Auto merge of #53578 - varkor:disappearing-turbofish-crater-run, r=<try>

[Do not merge] Optional turbofish breakage test

As an academic exercise, to satisfy ourselves that we completely understand the lay of the land, we'd like to see how often ambiguous cases arise in real-world Rust code. (Following on from #53511.)

@rust-lang/infra: is it possible to get a check-only crater run for this change?

r? @ghost

src/libsyntax/parse/parser.rs

@@ -1714,11 +1714,10 @@ impl<'a> Parser<'a> {
         } else if self.eat_keyword(keywords::Const) {
             Mutability::Immutable
         } else {
-            let span = self.prev_span;
-            self.span_err(span,
-                          "expected mut or const in raw pointer type (use \
-                           `*mut T` or `*const T` as appropriate)");
-            Mutability::Immutable
+            let mut err = self.fatal("expected mut or const in raw pointer type (use \
+            `*mut T` or `*const T` as appropriate)");
+            err.span_label(self.prev_span, "expected mut or const");
+            return Err(err);
         };
         let t = self.parse_ty_no_plus()?;
         Ok(MutTy { ty: t, mutbl: mutbl })
@@ -2022,20 +2021,31 @@ impl<'a> Parser<'a> {
                           -> PResult<'a, PathSegment> {
         let ident = self.parse_path_segment_ident()?;
 
-        let is_args_start = |token: &token::Token| match *token {
-            token::Lt | token::BinOp(token::Shl) | token::OpenDelim(token::Paren) => true,
+        let is_args_start = |token: &token::Token, include_paren: bool| match *token {
+            token::Lt | token::BinOp(token::Shl) => true,
+            token::OpenDelim(token::Paren) => include_paren,
             _ => false,
         };
-        let check_args_start = |this: &mut Self| {
-            this.expected_tokens.extend_from_slice(
-                &[TokenType::Token(token::Lt), TokenType::Token(token::OpenDelim(token::Paren))]
-            );
-            is_args_start(&this.token)
+        let check_args_start = |this: &mut Self, include_paren: bool| {
+            this.expected_tokens.push(TokenType::Token(token::Lt));
+            if include_paren {
+                this.expected_tokens.push(TokenType::Token(token::OpenDelim(token::Paren)));
+            }
+            is_args_start(&this.token, include_paren)
         };
 
-        Ok(if style == PathStyle::Type && check_args_start(self) ||
-              style != PathStyle::Mod && self.check(&token::ModSep)
-                                      && self.look_ahead(1, |t| is_args_start(t)) {
+        let mut parser_snapshot_before_generics = None;
+
+        Ok(if style == PathStyle::Type && check_args_start(self, true)
+            || style != PathStyle::Mod && self.check(&token::ModSep)
+                                       && self.look_ahead(1, |t| is_args_start(t, true))
+            || style == PathStyle::Expr && check_args_start(self, false) && {
+                // Check for generic arguments in an expression without a disambiguating `::`.
+                // We have to save a snapshot, because it could end up being an expression
+                // instead.
+                parser_snapshot_before_generics = Some(self.clone());
+                true
+            } {
             // Generic arguments are found - `<`, `(`, `::<` or `::(`.
             let lo = self.span;
             if self.eat(&token::ModSep) && style == PathStyle::Type && enable_warning {
@@ -2045,10 +2055,26 @@ impl<'a> Parser<'a> {
 
             let args = if self.eat_lt() {
                 // `<'a, T, A = U>`
-                let (args, bindings) = self.parse_generic_args()?;
-                self.expect_gt()?;
-                let span = lo.to(self.prev_span);
-                AngleBracketedArgs { args, bindings, span }.into()
+                let args: PResult<_> = do catch {
+                    let (args, bindings) = self.parse_generic_args()?;
+                    self.expect_gt()?;
+                    let span = lo.to(self.prev_span);
+                    AngleBracketedArgs { args, bindings, span }
+                };
+
+                match args {
+                    Err(mut err) => {
+                        if let Some(snapshot) = parser_snapshot_before_generics {
+                            err.cancel();
+                            mem::replace(self, snapshot);
+                            return Ok(PathSegment::from_ident(ident));
+                        }
+                        return Err(err);
+                    }
+                    _ => {
+                        args?.into()
+                    }
+                }
             } else {
                 // `(T, U) -> R`
                 self.bump(); // `(`

src/libsyntax/parse/token.rs

@@ -167,7 +167,7 @@ pub enum Token {
     Comma,
     Semi,
     Colon,
-    ModSep,
+    ModSep, // `::`
     RArrow,
     LArrow,
     FatArrow,

src/test/parse-fail/pat-ranges-2.rs

@@ -11,5 +11,5 @@
 // Parsing of range patterns
 
 fn main() {
-    let 10 ..= makropulos!() = 12; //~ error: expected one of `::`, `:`, `;`, or `=`, found `!`
+    let 10 ..= makropulos!() = 12; //~ error: expected one of `::`, `:`, `;`, `<`, or `=`, found `!`
 }

src/test/parse-fail/require-parens-for-chained-comparison.rs

@@ -18,9 +18,4 @@ fn main() {
 
     false == 0 < 2;
     //~^ ERROR: chained comparison operators require parentheses
-
-    f<X>();
-    //~^ ERROR: chained comparison operators require parentheses
-    //~| HELP: use `::<...>` instead of `<...>`
-    //~| HELP: or use `(...)`
 }

src/test/ui/did_you_mean/issue-40396.rs

@@ -9,16 +9,16 @@
 // except according to those terms.
 
 fn foo() {
-    println!("{:?}", (0..13).collect<Vec<i32>>()); //~ ERROR chained comparison
+    println!("{:?}", (0..13).collect<Vec<i32>>()); // ok
 }
 
 fn bar() {
-    println!("{:?}", Vec<i32>::new()); //~ ERROR chained comparison
+    println!("{:?}", Vec<i32>::new()); // ok
 }
 
 fn qux() {
-    println!("{:?}", (0..13).collect<Vec<i32>()); //~ ERROR chained comparison
-    //~^ ERROR chained comparison
+    println!("{:?}", (0..13).collect<Vec<i32>()); //~ ERROR expected function, found struct `Vec`
+    //~^ ERROR attempted to take value of method `collect`
 }
 
 fn main() {}

src/test/ui/did_you_mean/issue-40396.stderr

@@ -1,38 +1,18 @@
-error: chained comparison operators require parentheses
-  --> $DIR/issue-40396.rs:12:37
+error[E0423]: expected function, found struct `Vec`
+  --> $DIR/issue-40396.rs:20:38
    |
-LL |     println!("{:?}", (0..13).collect<Vec<i32>>()); //~ ERROR chained comparison
-   |                                     ^^^^^^^^
-   |
-   = help: use `::<...>` instead of `<...>` if you meant to specify type arguments
-   = help: or use `(...)` if you meant to specify fn arguments
-
-error: chained comparison operators require parentheses
-  --> $DIR/issue-40396.rs:16:25
-   |
-LL |     println!("{:?}", Vec<i32>::new()); //~ ERROR chained comparison
-   |                         ^^^^^^^
-   |
-   = help: use `::<...>` instead of `<...>` if you meant to specify type arguments
-   = help: or use `(...)` if you meant to specify fn arguments
-
-error: chained comparison operators require parentheses
-  --> $DIR/issue-40396.rs:20:37
-   |
-LL |     println!("{:?}", (0..13).collect<Vec<i32>()); //~ ERROR chained comparison
-   |                                     ^^^^^^^^
-   |
-   = help: use `::<...>` instead of `<...>` if you meant to specify type arguments
-   = help: or use `(...)` if you meant to specify fn arguments
+LL |     println!("{:?}", (0..13).collect<Vec<i32>()); //~ ERROR expected function, found struct `Vec`
+   |                                      ^^^^^^^^ did you mean `Vec { /* fields */ }`?
 
-error: chained comparison operators require parentheses
-  --> $DIR/issue-40396.rs:20:41
+error[E0615]: attempted to take value of method `collect` on type `std::ops::Range<{integer}>`
+  --> $DIR/issue-40396.rs:20:30
    |
-LL |     println!("{:?}", (0..13).collect<Vec<i32>()); //~ ERROR chained comparison
-   |                                         ^^^^^^
+LL |     println!("{:?}", (0..13).collect<Vec<i32>()); //~ ERROR expected function, found struct `Vec`
+   |                              ^^^^^^^
    |
-   = help: use `::<...>` instead of `<...>` if you meant to specify type arguments
-   = help: or use `(...)` if you meant to specify fn arguments
+   = help: maybe a `()` to call it is missing?
 
-error: aborting due to 4 previous errors
+error: aborting due to 2 previous errors
 
+Some errors occurred: E0423, E0615.
+For more information about an error, try `rustc --explain E0423`.

src/test/ui/issues/issue-6596-2.stderr

@@ -7,11 +7,11 @@ LL |         { $inp $nonexistent }
 LL |     g!(foo);
    |     -------- in this macro invocation
 
-error: expected one of `!`, `.`, `::`, `;`, `?`, `{`, `}`, or an operator, found `nonexistent`
+error: expected one of `!`, `.`, `::`, `;`, `<`, `?`, `{`, `}`, or an operator, found `nonexistent`
   --> $DIR/issue-6596-2.rs:15:16
    |
 LL |         { $inp $nonexistent }
-   |                ^^^^^^^^^^^^ expected one of 8 possible tokens here
+   |                ^^^^^^^^^^^^ expected one of 9 possible tokens here
 ...
 LL |     g!(foo);
    |     -------- in this macro invocation

src/test/ui/macro_backtrace/main.stderr

@@ -1,21 +1,21 @@
-error: expected one of `!`, `.`, `::`, `;`, `?`, `{`, `}`, or an operator, found `error`
+error: expected one of `!`, `.`, `::`, `;`, `<`, `?`, `{`, `}`, or an operator, found `error`
   --> $DIR/main.rs:19:20
    |
 LL | / macro_rules! pong {
 LL | |     () => { syntax error };
-   | |                    ^^^^^ expected one of 8 possible tokens here
+   | |                    ^^^^^ expected one of 9 possible tokens here
 LL | | }
    | |_- in this expansion of `pong!`
 ...
 LL |       pong!();
    |       -------- in this macro invocation
 
-error: expected one of `!`, `.`, `::`, `;`, `?`, `{`, `}`, or an operator, found `error`
+error: expected one of `!`, `.`, `::`, `;`, `<`, `?`, `{`, `}`, or an operator, found `error`
   --> $DIR/main.rs:19:20
    |
 LL | / macro_rules! pong {
 LL | |     () => { syntax error };
-   | |                    ^^^^^ expected one of 8 possible tokens here
+   | |                    ^^^^^ expected one of 9 possible tokens here
 LL | | }
    | |_- in this expansion of `pong!`
 ...
@@ -30,12 +30,12 @@ LL |   (  ) => { pong ! (  ) ; }
    |   |         in this macro invocation
    |   in this expansion of `ping!`
 
-error: expected one of `!`, `.`, `::`, `;`, `?`, `{`, `}`, or an operator, found `error`
+error: expected one of `!`, `.`, `::`, `;`, `<`, `?`, `{`, `}`, or an operator, found `error`
   --> $DIR/main.rs:19:20
    |
 LL | / macro_rules! pong {
 LL | |     () => { syntax error };
-   | |                    ^^^^^ expected one of 8 possible tokens here
+   | |                    ^^^^^ expected one of 9 possible tokens here
 LL | | }
    | |_- in this expansion of `pong!` (#5)
 ...

src/test/ui/raw/raw-literal-keywords.stderr

@@ -1,20 +1,20 @@
-error: expected one of `!`, `.`, `::`, `;`, `?`, `{`, `}`, or an operator, found `true`
+error: expected one of `!`, `.`, `::`, `;`, `<`, `?`, `{`, `}`, or an operator, found `true`
   --> $DIR/raw-literal-keywords.rs:14:10
    |
 LL |     r#if true { } //~ ERROR found `true`
-   |          ^^^^ expected one of 8 possible tokens here
+   |          ^^^^ expected one of 9 possible tokens here
 
-error: expected one of `!`, `.`, `::`, `;`, `?`, `{`, `}`, or an operator, found `Test`
+error: expected one of `!`, `.`, `::`, `;`, `<`, `?`, `{`, `}`, or an operator, found `Test`
   --> $DIR/raw-literal-keywords.rs:18:14
    |
 LL |     r#struct Test; //~ ERROR found `Test`
-   |              ^^^^ expected one of 8 possible tokens here
+   |              ^^^^ expected one of 9 possible tokens here
 
-error: expected one of `!`, `.`, `::`, `;`, `?`, `{`, `}`, or an operator, found `Test`
+error: expected one of `!`, `.`, `::`, `;`, `<`, `?`, `{`, `}`, or an operator, found `Test`
   --> $DIR/raw-literal-keywords.rs:22:13
    |
 LL |     r#union Test; //~ ERROR found `Test`
-   |             ^^^^ expected one of 8 possible tokens here
+   |             ^^^^ expected one of 9 possible tokens here
 
 error: aborting due to 3 previous errors