Auto merge of #109128 - chenyukang:yukang/remove-type-ascription, r=estebank

Remove type ascription from parser and diagnostics

Mostly based on https://github.com/rust-lang/rust/pull/106826

Part of #101728

r? `@estebank`

9 files changed, 255 insertions, 187 deletions

diff --git a/compiler/rustc_parse/src/errors.rs b/compiler/rustc_parse/src/errors.rs
index f286707a9c0..b445ccc7ad0 100644
--- a/compiler/rustc_parse/src/errors.rs
+++ b/compiler/rustc_parse/src/errors.rs
@@ -1341,6 +1341,28 @@ pub(crate) struct ExpectedFnPathFoundFnKeyword {
 }
 
 #[derive(Diagnostic)]
+#[diag(parse_path_single_colon)]
+pub(crate) struct PathSingleColon {
+    #[primary_span]
+    #[suggestion(applicability = "machine-applicable", code = "::")]
+    pub span: Span,
+
+    #[note(parse_type_ascription_removed)]
+    pub type_ascription: Option<()>,
+}
+
+#[derive(Diagnostic)]
+#[diag(parse_colon_as_semi)]
+pub(crate) struct ColonAsSemi {
+    #[primary_span]
+    #[suggestion(applicability = "machine-applicable", code = ";")]
+    pub span: Span,
+
+    #[note(parse_type_ascription_removed)]
+    pub type_ascription: Option<()>,
+}
+
+#[derive(Diagnostic)]
 #[diag(parse_where_clause_before_tuple_struct_body)]
 pub(crate) struct WhereClauseBeforeTupleStructBody {
     #[primary_span]
diff --git a/compiler/rustc_parse/src/parser/diagnostics.rs b/compiler/rustc_parse/src/parser/diagnostics.rs
index 0e041df898c..638a432cea5 100644
--- a/compiler/rustc_parse/src/parser/diagnostics.rs
+++ b/compiler/rustc_parse/src/parser/diagnostics.rs
@@ -4,7 +4,7 @@ use super::{
     TokenExpectType, TokenType,
 };
 use crate::errors::{
-    AmbiguousPlus, AttributeOnParamType, BadQPathStage2, BadTypePlus, BadTypePlusSub,
+    AmbiguousPlus, AttributeOnParamType, BadQPathStage2, BadTypePlus, BadTypePlusSub, ColonAsSemi,
     ComparisonOperatorsCannotBeChained, ComparisonOperatorsCannotBeChainedSugg,
     ConstGenericWithoutBraces, ConstGenericWithoutBracesSugg, DocCommentDoesNotDocumentAnything,
     DocCommentOnParamType, DoubleColonInBound, ExpectedIdentifier, ExpectedSemi, ExpectedSemiSugg,
@@ -84,6 +84,7 @@ impl RecoverQPath for Ty {
 }
 
 impl RecoverQPath for Pat {
+    const PATH_STYLE: PathStyle = PathStyle::Pat;
     fn to_ty(&self) -> Option<P<Ty>> {
         self.to_ty()
     }
@@ -663,7 +664,6 @@ impl<'a> Parser<'a> {
             err.span_label(sp, label_exp);
             err.span_label(self.token.span, "unexpected token");
         }
-        self.maybe_annotate_with_ascription(&mut err, false);
         Err(err)
     }
 
@@ -788,59 +788,6 @@ impl<'a> Parser<'a> {
         None
     }
 
-    pub fn maybe_annotate_with_ascription(
-        &mut self,
-        err: &mut Diagnostic,
-        maybe_expected_semicolon: bool,
-    ) {
-        if let Some((sp, likely_path)) = self.last_type_ascription.take() {
-            let sm = self.sess.source_map();
-            let next_pos = sm.lookup_char_pos(self.token.span.lo());
-            let op_pos = sm.lookup_char_pos(sp.hi());
-
-            let allow_unstable = self.sess.unstable_features.is_nightly_build();
-
-            if likely_path {
-                err.span_suggestion(
-                    sp,
-                    "maybe write a path separator here",
-                    "::",
-                    if allow_unstable {
-                        Applicability::MaybeIncorrect
-                    } else {
-                        Applicability::MachineApplicable
-                    },
-                );
-                self.sess.type_ascription_path_suggestions.borrow_mut().insert(sp);
-            } else if op_pos.line != next_pos.line && maybe_expected_semicolon {
-                err.span_suggestion(
-                    sp,
-                    "try using a semicolon",
-                    ";",
-                    Applicability::MaybeIncorrect,
-                );
-            } else if allow_unstable {
-                err.span_label(sp, "tried to parse a type due to this type ascription");
-            } else {
-                err.span_label(sp, "tried to parse a type due to this");
-            }
-            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>`",
-                );
-                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",
-                    );
-                }
-            }
-        }
-    }
-
     /// Eats and discards tokens until one of `kets` is encountered. Respects token trees,
     /// passes through any errors encountered. Used for error recovery.
     pub(super) fn eat_to_tokens(&mut self, kets: &[&TokenKind]) {
@@ -1622,12 +1569,36 @@ impl<'a> Parser<'a> {
     }
 
     pub(super) fn expect_semi(&mut self) -> PResult<'a, ()> {
-        if self.eat(&token::Semi) {
+        if self.eat(&token::Semi) || self.recover_colon_as_semi() {
             return Ok(());
         }
         self.expect(&token::Semi).map(drop) // Error unconditionally
     }
 
+    pub(super) fn recover_colon_as_semi(&mut self) -> bool {
+        let line_idx = |span: Span| {
+            self.sess
+                .source_map()
+                .span_to_lines(span)
+                .ok()
+                .and_then(|lines| Some(lines.lines.get(0)?.line_index))
+        };
+
+        if self.may_recover()
+            && self.token == token::Colon
+            && self.look_ahead(1, |next| line_idx(self.token.span) < line_idx(next.span))
+        {
+            self.sess.emit_err(ColonAsSemi {
+                span: self.token.span,
+                type_ascription: self.sess.unstable_features.is_nightly_build().then_some(()),
+            });
+            self.bump();
+            return true;
+        }
+
+        false
+    }
+
     /// Consumes alternative await syntaxes like `await!(<expr>)`, `await <expr>`,
     /// `await? <expr>`, `await(<expr>)`, and `await { <expr> }`.
     pub(super) fn recover_incorrect_await_syntax(
@@ -1790,24 +1761,6 @@ impl<'a> Parser<'a> {
         }
     }
 
-    pub(super) fn could_ascription_be_path(&self, node: &ast::ExprKind) -> bool {
-        (self.token == token::Lt && // `foo:<bar`, likely a typoed turbofish.
-            self.look_ahead(1, |t| t.is_ident() && !t.is_reserved_ident()))
-            || self.token.is_ident() &&
-            matches!(node, ast::ExprKind::Path(..) | ast::ExprKind::Field(..)) &&
-            !self.token.is_reserved_ident() &&           // v `foo:bar(baz)`
-            self.look_ahead(1, |t| t == &token::OpenDelim(Delimiter::Parenthesis))
-            || self.look_ahead(1, |t| t == &token::OpenDelim(Delimiter::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)
-                && (self.look_ahead(2, |t| t.is_ident()) ||   // `foo:bar::baz`
-            self.look_ahead(2, |t| t == &token::Lt)) // `foo:bar::<baz>`
-    }
-
     pub(super) fn recover_seq_parse_error(
         &mut self,
         delim: Delimiter,
@@ -1902,7 +1855,6 @@ impl<'a> Parser<'a> {
                         && brace_depth == 0
                         && bracket_depth == 0 =>
                 {
-                    debug!("recover_stmt_ return - Semi");
                     break;
                 }
                 _ => self.bump(),
diff --git a/compiler/rustc_parse/src/parser/expr.rs b/compiler/rustc_parse/src/parser/expr.rs
index bff9de5c652..02db4b095dc 100644
--- a/compiler/rustc_parse/src/parser/expr.rs
+++ b/compiler/rustc_parse/src/parser/expr.rs
@@ -174,10 +174,8 @@ impl<'a> Parser<'a> {
                 self.parse_expr_prefix(attrs)?
             }
         };
-        let last_type_ascription_set = self.last_type_ascription.is_some();
 
         if !self.should_continue_as_assoc_expr(&lhs) {
-            self.last_type_ascription = None;
             return Ok(lhs);
         }
 
@@ -301,9 +299,6 @@ impl<'a> Parser<'a> {
             if op == AssocOp::As {
                 lhs = self.parse_assoc_op_cast(lhs, lhs_span, ExprKind::Cast)?;
                 continue;
-            } else if op == AssocOp::Colon {
-                lhs = self.parse_assoc_op_ascribe(lhs, lhs_span)?;
-                continue;
             } else if op == AssocOp::DotDot || op == AssocOp::DotDotEq {
                 // If we didn't have to handle `x..`/`x..=`, it would be pretty easy to
                 // generalise it to the Fixity::None code.
@@ -364,7 +359,7 @@ impl<'a> Parser<'a> {
                     let aopexpr = self.mk_assign_op(source_map::respan(cur_op_span, aop), lhs, rhs);
                     self.mk_expr(span, aopexpr)
                 }
-                AssocOp::As | AssocOp::Colon | AssocOp::DotDot | AssocOp::DotDotEq => {
+                AssocOp::As | AssocOp::DotDot | AssocOp::DotDotEq => {
                     self.span_bug(span, "AssocOp should have been handled by special case")
                 }
             };
@@ -373,9 +368,7 @@ impl<'a> Parser<'a> {
                 break;
             }
         }
-        if last_type_ascription_set {
-            self.last_type_ascription = None;
-        }
+
         Ok(lhs)
     }
 
@@ -743,7 +736,7 @@ impl<'a> Parser<'a> {
                     (
                         // `foo: `
                         ExprKind::Path(None, ast::Path { segments, .. }),
-                        TokenKind::Ident(kw::For | kw::Loop | kw::While, false),
+                        token::Ident(kw::For | kw::Loop | kw::While, false),
                     ) if segments.len() == 1 => {
                         let snapshot = self.create_snapshot_for_diagnostic();
                         let label = Label {
@@ -838,21 +831,19 @@ impl<'a> Parser<'a> {
         &mut self,
         cast_expr: P<Expr>,
     ) -> PResult<'a, P<Expr>> {
+        if let ExprKind::Type(_, _) = cast_expr.kind {
+            panic!("ExprKind::Type must not be parsed");
+        }
+
         let span = cast_expr.span;
-        let (cast_kind, maybe_ascription_span) =
-            if let ExprKind::Type(ascripted_expr, _) = &cast_expr.kind {
-                ("type ascription", Some(ascripted_expr.span.shrink_to_hi().with_hi(span.hi())))
-            } else {
-                ("cast", None)
-            };
 
         let with_postfix = self.parse_expr_dot_or_call_with_(cast_expr, span)?;
 
         // Check if an illegal postfix operator has been added after the cast.
         // If the resulting expression is not a cast, it is an illegal postfix operator.
-        if !matches!(with_postfix.kind, ExprKind::Cast(_, _) | ExprKind::Type(_, _)) {
+        if !matches!(with_postfix.kind, ExprKind::Cast(_, _)) {
             let msg = format!(
-                "{cast_kind} cannot be followed by {}",
+                "cast cannot be followed by {}",
                 match with_postfix.kind {
                     ExprKind::Index(_, _) => "indexing",
                     ExprKind::Try(_) => "`?`",
@@ -878,44 +869,13 @@ impl<'a> Parser<'a> {
                 );
             };
 
-            // If type ascription is "likely an error", the user will already be getting a useful
-            // help message, and doesn't need a second.
-            if self.last_type_ascription.map_or(false, |last_ascription| last_ascription.1) {
-                self.maybe_annotate_with_ascription(&mut err, false);
-            } else if let Some(ascription_span) = maybe_ascription_span {
-                let is_nightly = self.sess.unstable_features.is_nightly_build();
-                if is_nightly {
-                    suggest_parens(&mut err);
-                }
-                err.span_suggestion(
-                    ascription_span,
-                    &format!(
-                        "{}remove the type ascription",
-                        if is_nightly { "alternatively, " } else { "" }
-                    ),
-                    "",
-                    if is_nightly {
-                        Applicability::MaybeIncorrect
-                    } else {
-                        Applicability::MachineApplicable
-                    },
-                );
-            } else {
-                suggest_parens(&mut err);
-            }
+            suggest_parens(&mut err);
+
             err.emit();
         };
         Ok(with_postfix)
     }
 
-    fn parse_assoc_op_ascribe(&mut self, lhs: P<Expr>, lhs_span: Span) -> PResult<'a, P<Expr>> {
-        let maybe_path = self.could_ascription_be_path(&lhs.kind);
-        self.last_type_ascription = Some((self.prev_token.span, maybe_path));
-        let lhs = self.parse_assoc_op_cast(lhs, lhs_span, ExprKind::Type)?;
-        self.sess.gated_spans.gate(sym::type_ascription, lhs.span);
-        Ok(lhs)
-    }
-
     /// Parse `& mut? <expr>` or `& raw [ const | mut ] <expr>`.
     fn parse_expr_borrow(&mut self, lo: Span) -> PResult<'a, (Span, ExprKind)> {
         self.expect_and()?;
@@ -1010,7 +970,7 @@ impl<'a> Parser<'a> {
             };
             if has_dot {
                 // expr.f
-                e = self.parse_expr_dot_suffix(lo, e)?;
+                e = self.parse_dot_suffix_expr(lo, e)?;
                 continue;
             }
             if self.expr_is_complete(&e) {
@@ -1024,13 +984,7 @@ impl<'a> Parser<'a> {
         }
     }
 
-    fn look_ahead_type_ascription_as_field(&mut self) -> bool {
-        self.look_ahead(1, |t| t.is_ident())
-            && self.look_ahead(2, |t| t == &token::Colon)
-            && self.look_ahead(3, |t| t.can_begin_expr())
-    }
-
-    fn parse_expr_dot_suffix(&mut self, lo: Span, base: P<Expr>) -> PResult<'a, P<Expr>> {
+    fn parse_dot_suffix_expr(&mut self, lo: Span, base: P<Expr>) -> PResult<'a, P<Expr>> {
         match self.token.uninterpolate().kind {
             token::Ident(..) => self.parse_dot_suffix(base, lo),
             token::Literal(token::Lit { kind: token::Integer, symbol, suffix }) => {
@@ -1183,9 +1137,7 @@ impl<'a> Parser<'a> {
 
     /// Parse a function call expression, `expr(...)`.
     fn parse_expr_fn_call(&mut self, lo: Span, fun: P<Expr>) -> P<Expr> {
-        let snapshot = if self.token.kind == token::OpenDelim(Delimiter::Parenthesis)
-            && self.look_ahead_type_ascription_as_field()
-        {
+        let snapshot = if self.token.kind == token::OpenDelim(Delimiter::Parenthesis) {
             Some((self.create_snapshot_for_diagnostic(), fun.kind.clone()))
         } else {
             None
@@ -1216,7 +1168,6 @@ impl<'a> Parser<'a> {
         if !self.may_recover() {
             return None;
         }
-
         match (seq.as_mut(), snapshot) {
             (Err(err), Some((mut snapshot, ExprKind::Path(None, path)))) => {
                 snapshot.bump(); // `(`
@@ -1260,9 +1211,7 @@ impl<'a> Parser<'a> {
                         return Some(self.mk_expr_err(span));
                     }
                     Ok(_) => {}
-                    Err(mut err) => {
-                        err.emit();
-                    }
+                    Err(err) => err.cancel(),
                 }
             }
             _ => {}
@@ -1516,7 +1465,6 @@ impl<'a> Parser<'a> {
             let mac = P(MacCall {
                 path,
                 args: self.parse_delim_args()?,
-                prior_type_ascription: self.last_type_ascription,
             });
             (lo.to(self.prev_token.span), ExprKind::MacCall(mac))
         } else if self.check(&token::OpenDelim(Delimiter::Brace))
@@ -1535,7 +1483,7 @@ impl<'a> Parser<'a> {
     }
 
     /// Parse `'label: $expr`. The label is already parsed.
-    fn parse_expr_labeled(
+    pub(super) fn parse_expr_labeled(
         &mut self,
         label_: Label,
         mut consume_colon: bool,
@@ -3013,6 +2961,11 @@ impl<'a> Parser<'a> {
                     } else {
                         e.span_label(pth.span, "while parsing this struct");
                     }
+
+                    if !recover {
+                        return Err(e);
+                    }
+
                     e.emit();
 
                     // If the next token is a comma, then try to parse
@@ -3024,6 +2977,7 @@ impl<'a> Parser<'a> {
                             break;
                         }
                     }
+
                     None
                 }
             };
diff --git a/compiler/rustc_parse/src/parser/item.rs b/compiler/rustc_parse/src/parser/item.rs
index 9e003bfc097..64ff7f1fb2c 100644
--- a/compiler/rustc_parse/src/parser/item.rs
+++ b/compiler/rustc_parse/src/parser/item.rs
@@ -443,7 +443,7 @@ impl<'a> Parser<'a> {
             Ok(args) => {
                 self.eat_semi_for_macro_if_needed(&args);
                 self.complain_if_pub_macro(vis, false);
-                Ok(MacCall { path, args, prior_type_ascription: self.last_type_ascription })
+                Ok(MacCall { path, args })
             }
 
             Err(mut err) => {
diff --git a/compiler/rustc_parse/src/parser/mod.rs b/compiler/rustc_parse/src/parser/mod.rs
index 1c34e491f21..b294e13402a 100644
--- a/compiler/rustc_parse/src/parser/mod.rs
+++ b/compiler/rustc_parse/src/parser/mod.rs
@@ -148,9 +148,6 @@ pub struct Parser<'a> {
     max_angle_bracket_count: u32,
 
     last_unexpected_token_span: Option<Span>,
-    /// Span pointing at the `:` for the last type ascription the parser has seen, and whether it
-    /// looked like it could have been a mistyped path or literal `Option:Some(42)`).
-    pub last_type_ascription: Option<(Span, bool /* likely path typo */)>,
     /// If present, this `Parser` is not parsing Rust code but rather a macro call.
     subparser_name: Option<&'static str>,
     capture_state: CaptureState,
@@ -165,7 +162,7 @@ pub struct Parser<'a> {
 // This type is used a lot, e.g. it's cloned when matching many declarative macro rules with nonterminals. Make sure
 // it doesn't unintentionally get bigger.
 #[cfg(all(target_arch = "x86_64", target_pointer_width = "64"))]
-rustc_data_structures::static_assert_size!(Parser<'_>, 288);
+rustc_data_structures::static_assert_size!(Parser<'_>, 272);
 
 /// Stores span information about a closure.
 #[derive(Clone)]
@@ -470,7 +467,6 @@ impl<'a> Parser<'a> {
             unmatched_angle_bracket_count: 0,
             max_angle_bracket_count: 0,
             last_unexpected_token_span: None,
-            last_type_ascription: None,
             subparser_name,
             capture_state: CaptureState {
                 capturing: Capturing::No,
@@ -941,10 +937,14 @@ impl<'a> Parser<'a> {
                                         // propagate the help message from sub error 'e' to main error 'expect_err;
                                         expect_err.children.push(xx.clone());
                                     }
-                                    expect_err.emit();
-
                                     e.cancel();
-                                    break;
+                                    if self.token == token::Colon {
+                                        // we will try to recover in `maybe_recover_struct_lit_bad_delims`
+                                        return Err(expect_err);
+                                    } else {
+                                        expect_err.emit();
+                                        break;
+                                    }
                                 }
                             }
                         }
diff --git a/compiler/rustc_parse/src/parser/pat.rs b/compiler/rustc_parse/src/parser/pat.rs
index f2422fe307c..3c4b2977af9 100644
--- a/compiler/rustc_parse/src/parser/pat.rs
+++ b/compiler/rustc_parse/src/parser/pat.rs
@@ -406,11 +406,11 @@ impl<'a> Parser<'a> {
             // Parse pattern starting with a path
             let (qself, path) = if self.eat_lt() {
                 // Parse a qualified path
-                let (qself, path) = self.parse_qpath(PathStyle::Expr)?;
+                let (qself, path) = self.parse_qpath(PathStyle::Pat)?;
                 (Some(qself), path)
             } else {
                 // Parse an unqualified path
-                (None, self.parse_path(PathStyle::Expr)?)
+                (None, self.parse_path(PathStyle::Pat)?)
             };
             let span = lo.to(self.prev_token.span);
 
@@ -666,7 +666,7 @@ impl<'a> Parser<'a> {
     fn parse_pat_mac_invoc(&mut self, path: Path) -> PResult<'a, PatKind> {
         self.bump();
         let args = self.parse_delim_args()?;
-        let mac = P(MacCall { path, args, prior_type_ascription: self.last_type_ascription });
+        let mac = P(MacCall { path, args });
         Ok(PatKind::MacCall(mac))
     }
 
@@ -789,11 +789,11 @@ impl<'a> Parser<'a> {
             let lo = self.token.span;
             let (qself, path) = if self.eat_lt() {
                 // Parse a qualified path
-                let (qself, path) = self.parse_qpath(PathStyle::Expr)?;
+                let (qself, path) = self.parse_qpath(PathStyle::Pat)?;
                 (Some(qself), path)
             } else {
                 // Parse an unqualified path
-                (None, self.parse_path(PathStyle::Expr)?)
+                (None, self.parse_path(PathStyle::Pat)?)
             };
             let hi = self.prev_token.span;
             Ok(self.mk_expr(lo.to(hi), ExprKind::Path(qself, path)))
diff --git a/compiler/rustc_parse/src/parser/path.rs b/compiler/rustc_parse/src/parser/path.rs
index ae73760bd8c..b9a2b141bce 100644
--- a/compiler/rustc_parse/src/parser/path.rs
+++ b/compiler/rustc_parse/src/parser/path.rs
@@ -1,5 +1,6 @@
 use super::ty::{AllowPlus, RecoverQPath, RecoverReturnSign};
 use super::{Parser, Restrictions, TokenType};
+use crate::errors::PathSingleColon;
 use crate::{errors, maybe_whole};
 use rustc_ast::ptr::P;
 use rustc_ast::token::{self, Delimiter, Token, TokenKind};
@@ -8,7 +9,7 @@ use rustc_ast::{
     AssocConstraintKind, BlockCheckMode, GenericArg, GenericArgs, Generics, ParenthesizedArgs,
     Path, PathSegment, QSelf,
 };
-use rustc_errors::{Applicability, PResult};
+use rustc_errors::{Applicability, IntoDiagnostic, PResult};
 use rustc_span::source_map::{BytePos, Span};
 use rustc_span::symbol::{kw, sym, Ident};
 use std::mem;
@@ -24,7 +25,19 @@ pub enum PathStyle {
     /// In all such contexts the non-path interpretation is preferred by default for practical
     /// reasons, but the path interpretation can be forced by the disambiguator `::`, e.g.
     /// `x<y>` - comparisons, `x::<y>` - unambiguously a path.
+    ///
+    /// Also, a path may never be followed by a `:`. This means that we can eagerly recover if
+    /// we encounter it.
     Expr,
+    /// The same as `Expr`, but may be followed by a `:`.
+    /// For example, this code:
+    /// ```rust
+    /// struct S;
+    ///
+    /// let S: S;
+    /// //  ^ Followed by a `:`
+    /// ```
+    Pat,
     /// In other contexts, notably in types, no ambiguity exists and paths can be written
     /// without the disambiguator, e.g., `x<y>` - unambiguously a path.
     /// Paths with disambiguators are still accepted, `x::<Y>` - unambiguously a path too.
@@ -38,6 +51,12 @@ pub enum PathStyle {
     Mod,
 }
 
+impl PathStyle {
+    fn has_generic_ambiguity(&self) -> bool {
+        matches!(self, Self::Expr | Self::Pat)
+    }
+}
+
 impl<'a> Parser<'a> {
     /// Parses a qualified path.
     /// Assumes that the leading `<` has been parsed already.
@@ -183,7 +202,6 @@ impl<'a> Parser<'a> {
             segments.push(PathSegment::path_root(lo.shrink_to_lo().with_ctxt(mod_sep_ctxt)));
         }
         self.parse_path_segments(&mut segments, style, ty_generics)?;
-
         Ok(Path { segments, span: lo.to(self.prev_token.span), tokens: None })
     }
 
@@ -195,7 +213,7 @@ impl<'a> Parser<'a> {
     ) -> PResult<'a, ()> {
         loop {
             let segment = self.parse_path_segment(style, ty_generics)?;
-            if style == PathStyle::Expr {
+            if style.has_generic_ambiguity() {
                 // In order to check for trailing angle brackets, we must have finished
                 // recursing (`parse_path_segment` can indirectly call this function),
                 // that is, the next token must be the highlighted part of the below example:
@@ -217,6 +235,29 @@ impl<'a> Parser<'a> {
             segments.push(segment);
 
             if self.is_import_coupler() || !self.eat(&token::ModSep) {
+                if style == PathStyle::Expr
+                    && self.may_recover()
+                    && self.token == token::Colon
+                    && self.look_ahead(1, |token| token.is_ident() && !token.is_reserved_ident())
+                {
+                    // Emit a special error message for `a::b:c` to help users
+                    // otherwise, `a: c` might have meant to introduce a new binding
+                    if self.token.span.lo() == self.prev_token.span.hi()
+                        && self.look_ahead(1, |token| self.token.span.hi() == token.span.lo())
+                    {
+                        self.bump(); // bump past the colon
+                        self.sess.emit_err(PathSingleColon {
+                            span: self.prev_token.span,
+                            type_ascription: self
+                                .sess
+                                .unstable_features
+                                .is_nightly_build()
+                                .then_some(()),
+                        });
+                    }
+                    continue;
+                }
+
                 return Ok(());
             }
         }
@@ -270,8 +311,25 @@ impl<'a> Parser<'a> {
                         ty_generics,
                     )?;
                     self.expect_gt().map_err(|mut err| {
+                        // Try to recover a `:` into a `::`
+                        if self.token == token::Colon
+                            && self.look_ahead(1, |token| {
+                                token.is_ident() && !token.is_reserved_ident()
+                            })
+                        {
+                            err.cancel();
+                            err = PathSingleColon {
+                                span: self.token.span,
+                                type_ascription: self
+                                    .sess
+                                    .unstable_features
+                                    .is_nightly_build()
+                                    .then_some(()),
+                            }
+                            .into_diagnostic(self.diagnostic());
+                        }
                         // Attempt to find places where a missing `>` might belong.
-                        if let Some(arg) = args
+                        else if let Some(arg) = args
                             .iter()
                             .rev()
                             .find(|arg| !matches!(arg, AngleBracketedArg::Constraint(_)))
diff --git a/compiler/rustc_parse/src/parser/stmt.rs b/compiler/rustc_parse/src/parser/stmt.rs
index fbe5b88c49e..0a571013d44 100644
--- a/compiler/rustc_parse/src/parser/stmt.rs
+++ b/compiler/rustc_parse/src/parser/stmt.rs
@@ -10,6 +10,8 @@ use super::{
 use crate::errors;
 use crate::maybe_whole;
 
+use crate::errors::MalformedLoopLabel;
+use ast::Label;
 use rustc_ast as ast;
 use rustc_ast::ptr::P;
 use rustc_ast::token::{self, Delimiter, TokenKind};
@@ -19,7 +21,8 @@ use rustc_ast::{Block, BlockCheckMode, Expr, ExprKind, HasAttrs, Local, Stmt};
 use rustc_ast::{StmtKind, DUMMY_NODE_ID};
 use rustc_errors::{Applicability, DiagnosticBuilder, ErrorGuaranteed, PResult};
 use rustc_span::source_map::{BytePos, Span};
-use rustc_span::symbol::{kw, sym};
+use rustc_span::symbol::{kw, sym, Ident};
+
 use std::mem;
 use thin_vec::{thin_vec, ThinVec};
 
@@ -186,7 +189,7 @@ impl<'a> Parser<'a> {
             _ => MacStmtStyle::NoBraces,
         };
 
-        let mac = P(MacCall { path, args, prior_type_ascription: self.last_type_ascription });
+        let mac = P(MacCall { path, args });
 
         let kind = if (style == MacStmtStyle::Braces
             && self.token != token::Dot
@@ -546,10 +549,36 @@ impl<'a> Parser<'a> {
             }
             let stmt = match self.parse_full_stmt(recover) {
                 Err(mut err) if recover.yes() => {
-                    self.maybe_annotate_with_ascription(&mut err, false);
                     if let Some(ref mut snapshot) = snapshot {
                         snapshot.recover_diff_marker();
                     }
+                    if self.token == token::Colon {
+                        // if next token is following a colon, it's likely a path
+                        // and we can suggest a path separator
+                        let ident_span = self.prev_token.span;
+                        self.bump();
+                        if self.token.span.lo() == self.prev_token.span.hi() {
+                            err.span_suggestion_verbose(
+                                self.prev_token.span,
+                                "maybe write a path separator here",
+                                "::",
+                                Applicability::MaybeIncorrect,
+                            );
+                        }
+                        if self.look_ahead(1, |token| token == &token::Eq) {
+                            err.span_suggestion_verbose(
+                                ident_span.shrink_to_lo(),
+                                "you might have meant to introduce a new binding",
+                                "let ",
+                                Applicability::MaybeIncorrect,
+                            );
+                        }
+                        if self.sess.unstable_features.is_nightly_build() {
+                            // FIXME(Nilstrieb): Remove this again after a few months.
+                            err.note("type ascription syntax has been removed, see issue #101728 <https://github.com/rust-lang/rust/issues/101728>");
+                        }
+                    }
+
                     err.emit();
                     self.recover_stmt_(SemiColonMode::Ignore, BlockMode::Ignore);
                     Some(self.mk_stmt_err(self.token.span))
@@ -580,19 +609,25 @@ impl<'a> Parser<'a> {
         };
 
         let mut eat_semi = true;
+        let mut add_semi_to_stmt = false;
+
         match &mut stmt.kind {
             // Expression without semicolon.
             StmtKind::Expr(expr)
                 if self.token != token::Eof && classify::expr_requires_semi_to_be_stmt(expr) => {
                 // Just check for errors and recover; do not eat semicolon yet.
                 // `expect_one_of` returns PResult<'a, bool /* recovered */>
-                let replace_with_err =
-                    match self.expect_one_of(&[], &[token::Semi, token::CloseDelim(Delimiter::Brace)]) {
+
+                let expect_result = self.expect_one_of(&[], &[token::Semi, token::CloseDelim(Delimiter::Brace)]);
+
+                let replace_with_err = 'break_recover: {
+                    match expect_result {
                     // Recover from parser, skip type error to avoid extra errors.
-                    Ok(true) => true,
-                    Err(mut e) => {
-                        if let TokenKind::DocComment(..) = self.token.kind &&
-                            let Ok(snippet) = self.span_to_snippet(self.token.span) {
+                        Ok(true) => true,
+                        Err(mut e) => {
+                            if let TokenKind::DocComment(..) = self.token.kind
+                                && let Ok(snippet) = self.span_to_snippet(self.token.span)
+                            {
                                 let sp = self.token.span;
                                 let marker = &snippet[..3];
                                 let (comment_marker, doc_comment_marker) = marker.split_at(2);
@@ -606,21 +641,72 @@ impl<'a> Parser<'a> {
                                     format!("{} {}", comment_marker, doc_comment_marker),
                                     Applicability::MaybeIncorrect,
                                 );
-                        }
+                            }
+
+                            if self.recover_colon_as_semi() {
+                                // recover_colon_as_semi has already emitted a nicer error.
+                                e.delay_as_bug();
+                                add_semi_to_stmt = true;
+                                eat_semi = false;
+
+                                break 'break_recover false;
+                            }
+
+                            match &expr.kind {
+                                ExprKind::Path(None, ast::Path { segments, .. }) if segments.len() == 1 => {
+                                    if self.token == token::Colon
+                                        && self.look_ahead(1, |token| {
+                                            token.is_whole_block() || matches!(
+                                                token.kind,
+                                                token::Ident(kw::For | kw::Loop | kw::While, false)
+                                                    | token::OpenDelim(Delimiter::Brace)
+                                            )
+                                        })
+                                    {
+                                        let snapshot = self.create_snapshot_for_diagnostic();
+                                        let label = Label {
+                                            ident: Ident::from_str_and_span(
+                                                &format!("'{}", segments[0].ident),
+                                                segments[0].ident.span,
+                                            ),
+                                        };
+                                        match self.parse_expr_labeled(label, false) {
+                                            Ok(labeled_expr) => {
+                                                e.delay_as_bug();
+                                                self.sess.emit_err(MalformedLoopLabel {
+                                                    span: label.ident.span,
+                                                    correct_label: label.ident,
+                                                });
+                                                *expr = labeled_expr;
+                                                break 'break_recover false;
+                                            }
+                                            Err(err) => {
+                                                err.cancel();
+                                                self.restore_snapshot(snapshot);
+                                            }
+                                        }
+                                    }
+                                }
+                                _ => {}
+                            }
 
-                        if let Err(mut e) =
-                            self.check_mistyped_turbofish_with_multiple_type_params(e, expr)
-                        {
-                            if recover.no() {
-                                return Err(e);
+                            if let Err(mut e) =
+                                self.check_mistyped_turbofish_with_multiple_type_params(e, expr)
+                            {
+                                if recover.no() {
+                                    return Err(e);
+                                }
+                                e.emit();
+                                self.recover_stmt();
                             }
-                            e.emit();
-                            self.recover_stmt();
+
+                            true
+
                         }
-                        true
+                        Ok(false) => false
                     }
-                    _ => false
                 };
+
                 if replace_with_err {
                     // We already emitted an error, so don't emit another type error
                     let sp = expr.span.to(self.prev_token.span);
@@ -643,9 +729,10 @@ impl<'a> Parser<'a> {
             StmtKind::Empty | StmtKind::Item(_) | StmtKind::Local(_) | StmtKind::Semi(_) => eat_semi = false,
         }
 
-        if eat_semi && self.eat(&token::Semi) {
+        if add_semi_to_stmt || (eat_semi && self.eat(&token::Semi)) {
             stmt = stmt.add_trailing_semicolon();
         }
+
         stmt.span = stmt.span.to(self.prev_token.span);
         Ok(Some(stmt))
     }
diff --git a/compiler/rustc_parse/src/parser/ty.rs b/compiler/rustc_parse/src/parser/ty.rs
index 3ceb3a2bef1..37c441fbecb 100644
--- a/compiler/rustc_parse/src/parser/ty.rs
+++ b/compiler/rustc_parse/src/parser/ty.rs
@@ -317,7 +317,6 @@ impl<'a> Parser<'a> {
             let msg = format!("expected type, found {}", super::token_descr(&self.token));
             let mut err = self.struct_span_err(self.token.span, &msg);
             err.span_label(self.token.span, "expected type");
-            self.maybe_annotate_with_ascription(&mut err, true);
             return Err(err);
         };
 
@@ -651,11 +650,7 @@ impl<'a> Parser<'a> {
         let path = self.parse_path_inner(PathStyle::Type, ty_generics)?;
         if self.eat(&token::Not) {
             // Macro invocation in type position
-            Ok(TyKind::MacCall(P(MacCall {
-                path,
-                args: self.parse_delim_args()?,
-                prior_type_ascription: self.last_type_ascription,
-            })))
+            Ok(TyKind::MacCall(P(MacCall { path, args: self.parse_delim_args()? })))
         } else if allow_plus == AllowPlus::Yes && self.check_plus() {
             // `Trait1 + Trait2 + 'a`
             self.parse_remaining_bounds_path(ThinVec::new(), path, lo, true)