Rollup merge of #84896 - estebank:issue-84772, r=jackh726

Handle incorrect placement of parentheses in trait bounds more gracefully

Fix #84772.

CC ``````@jonhoo``````

src/test/ui/parser/trait-object-delimiters.rs

@@ -0,0 +1,17 @@
+// edition:2018
+
+fn foo1(_: &dyn Drop + AsRef<str>) {} //~ ERROR ambiguous `+` in a type
+//~^ ERROR only auto traits can be used as additional traits in a trait object
+
+fn foo2(_: &dyn (Drop + AsRef<str>)) {} //~ ERROR incorrect braces around trait bounds
+
+fn foo3(_: &dyn {Drop + AsRef<str>}) {} //~ ERROR expected parameter name, found `{`
+//~^ ERROR expected one of `!`, `(`, `)`, `,`, `?`, `for`, lifetime, or path, found `{`
+//~| ERROR at least one trait is required for an object type
+
+fn foo4(_: &dyn <Drop + AsRef<str>>) {} //~ ERROR expected identifier, found `<`
+
+fn foo5(_: &(dyn Drop + dyn AsRef<str>)) {} //~ ERROR invalid `dyn` keyword
+//~^ ERROR only auto traits can be used as additional traits in a trait object
+
+fn main() {}

src/test/ui/parser/trait-object-delimiters.stderr

@@ -0,0 +1,77 @@
+error: ambiguous `+` in a type
+  --> $DIR/trait-object-delimiters.rs:3:13
+   |
+LL | fn foo1(_: &dyn Drop + AsRef<str>) {}
+   |             ^^^^^^^^^^^^^^^^^^^^^ help: use parentheses to disambiguate: `(dyn Drop + AsRef<str>)`
+
+error: incorrect braces around trait bounds
+  --> $DIR/trait-object-delimiters.rs:6:17
+   |
+LL | fn foo2(_: &dyn (Drop + AsRef<str>)) {}
+   |                 ^                 ^
+   |
+help: remove the parentheses
+   |
+LL | fn foo2(_: &dyn Drop + AsRef<str>) {}
+   |                --               --
+
+error: expected parameter name, found `{`
+  --> $DIR/trait-object-delimiters.rs:8:17
+   |
+LL | fn foo3(_: &dyn {Drop + AsRef<str>}) {}
+   |                 ^ expected parameter name
+
+error: expected one of `!`, `(`, `)`, `,`, `?`, `for`, lifetime, or path, found `{`
+  --> $DIR/trait-object-delimiters.rs:8:17
+   |
+LL | fn foo3(_: &dyn {Drop + AsRef<str>}) {}
+   |                -^ expected one of 8 possible tokens
+   |                |
+   |                help: missing `,`
+
+error: expected identifier, found `<`
+  --> $DIR/trait-object-delimiters.rs:12:17
+   |
+LL | fn foo4(_: &dyn <Drop + AsRef<str>>) {}
+   |                 ^ expected identifier
+
+error: invalid `dyn` keyword
+  --> $DIR/trait-object-delimiters.rs:14:25
+   |
+LL | fn foo5(_: &(dyn Drop + dyn AsRef<str>)) {}
+   |                         ^^^ help: remove this keyword
+   |
+   = help: `dyn` is only needed at the start of a trait `+`-separated list
+
+error[E0225]: only auto traits can be used as additional traits in a trait object
+  --> $DIR/trait-object-delimiters.rs:3:24
+   |
+LL | fn foo1(_: &dyn Drop + AsRef<str>) {}
+   |                 ----   ^^^^^^^^^^ additional non-auto trait
+   |                 |
+   |                 first non-auto trait
+   |
+   = help: consider creating a new trait with all of these as super-traits and using that trait here instead: `trait NewTrait: Drop + AsRef<str> {}`
+   = 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[E0224]: at least one trait is required for an object type
+  --> $DIR/trait-object-delimiters.rs:8:13
+   |
+LL | fn foo3(_: &dyn {Drop + AsRef<str>}) {}
+   |             ^^^
+
+error[E0225]: only auto traits can be used as additional traits in a trait object
+  --> $DIR/trait-object-delimiters.rs:14:29
+   |
+LL | fn foo5(_: &(dyn Drop + dyn AsRef<str>)) {}
+   |                  ----       ^^^^^^^^^^ additional non-auto trait
+   |                  |
+   |                  first non-auto trait
+   |
+   = help: consider creating a new trait with all of these as super-traits and using that trait here instead: `trait NewTrait: Drop + AsRef<str> {}`
+   = 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 9 previous errors
+
+Some errors have detailed explanations: E0224, E0225.
+For more information about an error, try `rustc --explain E0224`.