Auto merge of #80065 - b-naber:parse-angle-arg-diagnostics, r=petrochenkov

Improve diagnostics when parsing angle args

https://github.com/rust-lang/rust/pull/79266 introduced parsing of generic arguments in associated type constraints, this however resulted in possibly very confusing error messages in cases in which closing angle brackets were missing such as in `Vec<(u32, _, _) = vec![]`, which outputs an incorrectly parsed equality constraint error, as noted by `@cynecx.`

This PR tries to provide better error messages in such cases.

r? `@petrochenkov`

1 files changed, 11 insertions, 44 deletions

diff --git a/compiler/rustc_parse/src/parser/path.rs b/compiler/rustc_parse/src/parser/path.rs
index dd36122f6a1..4234740b2b1 100644
--- a/compiler/rustc_parse/src/parser/path.rs
+++ b/compiler/rustc_parse/src/parser/path.rs
@@ -185,7 +185,6 @@ impl<'a> Parser<'a> {
 
     pub(super) fn parse_path_segment(&mut self, style: PathStyle) -> PResult<'a, PathSegment> {
         let ident = self.parse_path_segment_ident()?;
-
         let is_args_start = |token: &Token| {
             matches!(
                 token.kind,
@@ -420,7 +419,10 @@ impl<'a> Parser<'a> {
         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 (ident, gen_args) = match self.get_ident_from_generic_arg(arg) {
+                        Ok(ident_gen_args) => ident_gen_args,
+                        Err(arg) => return Ok(Some(AngleBracketedArg::Arg(arg))),
+                    };
                     let kind = if self.eat(&token::Colon) {
                         // Parse associated type constraint bound.
 
@@ -561,50 +563,15 @@ impl<'a> Parser<'a> {
     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);
+    ) -> Result<(Ident, Option<GenericArgs>), GenericArg> {
+        if let GenericArg::Type(ty) = &gen_arg {
+            if let ast::TyKind::Path(qself, path) = &ty.kind {
+                if qself.is_none() && path.segments.len() == 1 {
+                    let seg = &path.segments[0];
+                    return Ok((seg.ident, seg.args.as_deref().cloned()));
                 }
-            },
-            _ => {
-                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);
             }
         }
+        Err(gen_arg)
     }
 }