Suggest enclosing const expression in block

When encountering `foo::<1 + 1>()`, suggest `foo::<{ 1 + 1 }>()`.

Fix rust-lang#61175.

Author: estebank

src/libsyntax/ast.rs

@@ -1254,6 +1254,23 @@ pub enum ExprKind {
     Err,
 }
 
+impl ExprKind {
+    /// Whether this expression can appear in a contst argument without being surrounded by braces.
+    ///
+    /// Only used in error recovery.
+    pub(crate) fn is_valid_const_on_its_own(&self) -> bool {
+        match self {
+            ExprKind::Tup(_) |
+            ExprKind::Lit(_) |
+            ExprKind::Type(..) |
+            ExprKind::Path(..) |
+            ExprKind::Unary(..) |
+            ExprKind::Err => true,
+            _ => false,
+        }
+    }
+}
+
 /// The explicit `Self` type in a "qualified path". The actual
 /// path, including the trait and the associated item, is stored
 /// separately. `position` represents the index of the associated

src/libsyntax/parse/parser.rs

@@ -38,8 +38,9 @@ use std::path::PathBuf;
 
 bitflags::bitflags! {
     struct Restrictions: u8 {
-        const STMT_EXPR         = 1 << 0;
-        const NO_STRUCT_LITERAL = 1 << 1;
+        const STMT_EXPR           = 1 << 0;
+        const NO_STRUCT_LITERAL   = 1 << 1;
+        const CONST_EXPR_RECOVERY = 1 << 2;
     }
 }
 

src/libsyntax/parse/parser/expr.rs

@@ -186,6 +186,12 @@ impl<'a> Parser<'a> {
                 self.last_type_ascription = None;
                 return Ok(lhs);
             }
+            (true, Some(AssocOp::Greater)) if self.restrictions.contains(
+                Restrictions::CONST_EXPR_RECOVERY,
+            ) => { // Recovering likely const argument without braces.
+                self.last_type_ascription = None;
+                return Ok(lhs);
+            }
             (true, Some(_)) => {
                 // We've found an expression that would be parsed as a statement, but the next
                 // token implies this should be parsed as an expression.
@@ -205,6 +211,11 @@ impl<'a> Parser<'a> {
         }
         self.expected_tokens.push(TokenType::Operator);
         while let Some(op) = AssocOp::from_token(&self.token) {
+            if let (AssocOp::Greater, true) = (&op, self.restrictions.contains(
+                Restrictions::CONST_EXPR_RECOVERY,
+            )) { // Recovering likely const argument without braces.
+                return Ok(lhs);
+            }
 
             // Adjust the span for interpolated LHS to point to the `$lhs` token and not to what
             // it refers to. Interpolated identifiers are unwrapped early and never show up here
@@ -341,8 +352,10 @@ impl<'a> Parser<'a> {
 
     /// Checks if this expression is a successfully parsed statement.
     fn expr_is_complete(&self, e: &Expr) -> bool {
-        self.restrictions.contains(Restrictions::STMT_EXPR) &&
-            !classify::expr_requires_semi_to_be_stmt(e)
+        (self.restrictions.contains(Restrictions::STMT_EXPR) &&
+            !classify::expr_requires_semi_to_be_stmt(e)) ||
+            (self.restrictions.contains(Restrictions::CONST_EXPR_RECOVERY) &&
+             (self.token == token::Lt || self.token == token::Gt || self.token == token::Comma))
     }
 
     fn is_at_start_of_range_notation_rhs(&self) -> bool {

src/libsyntax/parse/parser/path.rs

@@ -1,4 +1,4 @@
-use super::{Parser, PResult, TokenType};
+use super::{Parser, PResult, Restrictions, TokenType};
 
 use crate::{maybe_whole, ThinVec};
 use crate::ast::{self, QSelf, Path, PathSegment, Ident, ParenthesizedArgs, AngleBracketedArgs};
@@ -414,10 +414,52 @@ impl<'a> Parser<'a> {
                 assoc_ty_constraints.push(span);
             } else if self.check_const_arg() {
                 // Parse const argument.
+                let invalid = self.look_ahead(1, |t| t != &token::Lt && t != &token::Comma);
                 let expr = if let token::OpenDelim(token::Brace) = self.token.kind {
                     self.parse_block_expr(
                         None, self.token.span, BlockCheckMode::Default, ThinVec::new()
                     )?
+                } else if invalid {
+                    // This can't possibly be a valid const arg, it is likely missing braces.
+                    let snapshot = self.clone();
+                    match self.parse_expr_res(Restrictions::CONST_EXPR_RECOVERY, None) {
+                        Ok(expr) => {
+                            if self.token == token::Comma || self.token == token::Gt {
+                                // We parsed the whole const argument successfully without braces.
+                                if !expr.node.is_valid_const_on_its_own() {
+                                    // But it wasn't a literal, so we emit a custom error and
+                                    // suggest the appropriate code.
+                                    let msg =
+                                        "complex const arguments must be surrounded by braces";
+                                    let appl = Applicability::MachineApplicable;
+                                    self.span_fatal(expr.span, msg)
+                                        .multipart_suggestion(
+                                            "surround this const argument in braces",
+                                            vec![
+                                                (expr.span.shrink_to_lo(), "{ ".to_string()),
+                                                (expr.span.shrink_to_hi(), " }".to_string()),
+                                            ],
+                                            appl,
+                                        )
+                                        .emit();
+                                }
+                                expr
+                            } else {
+                                // We parsed *some* expression, but it isn't the whole argument
+                                // so we can't ensure it was a const argument with missing braces.
+                                // Roll-back and emit a regular parser error.
+                                mem::replace(self, snapshot);
+                                self.parse_literal_maybe_minus()?
+                            }
+                        }
+                        Err(mut err) => {
+                            // We couldn't parse an expression successfully.
+                            // Roll-back, hide the error and emit a regular parser error.
+                            err.cancel();
+                            mem::replace(self, snapshot);
+                            self.parse_literal_maybe_minus()?
+                        }
+                    }
                 } else if self.token.is_ident() {
                     // FIXME(const_generics): to distinguish between idents for types and consts,
                     // we should introduce a GenericArg::Ident in the AST and distinguish when

src/test/ui/const-generics/const-expression-parameter.rs

@@ -10,13 +10,30 @@ fn foo_a() {
 }
 
 fn foo_b() {
-    i32_identity::<1 + 2>(); //~ ERROR expected one of `,` or `>`, found `+`
+    i32_identity::<1 + 2>(); //~ ERROR complex const arguments must be surrounded by braces
 }
 
 fn foo_c() {
     i32_identity::< -1 >(); // ok
 }
 
+fn foo_d() {
+    i32_identity::<1 + 2, 3 + 4>();
+    //~^ ERROR complex const arguments must be surrounded by braces
+    //~| ERROR complex const arguments must be surrounded by braces
+    //~| ERROR wrong number of const arguments: expected 1, found 2
+}
+
+fn baz<const X: i32, const Y: i32>() -> i32 {
+    42
+}
+
+fn foo_e() {
+    baz::<1 + 2, 3 + 4>();
+    //~^ ERROR complex const arguments must be surrounded by braces
+    //~| ERROR complex const arguments must be surrounded by braces
+}
+
 fn main() {
     i32_identity::<5>(); // ok
 }

src/test/ui/const-generics/const-expression-parameter.stderr

@@ -1,8 +1,52 @@
-error: expected one of `,` or `>`, found `+`
-  --> $DIR/const-expression-parameter.rs:13:22
+error: complex const arguments must be surrounded by braces
+  --> $DIR/const-expression-parameter.rs:13:20
    |
 LL |     i32_identity::<1 + 2>();
-   |                      ^ expected one of `,` or `>` here
+   |                    ^^^^^
+help: surround this const argument in braces
+   |
+LL |     i32_identity::<{ 1 + 2 }>();
+   |                    ^       ^
+
+error: complex const arguments must be surrounded by braces
+  --> $DIR/const-expression-parameter.rs:21:20
+   |
+LL |     i32_identity::<1 + 2, 3 + 4>();
+   |                    ^^^^^
+help: surround this const argument in braces
+   |
+LL |     i32_identity::<{ 1 + 2 }, 3 + 4>();
+   |                    ^       ^
+
+error: complex const arguments must be surrounded by braces
+  --> $DIR/const-expression-parameter.rs:21:27
+   |
+LL |     i32_identity::<1 + 2, 3 + 4>();
+   |                           ^^^^^
+help: surround this const argument in braces
+   |
+LL |     i32_identity::<1 + 2, { 3 + 4 }>();
+   |                           ^       ^
+
+error: complex const arguments must be surrounded by braces
+  --> $DIR/const-expression-parameter.rs:32:11
+   |
+LL |     baz::<1 + 2, 3 + 4>();
+   |           ^^^^^
+help: surround this const argument in braces
+   |
+LL |     baz::<{ 1 + 2 }, 3 + 4>();
+   |           ^       ^
+
+error: complex const arguments must be surrounded by braces
+  --> $DIR/const-expression-parameter.rs:32:18
+   |
+LL |     baz::<1 + 2, 3 + 4>();
+   |                  ^^^^^
+help: surround this const argument in braces
+   |
+LL |     baz::<1 + 2, { 3 + 4 }>();
+   |                  ^       ^
 
 warning: the feature `const_generics` is incomplete and may cause the compiler to crash
   --> $DIR/const-expression-parameter.rs:1:12
@@ -12,5 +56,12 @@ LL | #![feature(const_generics)]
    |
    = note: `#[warn(incomplete_features)]` on by default
 
-error: aborting due to previous error
+error[E0107]: wrong number of const arguments: expected 1, found 2
+  --> $DIR/const-expression-parameter.rs:21:27
+   |
+LL |     i32_identity::<1 + 2, 3 + 4>();
+   |                           ^^^^^ unexpected const argument
+
+error: aborting due to 6 previous errors
 
+For more information about this error, try `rustc --explain E0107`.

src/test/ui/rfc-2497-if-let-chains/disallowed-positions.rs

@@ -239,6 +239,7 @@ fn inside_const_generic_arguments() {
     // admit non-IDENT expressions in const generic arguments.
 
     if A::<
-        true && let 1 = 1 //~ ERROR expected one of `,` or `>`, found `&&`
-    >::O == 5 {}
+        true && let 1 = 1 //~ ERROR complex const arguments must be surrounded by braces
+    >::O == 5 {}  //~ ERROR chained comparison operators require parentheses
+    //~^ ERROR expected one of `,`, `.`, `>`, `?`, or an operator, found `{`
 }

src/test/ui/rfc-2497-if-let-chains/disallowed-positions.stderr

@@ -1,8 +1,26 @@
-error: expected one of `,` or `>`, found `&&`
-  --> $DIR/disallowed-positions.rs:242:14
+error: chained comparison operators require parentheses
+  --> $DIR/disallowed-positions.rs:243:5
    |
-LL |         true && let 1 = 1
-   |              ^^ expected one of `,` or `>` here
+LL |     >::O == 5 {}
+   |     ^^^^^^^^^
+
+error: complex const arguments must be surrounded by braces
+  --> $DIR/disallowed-positions.rs:242:9
+   |
+LL | /         true && let 1 = 1
+LL | |     >::O == 5 {}
+   | |_____________^
+help: surround this const argument in braces
+   |
+LL |         { true && let 1 = 1
+LL |     >::O == 5 } {}
+   |
+
+error: expected one of `,`, `.`, `>`, `?`, or an operator, found `{`
+  --> $DIR/disallowed-positions.rs:243:15
+   |
+LL |     >::O == 5 {}
+   |               ^ expected one of `,`, `.`, `>`, `?`, or an operator here
 
 error: `let` expressions are not supported here
   --> $DIR/disallowed-positions.rs:32:9
@@ -983,7 +1001,7 @@ error[E0019]: constant contains unimplemented expression type
 LL |         true && let 1 = 1
    |                     ^
 
-error: aborting due to 109 previous errors
+error: aborting due to 111 previous errors
 
 Some errors have detailed explanations: E0019, E0277, E0308, E0600, E0614.
 For more information about an error, try `rustc --explain E0019`.