Rollup merge of rust-lang#139854 - fmease:modern-diag-for-lt-in-ty, r=davidtwco

Improve parse errors for stray lifetimes in type position

While technically & syntactically speaking lifetimes do begin[^1] types in type contexts (this essentially excludes generic argument lists) and require a following `+` to form a complete type (`'a +` denotes a bare trait object type), the likelihood that a user meant to write a lifetime-prefixed bare trait object type in *modern* editions (Rust ≥2021) when placing a lifetime into a type context is incredibly low (they would need to add at least three tokens to turn it into a *semantically* well-formed TOT: `'a` → `dyn 'a + Trait`).

Therefore let's *lie* in modern editions (just like in PR rust-lang#131239, a precedent if you will) by stating "*expected type, found lifetime*" in such cases which is a lot more a approachable, digestible and friendly compared to "*lifetime in trait object type must be followed by `+`*" (as added in PR rust-lang#69760).

I've also added recovery for "ampersand-less" reference types (e.g., `'a ()`, `'a mut Ty`) in modern editions because it was trivial to do and I think it's not unlikely to occur in practice.

Fixes rust-lang#133413.

[^1]: For example, in the context of decl macros, this implies that a lone `'a` always matches syntax fragment `ty` ("even if" there's a later macro matcher expecting syntax fragment `lifetime`). Rephrased, lifetimes (in type contexts) *commit* to the type parser.

Author: matthiaskrgr

compiler/rustc_parse/messages.ftl

@@ -543,7 +543,7 @@ parse_maybe_recover_from_bad_qpath_stage_2 =
     .suggestion = types that don't start with an identifier need to be surrounded with angle brackets in qualified paths
 
 parse_maybe_recover_from_bad_type_plus =
-    expected a path on the left-hand side of `+`, not `{$ty}`
+    expected a path on the left-hand side of `+`
 
 parse_maybe_report_ambiguous_plus =
     ambiguous `+` in a type
@@ -642,7 +642,9 @@ parse_mut_on_nested_ident_pattern = `mut` must be attached to each individual bi
     .suggestion = add `mut` to each binding
 parse_mut_on_non_ident_pattern = `mut` must be followed by a named binding
     .suggestion = remove the `mut` prefix
-parse_need_plus_after_trait_object_lifetime = lifetime in trait object type must be followed by `+`
+
+parse_need_plus_after_trait_object_lifetime = lifetimes must be followed by `+` to form a trait object type
+    .suggestion = consider adding a trait bound after the potential lifetime bound
 
 parse_nested_adt = `{$kw_str}` definition cannot be nested inside `{$keyword}`
     .suggestion = consider creating a new `{$kw_str}` definition instead of nesting

compiler/rustc_parse/src/errors.rs

@@ -30,7 +30,6 @@ pub(crate) struct AmbiguousPlus {
 #[derive(Diagnostic)]
 #[diag(parse_maybe_recover_from_bad_type_plus, code = E0178)]
 pub(crate) struct BadTypePlus {
-    pub ty: String,
     #[primary_span]
     pub span: Span,
     #[subdiagnostic]
@@ -2806,6 +2805,8 @@ pub(crate) struct ReturnTypesUseThinArrow {
 pub(crate) struct NeedPlusAfterTraitObjectLifetime {
     #[primary_span]
     pub span: Span,
+    #[suggestion(code = " + /* Trait */", applicability = "has-placeholders")]
+    pub suggestion: Span,
 }
 
 #[derive(Diagnostic)]

compiler/rustc_parse/src/parser/diagnostics.rs

@@ -1657,19 +1657,19 @@ impl<'a> Parser<'a> {
 
         self.bump(); // `+`
         let _bounds = self.parse_generic_bounds()?;
-        let sum_span = ty.span.to(self.prev_token.span);
-
         let sub = match &ty.kind {
             TyKind::Ref(_lifetime, mut_ty) => {
                 let lo = mut_ty.ty.span.shrink_to_lo();
                 let hi = self.prev_token.span.shrink_to_hi();
                 BadTypePlusSub::AddParen { suggestion: AddParen { lo, hi } }
             }
-            TyKind::Ptr(..) | TyKind::BareFn(..) => BadTypePlusSub::ForgotParen { span: sum_span },
-            _ => BadTypePlusSub::ExpectPath { span: sum_span },
+            TyKind::Ptr(..) | TyKind::BareFn(..) => {
+                BadTypePlusSub::ForgotParen { span: ty.span.to(self.prev_token.span) }
+            }
+            _ => BadTypePlusSub::ExpectPath { span: ty.span },
         };
 
-        self.dcx().emit_err(BadTypePlus { ty: pprust::ty_to_string(ty), span: sum_span, sub });
+        self.dcx().emit_err(BadTypePlus { span: ty.span, sub });
 
         Ok(())
     }

compiler/rustc_parse/src/parser/ty.rs

@@ -7,7 +7,7 @@ use rustc_ast::{
     Pinnedness, PolyTraitRef, PreciseCapturingArg, TraitBoundModifiers, TraitObjectSyntax, Ty,
     TyKind, UnsafeBinderTy,
 };
-use rustc_errors::{Applicability, PResult};
+use rustc_errors::{Applicability, Diag, PResult};
 use rustc_span::{ErrorGuaranteed, Ident, Span, kw, sym};
 use thin_vec::{ThinVec, thin_vec};
 
@@ -411,6 +411,9 @@ impl<'a> Parser<'a> {
                 TyKind::Path(None, path) if maybe_bounds => {
                     self.parse_remaining_bounds_path(ThinVec::new(), path, lo, true)
                 }
+                // For `('a) + …`, we know that `'a` in type position already lead to an error being
+                // emitted. To reduce output, let's indirectly suppress E0178 (bad `+` in type) and
+                // other irrelevant consequential errors.
                 TyKind::TraitObject(bounds, TraitObjectSyntax::None)
                     if maybe_bounds && bounds.len() == 1 && !trailing_plus =>
                 {
@@ -425,12 +428,60 @@ impl<'a> Parser<'a> {
     }
 
     fn parse_bare_trait_object(&mut self, lo: Span, allow_plus: AllowPlus) -> PResult<'a, TyKind> {
-        let lt_no_plus = self.check_lifetime() && !self.look_ahead(1, |t| t.is_like_plus());
-        let bounds = self.parse_generic_bounds_common(allow_plus)?;
-        if lt_no_plus {
-            self.dcx().emit_err(NeedPlusAfterTraitObjectLifetime { span: lo });
+        // A lifetime only begins a bare trait object type if it is followed by `+`!
+        if self.token.is_lifetime() && !self.look_ahead(1, |t| t.is_like_plus()) {
+            // In Rust 2021 and beyond, we assume that the user didn't intend to write a bare trait
+            // object type with a leading lifetime bound since that seems very unlikely given the
+            // fact that `dyn`-less trait objects are *semantically* invalid.
+            if self.psess.edition.at_least_rust_2021() {
+                let lt = self.expect_lifetime();
+                let mut err = self.dcx().struct_span_err(lo, "expected type, found lifetime");
+                err.span_label(lo, "expected type");
+                return Ok(match self.maybe_recover_ref_ty_no_leading_ampersand(lt, lo, err) {
+                    Ok(ref_ty) => ref_ty,
+                    Err(err) => TyKind::Err(err.emit()),
+                });
+            }
+
+            self.dcx().emit_err(NeedPlusAfterTraitObjectLifetime {
+                span: lo,
+                suggestion: lo.shrink_to_hi(),
+            });
+        }
+        Ok(TyKind::TraitObject(
+            self.parse_generic_bounds_common(allow_plus)?,
+            TraitObjectSyntax::None,
+        ))
+    }
+
+    fn maybe_recover_ref_ty_no_leading_ampersand<'cx>(
+        &mut self,
+        lt: Lifetime,
+        lo: Span,
+        mut err: Diag<'cx>,
+    ) -> Result<TyKind, Diag<'cx>> {
+        if !self.may_recover() {
+            return Err(err);
+        }
+        let snapshot = self.create_snapshot_for_diagnostic();
+        let mutbl = self.parse_mutability();
+        match self.parse_ty_no_plus() {
+            Ok(ty) => {
+                err.span_suggestion_verbose(
+                    lo.shrink_to_lo(),
+                    "you might have meant to write a reference type here",
+                    "&",
+                    Applicability::MaybeIncorrect,
+                );
+                err.emit();
+                Ok(TyKind::Ref(Some(lt), MutTy { ty, mutbl }))
+            }
+            Err(diag) => {
+                diag.cancel();
+                self.restore_snapshot(snapshot);
+                Err(err)
+            }
         }
-        Ok(TyKind::TraitObject(bounds, TraitObjectSyntax::None))
     }
 
     fn parse_remaining_bounds_path(

tests/ui/did_you_mean/E0178.stderr

@@ -1,41 +1,43 @@
-error[E0178]: expected a path on the left-hand side of `+`, not `&'a Foo`
+error[E0178]: expected a path on the left-hand side of `+`
   --> $DIR/E0178.rs:6:8
    |
 LL |     w: &'a Foo + Copy,
-   |        ^^^^^^^^^^^^^^
+   |        ^^^^^^^
    |
 help: try adding parentheses
    |
 LL |     w: &'a (Foo + Copy),
    |            +          +
 
-error[E0178]: expected a path on the left-hand side of `+`, not `&'a Foo`
+error[E0178]: expected a path on the left-hand side of `+`
   --> $DIR/E0178.rs:7:8
    |
 LL |     x: &'a Foo + 'a,
-   |        ^^^^^^^^^^^^
+   |        ^^^^^^^
    |
 help: try adding parentheses
    |
 LL |     x: &'a (Foo + 'a),
    |            +        +
 
-error[E0178]: expected a path on the left-hand side of `+`, not `&'a mut Foo`
+error[E0178]: expected a path on the left-hand side of `+`
   --> $DIR/E0178.rs:8:8
    |
 LL |     y: &'a mut Foo + 'a,
-   |        ^^^^^^^^^^^^^^^^
+   |        ^^^^^^^^^^^
    |
 help: try adding parentheses
    |
 LL |     y: &'a mut (Foo + 'a),
    |                +        +
 
-error[E0178]: expected a path on the left-hand side of `+`, not `fn() -> Foo`
+error[E0178]: expected a path on the left-hand side of `+`
   --> $DIR/E0178.rs:9:8
    |
 LL |     z: fn() -> Foo + 'a,
-   |        ^^^^^^^^^^^^^^^^ perhaps you forgot parentheses?
+   |        ^^^^^^^^^^^-----
+   |        |
+   |        perhaps you forgot parentheses?
 
 error: aborting due to 4 previous errors
 

tests/ui/did_you_mean/trait-object-reference-without-parens-suggestion.stderr

@@ -1,19 +1,19 @@
-error[E0178]: expected a path on the left-hand side of `+`, not `&Copy`
+error[E0178]: expected a path on the left-hand side of `+`
   --> $DIR/trait-object-reference-without-parens-suggestion.rs:4:12
    |
 LL |     let _: &Copy + 'static;
-   |            ^^^^^^^^^^^^^^^
+   |            ^^^^^
    |
 help: try adding parentheses
    |
 LL |     let _: &(Copy + 'static);
    |             +              +
 
-error[E0178]: expected a path on the left-hand side of `+`, not `&'static Copy`
+error[E0178]: expected a path on the left-hand side of `+`
   --> $DIR/trait-object-reference-without-parens-suggestion.rs:6:12
    |
 LL |     let _: &'static Copy + 'static;
-   |            ^^^^^^^^^^^^^^^^^^^^^^^
+   |            ^^^^^^^^^^^^^
    |
 help: try adding parentheses
    |

tests/ui/generic-associated-types/gat-trait-path-parenthesised-args.rs

@@ -3,7 +3,7 @@ trait X {
 }
 
 fn foo<'a>(arg: Box<dyn X<Y('a) = &'a ()>>) {}
-  //~^ ERROR: lifetime in trait object type must be followed by `+`
+  //~^ ERROR: lifetimes must be followed by `+` to form a trait object type
   //~| ERROR: parenthesized generic arguments cannot be used
   //~| ERROR associated type takes 0 generic arguments but 1 generic argument
   //~| ERROR associated type takes 1 lifetime argument but 0 lifetime arguments

tests/ui/generic-associated-types/gat-trait-path-parenthesised-args.stderr

@@ -1,8 +1,13 @@
-error: lifetime in trait object type must be followed by `+`
+error: lifetimes must be followed by `+` to form a trait object type
   --> $DIR/gat-trait-path-parenthesised-args.rs:5:29
    |
 LL | fn foo<'a>(arg: Box<dyn X<Y('a) = &'a ()>>) {}
    |                             ^^
+   |
+help: consider adding a trait bound after the potential lifetime bound
+   |
+LL | fn foo<'a>(arg: Box<dyn X<Y('a + /* Trait */) = &'a ()>>) {}
+   |                                +++++++++++++
 
 error: parenthesized generic arguments cannot be used in associated type constraints
   --> $DIR/gat-trait-path-parenthesised-args.rs:5:27

tests/ui/impl-trait/impl-trait-plus-priority.rs

@@ -27,7 +27,7 @@ type A = fn() -> impl A + B;
 type A = fn() -> dyn A + B;
 //~^ ERROR ambiguous `+` in a type
 type A = fn() -> A + B;
-//~^ ERROR expected a path on the left-hand side of `+`, not `fn() -> A`
+//~^ ERROR expected a path on the left-hand side of `+`
 
 type A = Fn() -> impl A +;
 //~^ ERROR ambiguous `+` in a type
@@ -44,6 +44,6 @@ type A = &impl A + B;
 type A = &dyn A + B;
 //~^ ERROR ambiguous `+` in a type
 type A = &A + B;
-//~^ ERROR expected a path on the left-hand side of `+`, not `&A`
+//~^ ERROR expected a path on the left-hand side of `+`
 
 fn main() {}

tests/ui/impl-trait/impl-trait-plus-priority.stderr

@@ -31,11 +31,13 @@ help: try adding parentheses
 LL | type A = fn() -> (dyn A + B);
    |                  +         +
 
-error[E0178]: expected a path on the left-hand side of `+`, not `fn() -> A`
+error[E0178]: expected a path on the left-hand side of `+`
   --> $DIR/impl-trait-plus-priority.rs:29:10
    |
 LL | type A = fn() -> A + B;
-   |          ^^^^^^^^^^^^^ perhaps you forgot parentheses?
+   |          ^^^^^^^^^----
+   |          |
+   |          perhaps you forgot parentheses?
 
 error: ambiguous `+` in a type
   --> $DIR/impl-trait-plus-priority.rs:32:18
@@ -103,11 +105,11 @@ help: try adding parentheses
 LL | type A = &(dyn A + B);
    |           +         +
 
-error[E0178]: expected a path on the left-hand side of `+`, not `&A`
+error[E0178]: expected a path on the left-hand side of `+`
   --> $DIR/impl-trait-plus-priority.rs:46:10
    |
 LL | type A = &A + B;
-   |          ^^^^^^
+   |          ^^
    |
 help: try adding parentheses
    |