Auto merge of #79266 - b-naber:gat_trait_path_parser, r=petrochenkov

Generic Associated Types in Trait Paths - Ast part

The Ast part of https://github.com/rust-lang/rust/pull/78978

r? `@petrochenkov`

compiler/rustc_parse/src/parser/path.rs

@@ -3,10 +3,9 @@ use super::{Parser, TokenType};
 use crate::maybe_whole;
 use rustc_ast::ptr::P;
 use rustc_ast::token::{self, Token};
-use rustc_ast::{
-    self as ast, AngleBracketedArg, AngleBracketedArgs, GenericArg, ParenthesizedArgs,
-};
+use rustc_ast::{self as ast, AngleBracketedArg, AngleBracketedArgs, ParenthesizedArgs};
 use rustc_ast::{AnonConst, AssocTyConstraint, AssocTyConstraintKind, BlockCheckMode};
+use rustc_ast::{GenericArg, GenericArgs};
 use rustc_ast::{Path, PathSegment, QSelf};
 use rustc_errors::{pluralize, Applicability, PResult};
 use rustc_span::source_map::{BytePos, Span};
@@ -414,32 +413,40 @@ impl<'a> Parser<'a> {
 
     /// Parses a single argument in the angle arguments `<...>` of a path segment.
     fn parse_angle_arg(&mut self) -> PResult<'a, Option<AngleBracketedArg>> {
-        if self.check_ident() && self.look_ahead(1, |t| matches!(t.kind, token::Eq | token::Colon))
-        {
-            // Parse associated type constraint.
-            let lo = self.token.span;
-            let ident = self.parse_ident()?;
-            let kind = if self.eat(&token::Eq) {
-                let ty = self.parse_assoc_equality_term(ident, self.prev_token.span)?;
-                AssocTyConstraintKind::Equality { ty }
-            } else if self.eat(&token::Colon) {
-                let bounds = self.parse_generic_bounds(Some(self.prev_token.span))?;
-                AssocTyConstraintKind::Bound { bounds }
-            } else {
-                unreachable!();
-            };
+        let lo = self.token.span;
+        let arg = self.parse_generic_arg()?;
+        match arg {
+            Some(arg) => {
+                if self.check(&token::Colon) | self.check(&token::Eq) {
+                    let (ident, gen_args) = self.get_ident_from_generic_arg(arg, lo)?;
+                    let kind = if self.eat(&token::Colon) {
+                        // Parse associated type constraint bound.
 
-            let span = lo.to(self.prev_token.span);
+                        let bounds = self.parse_generic_bounds(Some(self.prev_token.span))?;
+                        AssocTyConstraintKind::Bound { bounds }
+                    } else if self.eat(&token::Eq) {
+                        // Parse associated type equality constraint
 
-            // Gate associated type bounds, e.g., `Iterator<Item: Ord>`.
-            if let AssocTyConstraintKind::Bound { .. } = kind {
-                self.sess.gated_spans.gate(sym::associated_type_bounds, span);
+                        let ty = self.parse_assoc_equality_term(ident, self.prev_token.span)?;
+                        AssocTyConstraintKind::Equality { ty }
+                    } else {
+                        unreachable!();
+                    };
+
+                    let span = lo.to(self.prev_token.span);
+
+                    // Gate associated type bounds, e.g., `Iterator<Item: Ord>`.
+                    if let AssocTyConstraintKind::Bound { .. } = kind {
+                        self.sess.gated_spans.gate(sym::associated_type_bounds, span);
+                    }
+                    let constraint =
+                        AssocTyConstraint { id: ast::DUMMY_NODE_ID, ident, gen_args, kind, span };
+                    Ok(Some(AngleBracketedArg::Constraint(constraint)))
+                } else {
+                    Ok(Some(AngleBracketedArg::Arg(arg)))
+                }
             }
-
-            let constraint = AssocTyConstraint { id: ast::DUMMY_NODE_ID, ident, kind, span };
-            Ok(Some(AngleBracketedArg::Constraint(constraint)))
-        } else {
-            Ok(self.parse_generic_arg()?.map(AngleBracketedArg::Arg))
+            _ => Ok(None),
         }
     }
 
@@ -542,4 +549,54 @@ impl<'a> Parser<'a> {
         };
         Ok(Some(arg))
     }
+
+    fn get_ident_from_generic_arg(
+        &self,
+        gen_arg: GenericArg,
+        lo: Span,
+    ) -> PResult<'a, (Ident, Option<GenericArgs>)> {
+        let gen_arg_span = gen_arg.span();
+        match gen_arg {
+            GenericArg::Type(t) => match t.into_inner().kind {
+                ast::TyKind::Path(qself, mut path) => {
+                    if let Some(qself) = qself {
+                        let mut err = self.struct_span_err(
+                            gen_arg_span,
+                            "qualified paths cannot be used in associated type constraints",
+                        );
+                        err.span_label(
+                            qself.path_span,
+                            "not allowed in associated type constraints",
+                        );
+                        return Err(err);
+                    }
+                    if path.segments.len() == 1 {
+                        let path_seg = path.segments.remove(0);
+                        let ident = path_seg.ident;
+                        let gen_args = path_seg.args.map(|args| args.into_inner());
+                        return Ok((ident, gen_args));
+                    }
+                    let err = self.struct_span_err(
+                        path.span,
+                        "paths with multiple segments cannot be used in associated type constraints",
+                    );
+                    return Err(err);
+                }
+                _ => {
+                    let span = lo.to(self.prev_token.span);
+                    let err = self.struct_span_err(
+                        span,
+                        "only path types can be used in associated type constraints",
+                    );
+                    return Err(err);
+                }
+            },
+            _ => {
+                let span = lo.to(self.prev_token.span);
+                let err = self
+                    .struct_span_err(span, "only types can be used in associated type constraints");
+                return Err(err);
+            }
+        }
+    }
 }