --------   ^^^^^^^^ additional non-auto trait
+   |                     |
    |                     first non-auto trait
-   |                     trait alias used in trait object type (first use)
+   |
+   = help: consider creating a new trait with all of these as super-traits and using that trait here instead: `trait NewTrait: std::ops::Add<Rhs> + std::ops::Sub<Rhs> + Y<Rhs> {}`
+   = note: auto-traits like `Send` and `Sync` are traits that have special properties; for more information on them, visit <https://doc.rust-lang.org/reference/special-types-and-traits.html#auto-traits>
 
 error[E0191]: the value of the associated types `A` (from trait `Y`), `Output` (from trait `std::ops::Add`), `Output` (from trait `std::ops::Sub`) must be specified
   --> $DIR/missing-associated-types.rs:18:21
@@ -95,12 +95,12 @@ error[E0225]: only auto traits can be used as additional traits in a trait objec
   --> $DIR/missing-associated-types.rs:21:32
    |
 LL | type Bat<Rhs> = dyn Add<Rhs> + Sub<Rhs> + Fine<Rhs>;
-   |                     --------   ^^^^^^^^
-   |                     |          |
-   |                     |          additional non-auto trait
-   |                     |          trait alias used in trait object type (additional use)
+   |                     --------   ^^^^^^^^ additional non-auto trait
+   |                     |
    |                     first non-auto trait
-   |                     trait alias used in trait object type (first use)
+   |
+   = help: consider creating a new trait with all of these as super-traits and using that trait here instead: `trait NewTrait: std::ops::Add<Rhs> + std::ops::Sub<Rhs> + Fine<Rhs> {}`
+   = note: auto-traits like `Send` and `Sync` are traits that have special properties; for more information on them, visit <https://doc.rust-lang.org/reference/special-types-and-traits.html#auto-traits>
 
 error[E0191]: the value of the associated types `Output` (from trait `std::ops::Add`), `Output` (from trait `std::ops::Sub`) must be specified
   --> $DIR/missing-associated-types.rs:21:21

diff --git a/src/test/ui/bad/bad-sized.stderr b/src/test/ui/bad/bad-sized.stderr
@@ -2,12 +2,12 @@ error[E0225]: only auto traits can be used as additional traits in a trait objec
   --> $DIR/bad-sized.rs:4:28
    |
 LL |     let x: Vec<dyn Trait + Sized> = Vec::new();
-   |                    -----   ^^^^^
-   |                    |       |
-   |                    |       additional non-auto trait
-   |                    |       trait alias used in trait object type (additional use)
+   |                    -----   ^^^^^ additional non-auto trait
+   |                    |
    |                    first non-auto trait
-   |                    trait alias used in trait object type (first use)
+   |
+   = help: consider creating a new trait with all of these as super-traits and using that trait here instead: `trait NewTrait: Trait + std::marker::Sized {}`
+   = note: auto-traits like `Send` and `Sync` are traits that have special properties; for more information on them, visit <https://doc.rust-lang.org/reference/special-types-and-traits.html#auto-traits>
 
 error[E0277]: the size for values of type `dyn Trait` cannot be known at compilation time
   --> $DIR/bad-sized.rs:4:12

diff --git a/src/test/ui/error-codes/E0225.stderr b/src/test/ui/error-codes/E0225.stderr
@@ -2,12 +2,12 @@ error[E0225]: only auto traits can be used as additional traits in a trait objec
   --> $DIR/E0225.rs:6:36
    |
 LL |     let _: Box<dyn std::io::Read + std::io::Write>;
-   |                    -------------   ^^^^^^^^^^^^^^
-   |                    |               |
-   |                    |               additional non-auto trait
-   |                    |               trait alias used in trait object type (additional use)
+   |                    -------------   ^^^^^^^^^^^^^^ additional non-auto trait
+   |                    |
    |                    first non-auto trait
-   |                    trait alias used in trait object type (first use)
+   |
+   = help: consider creating a new trait with all of these as super-traits and using that trait here instead: `trait NewTrait: std::io::Read + std::io::Write {}`
+   = note: auto-traits like `Send` and `Sync` are traits that have special properties; for more information on them, visit <https://doc.rust-lang.org/reference/special-types-and-traits.html#auto-traits>
 
 error[E0225]: only auto traits can be used as additional traits in a trait object
   --> $DIR/E0225.rs:8:20
@@ -22,6 +22,9 @@ LL |     let _: Box<dyn Foo>;
    |                    |
    |                    trait alias used in trait object type (additional use)
    |                    trait alias used in trait object type (first use)
+   |
+   = help: consider creating a new trait with all of these as super-traits and using that trait here instead: `trait NewTrait: std::io::Read + std::io::Write {}`
+   = note: auto-traits like `Send` and `Sync` are traits that have special properties; for more information on them, visit <https://doc.rust-lang.org/reference/special-types-and-traits.html#auto-traits>
 
 error: aborting due to 2 previous errors
 

diff --git a/src/test/ui/issues/issue-22560.stderr b/src/test/ui/issues/issue-22560.stderr
@@ -28,12 +28,12 @@ error[E0225]: only auto traits can be used as additional traits in a trait objec
   --> $DIR/issue-22560.rs:9:23
    |
 LL | type Test = dyn Add + Sub;
-   |                 ---   ^^^
-   |                 |     |
-   |                 |     additional non-auto trait
-   |                 |     trait alias used in trait object type (additional use)
+   |                 ---   ^^^ additional non-auto trait
+   |                 |
    |                 first non-auto trait
-   |                 trait alias used in trait object type (first use)
+   |
+   = help: consider creating a new trait with all of these as super-traits and using that trait here instead: `trait NewTrait: Add<[type error]> + Sub<[type error]> {}`
+   = note: auto-traits like `Send` and `Sync` are traits that have special properties; for more information on them, visit <https://doc.rust-lang.org/reference/special-types-and-traits.html#auto-traits>
 
 error[E0191]: the value of the associated types `Output` (from trait `Add`), `Output` (from trait `Sub`) must be specified
   --> $DIR/issue-22560.rs:9:17

diff --git a/src/test/ui/issues/issue-32963.stderr b/src/test/ui/issues/issue-32963.stderr
@@ -2,23 +2,23 @@ error[E0225]: only auto traits can be used as additional traits in a trait objec
   --> $DIR/issue-32963.rs:8:31
    |
 LL |     size_of_copy::<dyn Misc + Copy>();
-   |                        ----   ^^^^
-   |                        |      |
-   |                        |      additional non-auto trait
-   |                        |      trait alias used in trait object type (additional use)
+   |                        ----   ^^^^ additional non-auto trait
+   |                        |
    |                        first non-auto trait
-   |                        trait alias used in trait object type (first use)
+   |
+   = help: consider creating a new trait with all of these as super-traits and using that trait here instead: `trait NewTrait: Misc + std::marker::Copy {}`
+   = note: auto-traits like `Send` and `Sync` are traits that have special properties; for more information on them, visit <https://doc.rust-lang.org/reference/special-types-and-traits.html#auto-traits>
 
 error[E0225]: only auto traits can be used as additional traits in a trait object
   --> $DIR/issue-32963.rs:8:31
    |
 LL |     size_of_copy::<dyn Misc + Copy>();
-   |                        ----   ^^^^
-   |                        |      |
-   |                        |      additional non-auto trait
-   |                        |      trait alias used in trait object type (additional use)
+   |                        ----   ^^^^ additional non-auto trait
+   |                        |
    |                        first non-auto trait
-   |                        trait alias used in trait object type (first use)
+   |
+   = help: consider creating a new trait with all of these as super-traits and using that trait here instead: `trait NewTrait: Misc + std::marker::Copy {}`
+   = note: auto-traits like `Send` and `Sync` are traits that have special properties; for more information on them, visit <https://doc.rust-lang.org/reference/special-types-and-traits.html#auto-traits>
 
 error[E0277]: the trait bound `dyn Misc: std::marker::Copy` is not satisfied
   --> $DIR/issue-32963.rs:8:5

diff --git a/src/test/ui/parser/trait-object-trait-parens.stderr b/src/test/ui/parser/trait-object-trait-parens.stderr
@@ -40,34 +40,34 @@ error[E0225]: only auto traits can be used as additional traits in a trait objec
   --> $DIR/trait-object-trait-parens.rs:8:35
    |
 LL |     let _: Box<(Obj) + (?Sized) + (for<'a> Trait<'a>)>;
-   |                -----              ^^^^^^^^^^^^^^^^^^^
-   |                |                  |
-   |                |                  additional non-auto trait
-   |                |                  trait alias used in trait object type (additional use)
+   |                -----              ^^^^^^^^^^^^^^^^^^^ additional non-auto trait
+   |                |
    |                first non-auto trait
-   |                trait alias used in trait object type (first use)
+   |
+   = help: consider creating a new trait with all of these as super-traits and using that trait here instead: `trait NewTrait: Obj + for<'a> Trait<'a> {}`
+   = note: auto-traits like `Send` and `Sync` are traits that have special properties; for more information on them, visit <https://doc.rust-lang.org/reference/special-types-and-traits.html#auto-traits>
 
 error[E0225]: only auto traits can be used as additional traits in a trait object
   --> $DIR/trait-object-trait-parens.rs:12:49
    |
 LL |     let _: Box<(?Sized) + (for<'a> Trait<'a>) + (Obj)>;
-   |                           -------------------   ^^^^^
-   |                           |                     |
-   |                           |                     additional non-auto trait
-   |                           |                     trait alias used in trait object type (additional use)
+   |                           -------------------   ^^^^^ additional non-auto trait
+   |                           |
    |                           first non-auto trait
-   |                           trait alias used in trait object type (first use)
+   |
+   = help: consider creating a new trait with all of these as super-traits and using that trait here instead: `trait NewTrait: for<'a> Trait<'a> + Obj {}`
+   = note: auto-traits like `Send` and `Sync` are traits that have special properties; for more information on them, visit <https://doc.rust-lang.org/reference/special-types-and-traits.html#auto-traits>
 
 error[E0225]: only auto traits can be used as additional traits in a trait object
   --> $DIR/trait-object-trait-parens.rs:16:38
    |
 LL |     let _: Box<(for<'a> Trait<'a>) + (Obj) + (?Sized)>;
-   |                 -----------------    ^^^^^
-   |                 |                    |
-   |                 |                    additional non-auto trait
-   |                 |                    trait alias used in trait object type (additional use)
+   |                 -----------------    ^^^^^ additional non-auto trait
+   |                 |
    |                 first non-auto trait
-   |                 trait alias used in trait object type (first use)
+   |
+   = help: consider creating a new trait with all of these as super-traits and using that trait here instead: `trait NewTrait: for<'a> Trait<'a> + Obj {}`
+   = note: auto-traits like `Send` and `Sync` are traits that have special properties; for more information on them, visit <https://doc.rust-lang.org/reference/special-types-and-traits.html#auto-traits>
 
 error: aborting due to 6 previous errors; 3 warnings emitted