Suggest assoc ty bound on lifetime in eq constraint

compiler/rustc_parse/messages.ftl

@@ -14,10 +14,6 @@ parse_array_index_offset_of = array indexing not supported in offset_of
 
 parse_assignment_else_not_allowed = <assignment> ... else {"{"} ... {"}"} is not allowed
 
-parse_assoc_lifetime = associated lifetimes are not supported
-    .label = the lifetime is given here
-    .help = if you meant to specify a trait object, write `dyn Trait + 'lifetime`
-
 parse_associated_static_item_not_allowed = associated `static` items are not allowed
 
 parse_async_block_in_2015 = `async` blocks are only allowed in Rust 2018 or later
@@ -450,6 +446,12 @@ parse_lifetime_in_borrow_expression = borrow expressions cannot be annotated wit
     .suggestion = remove the lifetime annotation
     .label = annotated with lifetime here
 
+parse_lifetime_in_eq_constraint = lifetimes are not permitted in this context
+    .label = lifetime is not allowed here
+    .context_label = this introduces an associated item binding
+    .help = if you meant to specify a trait object, write `dyn /*Trait*/ + {$lifetime}`
+    .colon_sugg = you might have meant to write a bound here
+
 parse_lone_slash = invalid trailing slash in literal
     .label = {parse_lone_slash}
 

compiler/rustc_parse/src/errors.rs

@@ -2618,13 +2618,22 @@ pub(crate) struct GenericsInPath {
 }
 
 #[derive(Diagnostic)]
-#[diag(parse_assoc_lifetime)]
+#[diag(parse_lifetime_in_eq_constraint)]
 #[help]
-pub(crate) struct AssocLifetime {
+pub(crate) struct LifetimeInEqConstraint {
     #[primary_span]
-    pub span: Span,
     #[label]
-    pub lifetime: Span,
+    pub span: Span,
+    pub lifetime: Ident,
+    #[label(parse_context_label)]
+    pub binding_label: Span,
+    #[suggestion(
+        parse_colon_sugg,
+        style = "verbose",
+        applicability = "maybe-incorrect",
+        code = ": "
+    )]
+    pub colon_sugg: Span,
 }
 
 #[derive(Diagnostic)]

compiler/rustc_parse/src/parser/path.rs

@@ -626,7 +626,11 @@ impl<'a> Parser<'a> {
                         let bounds = self.parse_generic_bounds()?;
                         AssocConstraintKind::Bound { bounds }
                     } else if self.eat(&token::Eq) {
-                        self.parse_assoc_equality_term(ident, self.prev_token.span)?
+                        self.parse_assoc_equality_term(
+                            ident,
+                            gen_args.as_ref(),
+                            self.prev_token.span,
+                        )?
                     } else {
                         unreachable!();
                     };
@@ -663,11 +667,13 @@ impl<'a> Parser<'a> {
     }
 
     /// Parse the term to the right of an associated item equality constraint.
-    /// That is, parse `<term>` in `Item = <term>`.
-    /// Right now, this only admits types in `<term>`.
+    ///
+    /// That is, parse `$term` in `Item = $term` where `$term` is a type or
+    /// a const expression (wrapped in curly braces if complex).
     fn parse_assoc_equality_term(
         &mut self,
         ident: Ident,
+        gen_args: Option<&GenericArgs>,
         eq: Span,
     ) -> PResult<'a, AssocConstraintKind> {
         let arg = self.parse_generic_arg(None)?;
@@ -679,9 +685,15 @@ impl<'a> Parser<'a> {
                 c.into()
             }
             Some(GenericArg::Lifetime(lt)) => {
-                let guar =
-                    self.dcx().emit_err(errors::AssocLifetime { span, lifetime: lt.ident.span });
-                self.mk_ty(span, ast::TyKind::Err(guar)).into()
+                let guar = self.dcx().emit_err(errors::LifetimeInEqConstraint {
+                    span: lt.ident.span,
+                    lifetime: lt.ident,
+                    binding_label: span,
+                    colon_sugg: gen_args
+                        .map_or(ident.span, |args| args.span())
+                        .between(lt.ident.span),
+                });
+                self.mk_ty(lt.ident.span, ast::TyKind::Err(guar)).into()
             }
             None => {
                 let after_eq = eq.shrink_to_hi();

tests/ui/parser/recover/recover-assoc-lifetime-constraint.rs

@@ -1,6 +1,6 @@
 #[cfg(FALSE)]
 fn syntax() {
-    bar::<Item = 'a>(); //~ ERROR associated lifetimes are not supported
+    bar::<Item = 'a>(); //~ ERROR lifetimes are not permitted in this context
 }
 
 fn main() {}

tests/ui/parser/recover/recover-assoc-lifetime-constraint.stderr

@@ -1,12 +1,17 @@
-error: associated lifetimes are not supported
-  --> $DIR/recover-assoc-lifetime-constraint.rs:3:11
+error: lifetimes are not permitted in this context
+  --> $DIR/recover-assoc-lifetime-constraint.rs:3:18
    |
 LL |     bar::<Item = 'a>();
-   |           ^^^^^^^--
-   |                  |
-   |                  the lifetime is given here
+   |           -------^^
+   |           |      |
+   |           |      lifetime is not allowed here
+   |           this introduces an associated item binding
    |
-   = help: if you meant to specify a trait object, write `dyn Trait + 'lifetime`
+   = help: if you meant to specify a trait object, write `dyn /*Trait*/ + 'a`
+help: you might have meant to write a bound here
+   |
+LL |     bar::<Item: 'a>();
+   |               ~
 
 error: aborting due to 1 previous error