Auto merge of #77502 - varkor:const-generics-suggest-enclosing-braces, r=petrochenkov

Suggest that expressions that look like const generic arguments should be enclosed in brackets

I pulled out the changes for const expressions from https://github.com/rust-lang/rust/pull/71592 (without the trait object diagnostic changes) and made some small changes; the implementation is `@estebank's.`

We're also going to want to make some changes separately to account for trait objects (they result in poor diagnostics, as is evident from one of the test cases here), such as an adaption of https://github.com/rust-lang/rust/pull/72273.

Fixes https://github.com/rust-lang/rust/issues/70753.

r? `@petrochenkov`

compiler/rustc_parse/src/parser/diagnostics.rs

@@ -1,13 +1,14 @@
 use super::ty::AllowPlus;
-use super::{BlockMode, Parser, PathStyle, SemiColonMode, SeqSep, TokenExpectType, TokenType};
+use super::TokenType;
+use super::{BlockMode, Parser, PathStyle, Restrictions, SemiColonMode, SeqSep, TokenExpectType};
 
 use rustc_ast::ptr::P;
 use rustc_ast::token::{self, Lit, LitKind, TokenKind};
 use rustc_ast::util::parser::AssocOp;
 use rustc_ast::{
-    self as ast, AngleBracketedArgs, AttrVec, BinOpKind, BindingMode, Block, BlockCheckMode, Expr,
-    ExprKind, Item, ItemKind, Mutability, Param, Pat, PatKind, Path, PathSegment, QSelf, Ty,
-    TyKind,
+    self as ast, AngleBracketedArg, AngleBracketedArgs, AnonConst, AttrVec, BinOpKind, BindingMode,
+    Block, BlockCheckMode, Expr, ExprKind, GenericArg, Item, ItemKind, Mutability, Param, Pat,
+    PatKind, Path, PathSegment, QSelf, Ty, TyKind,
 };
 use rustc_ast_pretty::pprust;
 use rustc_data_structures::fx::FxHashSet;
@@ -1780,4 +1781,142 @@ impl<'a> Parser<'a> {
             }
         }
     }
+
+    /// Handle encountering a symbol in a generic argument list that is not a `,` or `>`. In this
+    /// case, we emit an error and try to suggest enclosing a const argument in braces if it looks
+    /// like the user has forgotten them.
+    pub fn handle_ambiguous_unbraced_const_arg(
+        &mut self,
+        args: &mut Vec<AngleBracketedArg>,
+    ) -> PResult<'a, bool> {
+        // If we haven't encountered a closing `>`, then the argument is malformed.
+        // It's likely that the user has written a const expression without enclosing it
+        // in braces, so we try to recover here.
+        let arg = args.pop().unwrap();
+        // FIXME: for some reason using `unexpected` or `expected_one_of_not_found` has
+        // adverse side-effects to subsequent errors and seems to advance the parser.
+        // We are causing this error here exclusively in case that a `const` expression
+        // could be recovered from the current parser state, even if followed by more
+        // arguments after a comma.
+        let mut err = self.struct_span_err(
+            self.token.span,
+            &format!("expected one of `,` or `>`, found {}", super::token_descr(&self.token)),
+        );
+        err.span_label(self.token.span, "expected one of `,` or `>`");
+        match self.recover_const_arg(arg.span(), err) {
+            Ok(arg) => {
+                args.push(AngleBracketedArg::Arg(arg));
+                if self.eat(&token::Comma) {
+                    return Ok(true); // Continue
+                }
+            }
+            Err(mut err) => {
+                args.push(arg);
+                // We will emit a more generic error later.
+                err.delay_as_bug();
+            }
+        }
+        return Ok(false); // Don't continue.
+    }
+
+    /// Handle a generic const argument that had not been enclosed in braces, and suggest enclosing
+    /// it braces. In this situation, unlike in `handle_ambiguous_unbraced_const_arg`, this is
+    /// almost certainly a const argument, so we always offer a suggestion.
+    pub fn handle_unambiguous_unbraced_const_arg(&mut self) -> PResult<'a, P<Expr>> {
+        let start = self.token.span;
+        let expr = self.parse_expr_res(Restrictions::CONST_EXPR, None).map_err(|mut err| {
+            err.span_label(
+                start.shrink_to_lo(),
+                "while parsing a const generic argument starting here",
+            );
+            err
+        })?;
+        if !self.expr_is_valid_const_arg(&expr) {
+            self.struct_span_err(
+                expr.span,
+                "expressions must be enclosed in braces to be used as const generic \
+                    arguments",
+            )
+            .multipart_suggestion(
+                "enclose the `const` expression in braces",
+                vec![
+                    (expr.span.shrink_to_lo(), "{ ".to_string()),
+                    (expr.span.shrink_to_hi(), " }".to_string()),
+                ],
+                Applicability::MachineApplicable,
+            )
+            .emit();
+        }
+        Ok(expr)
+    }
+
+    /// Try to recover from possible generic const argument without `{` and `}`.
+    ///
+    /// When encountering code like `foo::< bar + 3 >` or `foo::< bar - baz >` we suggest
+    /// `foo::<{ bar + 3 }>` and `foo::<{ bar - baz }>`, respectively. We only provide a suggestion
+    /// if we think that that the resulting expression would be well formed.
+    pub fn recover_const_arg(
+        &mut self,
+        start: Span,
+        mut err: DiagnosticBuilder<'a>,
+    ) -> PResult<'a, GenericArg> {
+        let is_op = AssocOp::from_token(&self.token)
+            .and_then(|op| {
+                if let AssocOp::Greater
+                | AssocOp::Less
+                | AssocOp::ShiftRight
+                | AssocOp::GreaterEqual
+                // Don't recover from `foo::<bar = baz>`, because this could be an attempt to
+                // assign a value to a defaulted generic parameter.
+                | AssocOp::Assign
+                | AssocOp::AssignOp(_) = op
+                {
+                    None
+                } else {
+                    Some(op)
+                }
+            })
+            .is_some();
+        // This will be true when a trait object type `Foo +` or a path which was a `const fn` with
+        // type params has been parsed.
+        let was_op =
+            matches!(self.prev_token.kind, token::BinOp(token::Plus | token::Shr) | token::Gt);
+        if !is_op && !was_op {
+            // We perform these checks and early return to avoid taking a snapshot unnecessarily.
+            return Err(err);
+        }
+        let snapshot = self.clone();
+        if is_op {
+            self.bump();
+        }
+        match self.parse_expr_res(Restrictions::CONST_EXPR, None) {
+            Ok(expr) => {
+                if token::Comma == self.token.kind || self.token.kind.should_end_const_arg() {
+                    // Avoid the following output by checking that we consumed a full const arg:
+                    // help: expressions must be enclosed in braces to be used as const generic
+                    //       arguments
+                    //    |
+                    // LL |     let sr: Vec<{ (u32, _, _) = vec![] };
+                    //    |                 ^                      ^
+                    err.multipart_suggestion(
+                        "expressions must be enclosed in braces to be used as const generic \
+                         arguments",
+                        vec![
+                            (start.shrink_to_lo(), "{ ".to_string()),
+                            (expr.span.shrink_to_hi(), " }".to_string()),
+                        ],
+                        Applicability::MaybeIncorrect,
+                    );
+                    let value = self.mk_expr_err(start.to(expr.span));
+                    err.emit();
+                    return Ok(GenericArg::Const(AnonConst { id: ast::DUMMY_NODE_ID, value }));
+                }
+            }
+            Err(mut err) => {
+                err.cancel();
+            }
+        }
+        *self = snapshot;
+        Err(err)
+    }
 }