Tweak :: -> : typo heuristic and reduce verbosity

Do not trigger on correct type ascription expressions with trailing
operators and _do_ trigger on likely path typos where a turbofish is
used.

On likely path typos, remove note explaining type ascription.

src/librustc_parse/parser/diagnostics.rs

@@ -346,13 +346,16 @@ impl<'a> Parser<'a> {
             if allow_unstable {
                 // Give extra information about type ascription only if it's a nightly compiler.
                 err.note(
-                    "`#![feature(type_ascription)]` lets you annotate an expression with a \
-                          type: `<expr>: <type>`",
-                );
-                err.note(
-                    "see issue #23416 <https://github.com/rust-lang/rust/issues/23416> \
-                     for more information",
+                    "`#![feature(type_ascription)]` lets you annotate an expression with a type: \
+                     `<expr>: <type>`",
                 );
+                if !likely_path {
+                    // Avoid giving too much info when it was likely an unrelated typo.
+                    err.note(
+                        "see issue #23416 <https://github.com/rust-lang/rust/issues/23416> \
+                        for more information",
+                    );
+                }
             }
         }
     }
@@ -1152,8 +1155,10 @@ impl<'a> Parser<'a> {
             } &&
             !self.token.is_reserved_ident() &&           // v `foo:bar(baz)`
             self.look_ahead(1, |t| t == &token::OpenDelim(token::Paren))
-            || self.look_ahead(1, |t| t == &token::Lt) &&     // `foo:bar<baz`
-            self.look_ahead(2, |t| t.is_ident())
+            || self.look_ahead(1, |t| t == &token::OpenDelim(token::Brace)) // `foo:bar {`
+            || self.look_ahead(1, |t| t == &token::Colon) &&     // `foo:bar::<baz`
+            self.look_ahead(2, |t| t == &token::Lt) &&
+            self.look_ahead(3, |t| t.is_ident())
             || self.look_ahead(1, |t| t == &token::Colon) &&  // `foo:bar:baz`
             self.look_ahead(2, |t| t.is_ident())
             || self.look_ahead(1, |t| t == &token::ModSep)

src/test/ui/lifetime_starts_expressions.stderr

@@ -15,10 +15,9 @@ error: expected type, found keyword `loop`
 LL |     loop { break 'label: loop { break 'label 42; }; }
    |                        - ^^^^ expected type
    |                        |
-   |                        tried to parse a type due to this type ascription
+   |                        help: maybe write a path separator here: `::`
    |
    = note: `#![feature(type_ascription)]` lets you annotate an expression with a type: `<expr>: <type>`
-   = note: see issue #23416 <https://github.com/rust-lang/rust/issues/23416> for more information
 
 error: aborting due to 2 previous errors
 

src/test/ui/parser/issue-35813-postfix-after-cast.stderr

@@ -280,12 +280,12 @@ error: casts cannot be followed by ?
   --> $DIR/issue-35813-postfix-after-cast.rs:121:5
    |
 LL |     Err(0u64): Result<u64,u64>?;
-   |     ^^^^^^^^^-^^^^^^^^^^^^^^^^
-   |              |
-   |              help: maybe write a path separator here: `::`
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
-   = note: `#![feature(type_ascription)]` lets you annotate an expression with a type: `<expr>: <type>`
-   = note: see issue #23416 <https://github.com/rust-lang/rust/issues/23416> for more information
+help: try surrounding the expression in parentheses
+   |
+LL |     (Err(0u64): Result<u64,u64>)?;
+   |     ^                          ^
 
 error: casts cannot be followed by a function call
   --> $DIR/issue-35813-postfix-after-cast.rs:145:5
@@ -324,12 +324,12 @@ error: casts cannot be followed by `.await`
   --> $DIR/issue-35813-postfix-after-cast.rs:155:5
    |
 LL |     Box::pin(noop()): Pin<Box<_>>.await;
-   |     ^^^^^^^^^^^^^^^^-^^^^^^^^^^^^
-   |                     |
-   |                     help: maybe write a path separator here: `::`
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+help: try surrounding the expression in parentheses
    |
-   = note: `#![feature(type_ascription)]` lets you annotate an expression with a type: `<expr>: <type>`
-   = note: see issue #23416 <https://github.com/rust-lang/rust/issues/23416> for more information
+LL |     (Box::pin(noop()): Pin<Box<_>>).await;
+   |     ^                             ^
 
 error: casts cannot be followed by a field access
   --> $DIR/issue-35813-postfix-after-cast.rs:167:5

src/test/ui/suggestions/type-ascription-instead-of-method.stderr

@@ -7,7 +7,6 @@ LL |     Box:new("foo".to_string())
    |        help: maybe write a path separator here: `::`
    |
    = note: `#![feature(type_ascription)]` lets you annotate an expression with a type: `<expr>: <type>`
-   = note: see issue #23416 <https://github.com/rust-lang/rust/issues/23416> for more information
 
 error: aborting due to previous error
 

src/test/ui/suggestions/type-ascription-instead-of-path-2.stderr

@@ -7,7 +7,6 @@ LL |     vec![Ok(2)].into_iter().collect:<Result<Vec<_>,_>>()?;
    |                                    help: maybe write a path separator here: `::`
    |
    = note: `#![feature(type_ascription)]` lets you annotate an expression with a type: `<expr>: <type>`
-   = note: see issue #23416 <https://github.com/rust-lang/rust/issues/23416> for more information
 
 error: aborting due to previous error
 

src/test/ui/suggestions/type-ascription-instead-of-variant.stderr

@@ -7,7 +7,6 @@ LL |     let _ = Option:Some("");
    |                   help: maybe write a path separator here: `::`
    |
    = note: `#![feature(type_ascription)]` lets you annotate an expression with a type: `<expr>: <type>`
-   = note: see issue #23416 <https://github.com/rust-lang/rust/issues/23416> for more information
 
 error: aborting due to previous error
 

src/test/ui/type/ascription/issue-47666.stderr

@@ -10,7 +10,6 @@ LL |     let _ = Option:Some(vec![0, 1]);
    |                   help: maybe write a path separator here: `::`
    |
    = note: `#![feature(type_ascription)]` lets you annotate an expression with a type: `<expr>: <type>`
-   = note: see issue #23416 <https://github.com/rust-lang/rust/issues/23416> for more information
    = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error[E0423]: expected value, found enum `Option`

src/test/ui/type/ascription/issue-54516.stderr

@@ -7,7 +7,6 @@ LL |     println!("{}", std::mem:size_of::<BTreeMap<u32, u32>>());
    |                            help: maybe write a path separator here: `::`
    |
    = note: `#![feature(type_ascription)]` lets you annotate an expression with a type: `<expr>: <type>`
-   = note: see issue #23416 <https://github.com/rust-lang/rust/issues/23416> for more information
 
 error[E0423]: expected value, found module `std::mem`
   --> $DIR/issue-54516.rs:4:20

src/test/ui/type/ascription/issue-60933.stderr

@@ -7,7 +7,6 @@ LL |     let u: usize = std::mem:size_of::<u32>();
    |                            help: maybe write a path separator here: `::`
    |
    = note: `#![feature(type_ascription)]` lets you annotate an expression with a type: `<expr>: <type>`
-   = note: see issue #23416 <https://github.com/rust-lang/rust/issues/23416> for more information
 
 error[E0423]: expected value, found module `std::mem`
   --> $DIR/issue-60933.rs:2:20