Deny bare const expressions

src/librustc_parse/parser/path.rs

@@ -593,13 +593,39 @@ impl<'a> Parser<'a> {
                 // we should introduce a GenericArg::Ident in the AST and distinguish when
                 // lowering to the HIR. For now, idents for const args are not permitted.
                 let start = self.token.span;
-                self.parse_expr_res(Restrictions::CONST_EXPR, None).map_err(|mut err| {
-                    err.span_label(
-                        start.shrink_to_lo(),
-                        "while parsing a `const` argument starting here",
-                    );
-                    err
-                })?
+                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` argument starting here",
+                        );
+                        err
+                    })?;
+                let valid_const = match &expr.kind {
+                    ast::ExprKind::Block(_, _) | ast::ExprKind::Lit(_) => true,
+                    ast::ExprKind::Unary(ast::UnOp::Neg, expr) => match &expr.kind {
+                        ast::ExprKind::Lit(_) => true,
+                        _ => false,
+                    },
+                    _ => false,
+                };
+                if !valid_const {
+                    self.struct_span_err(
+                        expr.span,
+                        "`const` generic expressions without braces are not supported",
+                    )
+                    .note("only literals are supported as `const` generic without braces")
+                    .multipart_suggestion(
+                        "surround `const` expressions with braces",
+                        vec![
+                            (expr.span.shrink_to_lo(), "{ ".to_string()),
+                            (expr.span.shrink_to_hi(), " }".to_string()),
+                        ],
+                        Applicability::MachineApplicable,
+                    )
+                    .emit();
+                }
+                expr
             };
             GenericArg::Const(AnonConst { id: ast::DUMMY_NODE_ID, value })
         } else if self.check_type() {

src/test/ui/const-generics/const-expression-missing-braces.rs

@@ -6,26 +6,22 @@ fn foo<const C: usize>() {}
 const BAR: usize = 42;
 
 fn a() {
-    foo::<BAR + 3>();
-    //~^ ERROR expected one of
+    foo::<BAR + 3>(); //~ ERROR expected one of
 }
 fn b() {
-    foo::<BAR + BAR>();
-    //~^ ERROR likely `const` expression parsed as trait bounds
+    foo::<BAR + BAR>(); //~ ERROR likely `const` expression parsed as trait bounds
 }
 fn c() {
-    foo::<3 + 3>(); // ok
+    foo::<3 + 3>(); //~ ERROR `const` generic expressions without braces are not supported
 }
 fn d() {
-    foo::<BAR - 3>();
-    //~^ ERROR expected one of
+    foo::<BAR - 3>(); //~ ERROR expected one of
 }
 fn e() {
-    foo::<BAR - BAR>();
-    //~^ ERROR expected one of
+    foo::<BAR - BAR>(); //~ ERROR expected one of
 }
 fn f() {
-    foo::<100 - BAR>(); // ok
+    foo::<100 - BAR>(); //~ ERROR `const` generic expressions without braces are not supported
 }
 fn g() {
     foo::<bar<i32>()>(); //~ ERROR expected one of

src/test/ui/const-generics/const-expression-missing-braces.stderr

@@ -9,8 +9,20 @@ help: to write a `const` expression, surround it with braces for it to be unambi
 LL |     foo::<{ BAR + 3 }>();
    |           ^         ^
 
+error: `const` generic expressions without braces are not supported
+  --> $DIR/const-expression-missing-braces.rs:15:11
+   |
+LL |     foo::<3 + 3>();
+   |           ^^^^^
+   |
+   = note: only literals are supported as `const` generic without braces
+help: surround `const` expressions with braces
+   |
+LL |     foo::<{ 3 + 3 }>();
+   |           ^       ^
+
 error: expected one of `,` or `>`, found `-`
-  --> $DIR/const-expression-missing-braces.rs:20:15
+  --> $DIR/const-expression-missing-braces.rs:18:15
    |
 LL |     foo::<BAR - 3>();
    |               ^ expected one of `,` or `>`
@@ -21,7 +33,7 @@ LL |     foo::<{ BAR - 3 }>();
    |           ^         ^
 
 error: expected one of `,` or `>`, found `-`
-  --> $DIR/const-expression-missing-braces.rs:24:15
+  --> $DIR/const-expression-missing-braces.rs:21:15
    |
 LL |     foo::<BAR - BAR>();
    |               ^ expected one of `,` or `>`
@@ -31,8 +43,20 @@ help: to write a `const` expression, surround it with braces for it to be unambi
 LL |     foo::<{ BAR - BAR }>();
    |           ^           ^
 
+error: `const` generic expressions without braces are not supported
+  --> $DIR/const-expression-missing-braces.rs:24:11
+   |
+LL |     foo::<100 - BAR>();
+   |           ^^^^^^^^^
+   |
+   = note: only literals are supported as `const` generic without braces
+help: surround `const` expressions with braces
+   |
+LL |     foo::<{ 100 - BAR }>();
+   |           ^           ^
+
 error: expected one of `,` or `>`, found `(`
-  --> $DIR/const-expression-missing-braces.rs:31:19
+  --> $DIR/const-expression-missing-braces.rs:27:19
    |
 LL |     foo::<bar<i32>()>();
    |                   ^ expected one of `,` or `>`
@@ -43,7 +67,7 @@ LL |     foo::<{ bar<i32>() }>();
    |           ^            ^
 
 error: expected one of `,` or `>`, found `(`
-  --> $DIR/const-expression-missing-braces.rs:34:21
+  --> $DIR/const-expression-missing-braces.rs:30:21
    |
 LL |     foo::<bar::<i32>()>();
    |                     ^ expected one of `,` or `>`
@@ -54,7 +78,7 @@ LL |     foo::<{ bar::<i32>() }>();
    |           ^              ^
 
 error: expected one of `,` or `>`, found `(`
-  --> $DIR/const-expression-missing-braces.rs:37:21
+  --> $DIR/const-expression-missing-braces.rs:33:21
    |
 LL |     foo::<bar::<i32>() + BAR>();
    |                     ^ expected one of `,` or `>`
@@ -65,7 +89,7 @@ LL |     foo::<{ bar::<i32>() + BAR }>();
    |           ^                    ^
 
 error: expected one of `,` or `>`, found `(`
-  --> $DIR/const-expression-missing-braces.rs:40:21
+  --> $DIR/const-expression-missing-braces.rs:36:21
    |
 LL |     foo::<bar::<i32>() - BAR>();
    |                     ^ expected one of `,` or `>`
@@ -76,7 +100,7 @@ LL |     foo::<{ bar::<i32>() - BAR }>();
    |           ^                    ^
 
 error: expected one of `,` or `>`, found `-`
-  --> $DIR/const-expression-missing-braces.rs:43:15
+  --> $DIR/const-expression-missing-braces.rs:39:15
    |
 LL |     foo::<BAR - bar::<i32>()>();
    |               ^ expected one of `,` or `>`
@@ -87,7 +111,7 @@ LL |     foo::<{ BAR - bar::<i32>() }>();
    |           ^                    ^
 
 error: expected one of `,` or `>`, found `-`
-  --> $DIR/const-expression-missing-braces.rs:46:15
+  --> $DIR/const-expression-missing-braces.rs:42:15
    |
 LL |     foo::<BAR - bar::<i32>()>();
    |               ^ expected one of `,` or `>`
@@ -98,7 +122,7 @@ LL |     foo::<{ BAR - bar::<i32>() }>();
    |           ^                    ^
 
 error: likely `const` expression parsed as trait bounds
-  --> $DIR/const-expression-missing-braces.rs:13:11
+  --> $DIR/const-expression-missing-braces.rs:12:11
    |
 LL |     foo::<BAR + BAR>();
    |           ^^^^^^^^^ parsed as trait bounds but traits weren't found
@@ -108,5 +132,5 @@ help: if you meant to write a `const` expression, surround the expression with b
 LL |     foo::<{ BAR + BAR }>();
    |           ^           ^
 
-error: aborting due to 10 previous errors
+error: aborting due to 12 previous errors
 

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

@@ -1,4 +1,3 @@
-// check-pass
 #![allow(incomplete_features)]
 #![feature(const_generics)]
 
@@ -11,7 +10,7 @@ fn foo_a() {
 }
 
 fn foo_b() {
-    i32_identity::<1 + 2>(); // ok
+    i32_identity::<1 + 2>(); //~ ERROR  `const` generic expressions without braces are not supported
 }
 
 fn foo_c() {

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

@@ -0,0 +1,14 @@
+error: `const` generic expressions without braces are not supported
+  --> $DIR/const-expression-parameter.rs:13:20
+   |
+LL |     i32_identity::<1 + 2>();
+   |                    ^^^^^
+   |
+   = note: only literals are supported as `const` generic without braces
+help: surround `const` expressions with braces
+   |
+LL |     i32_identity::<{ 1 + 2 }>();
+   |                    ^       ^
+
+error: aborting due to previous error
+

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

@@ -237,5 +237,6 @@ fn inside_const_generic_arguments() {
     if A::<
         true && let 1 = 1 //~ ERROR `let` expressions are not supported here
         //~^ ERROR `match` is not allowed in a `const`
+        //~| ERROR  `const` generic expressions without braces are not supported
     >::O == 5 {}
 }

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

@@ -1,3 +1,15 @@
+error: `const` generic expressions without braces are not supported
+  --> $DIR/disallowed-positions.rs:238:9
+   |
+LL |         true && let 1 = 1
+   |         ^^^^^^^^^^^^^^^^^
+   |
+   = note: only literals are supported as `const` generic without braces
+help: surround `const` expressions with braces
+   |
+LL |         { true && let 1 = 1 }
+   |         ^                   ^
+
 error: `let` expressions are not supported here
   --> $DIR/disallowed-positions.rs:32:9
    |
@@ -992,7 +1004,7 @@ LL |         let 0 = 0?;
    = help: the trait `std::ops::Try` is not implemented for `{integer}`
    = note: required by `std::ops::Try::into_result`
 
-error: aborting due to 107 previous errors; 2 warnings emitted
+error: aborting due to 108 previous errors; 2 warnings emitted
 
 Some errors have detailed explanations: E0277, E0308, E0600, E0614, E0658.
 For more information about an error, try `rustc --explain E0277`.