Improve diagnostics for parenthesized type arguments

compiler/rustc_parse/src/parser/path.rs

@@ -1,6 +1,7 @@
 use super::ty::{AllowPlus, RecoverQPath, RecoverReturnSign};
 use super::{Parser, Restrictions, TokenType};
 use crate::errors::PathSingleColon;
+use crate::parser::{CommaRecoveryMode, RecoverColon, RecoverComma};
 use crate::{errors, maybe_whole};
 use ast::token::IdentIsRaw;
 use rustc_ast::ptr::P;
@@ -373,7 +374,52 @@ impl<'a> Parser<'a> {
                     .into()
                 } else {
                     // `(T, U) -> R`
-                    let (inputs, _) = self.parse_paren_comma_seq(|p| p.parse_ty())?;
+                    let prev_token_before_parsing = self.prev_token.clone();
+
+                    let mut snapshot = None;
+                    if self.may_recover()
+                        && (self.token.can_begin_expr() || self.token.can_begin_pattern())
+                    {
+                        snapshot = Some(self.create_snapshot_for_diagnostic());
+                    }
+
+                    let (inputs, _) = match self.parse_paren_comma_seq(|p| p.parse_ty()) {
+                        Ok(output) => output,
+                        Err(mut error) if prev_token_before_parsing.kind == token::ModSep => {
+                            error.span_label(prev_token_before_parsing.span.with_hi(prev_token_before_parsing.span.hi() + BytePos(1)), "while parsing this list of parenthesized type arguments starting here");
+
+                            if let Some(mut snapshot) = snapshot {
+                                if ((style == PathStyle::Expr
+                                    && snapshot.parse_paren_comma_seq(|p| p.parse_expr()).is_ok())
+                                    || (style == PathStyle::Pat
+                                        && snapshot
+                                            .parse_paren_comma_seq(|p| {
+                                                p.parse_pat_allow_top_alt(
+                                                    None,
+                                                    RecoverComma::No,
+                                                    RecoverColon::No,
+                                                    CommaRecoveryMode::LikelyTuple,
+                                                )
+                                            })
+                                            .is_ok()))
+                                    && snapshot.token.kind != token::ModSep
+                                    && snapshot.token.kind != token::RArrow
+                                {
+                                    {
+                                        error.span_suggestion_verbose(
+                                            prev_token_before_parsing.span,
+                                            "consider removing `::`",
+                                            "",
+                                            Applicability::Unspecified,
+                                        );
+                                    }
+                                }
+                            }
+
+                            return Err(error);
+                        }
+                        Err(error) => return Err(error),
+                    };
                     let inputs_span = lo.to(self.prev_token.span);
                     let output =
                         self.parse_ret_ty(AllowPlus::No, RecoverQPath::No, RecoverReturnSign::No)?;

tests/ui/parser/issues/diagnostics-parenthesized-type-arguments-issue-120892-1.rs

@@ -0,0 +1,14 @@
+fn main() {
+  foo::( //~ HELP: consider removing `::`
+    //~^ NOTE: while parsing this list of parenthesized type arguments starting here
+    bar(x, y, z),
+    bar(x, y, z),
+    bar(x, y, z),
+    bar(x, y, z),
+    bar(x, y, z),
+    bar(x, y, z),
+    bar(x, y, z),
+    baz("test"), //~ ERROR: expected type, found `"test"`
+    //~^ NOTE: expected type
+  )
+}

tests/ui/parser/issues/diagnostics-parenthesized-type-arguments-issue-120892-1.stderr

@@ -0,0 +1,17 @@
+error: expected type, found `"test"`
+  --> $DIR/issue-120892-1.rs:11:9
+   |
+LL |   foo::(
+   |      --- while parsing this list of parenthesized type arguments starting here
+...
+LL |     baz("test"),
+   |         ^^^^^^ expected type
+   |
+help: consider removing `::`
+   |
+LL -   foo::(
+LL +   foo(
+   |
+
+error: aborting due to 1 previous error
+

tests/ui/parser/issues/diagnostics-parenthesized-type-arguments-issue-120892-2.rs

@@ -0,0 +1,5 @@
+fn main() {
+  foo::(123, "foo") -> (u32); //~ ERROR: expected type, found `123`
+  //~^ NOTE: while parsing this list of parenthesized type arguments starting here
+  //~^^ NOTE: expected type
+}

tests/ui/parser/issues/diagnostics-parenthesized-type-arguments-issue-120892-2.stderr

@@ -0,0 +1,10 @@
+error: expected type, found `123`
+  --> $DIR/issue-120892-2.rs:2:9
+   |
+LL |   foo::(123, "foo") -> (u32);
+   |      ---^^^ expected type
+   |      |
+   |      while parsing this list of parenthesized type arguments starting here
+
+error: aborting due to 1 previous error
+

tests/ui/parser/issues/diagnostics-parenthesized-type-arguments-issue-120892-3.rs

@@ -0,0 +1,14 @@
+struct Foo(u32, u32);
+impl Foo {
+    fn foo(&self) {
+        match *self {
+            Foo::(1, 2) => {}, //~ HELP: consider removing `::`
+            //~^ NOTE: while parsing this list of parenthesized type arguments starting here
+            //~^^ ERROR: expected type, found `1`
+            //~^^^ NOTE: expected type
+             _ => {},
+        }
+    }
+}
+
+fn main() {}

tests/ui/parser/issues/diagnostics-parenthesized-type-arguments-issue-120892-3.stderr

@@ -0,0 +1,16 @@
+error: expected type, found `1`
+  --> $DIR/issue-120892-3.rs:5:19
+   |
+LL |             Foo::(1, 2) => {},
+   |                ---^ expected type
+   |                |
+   |                while parsing this list of parenthesized type arguments starting here
+   |
+help: consider removing `::`
+   |
+LL -             Foo::(1, 2) => {},
+LL +             Foo(1, 2) => {},
+   |
+
+error: aborting due to 1 previous error
+