comptime fn error: suggest wrapping in const block

compiler/rustc_hir_typeck/src/callee.rs

@@ -1115,7 +1115,13 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             match const_context {
                 Some(hir::ConstContext::Const { .. } | hir::ConstContext::Static(_)) => {}
                 Some(hir::ConstContext::ConstFn) | None => {
-                    self.dcx().span_err(span, "comptime fns can only be called at compile time");
+                    self.dcx().emit_err(diagnostics::ComptimeFnCalledOutsideConstContext {
+                        span,
+                        const_block: diagnostics::WrapConstBlock {
+                            left: span.shrink_to_lo(),
+                            right: span.shrink_to_hi(),
+                        },
+                    });
                 }
             }
         }

compiler/rustc_hir_typeck/src/diagnostics.rs

@@ -1349,3 +1349,24 @@ impl ExprParenthesesNeeded {
 #[derive(Diagnostic)]
 #[diag("`#[panic_handler]` functions can't be `unsafe`")]
 pub(crate) struct UnsafePanicHandlers;
+
+#[derive(Diagnostic)]
+#[diag("comptime functions can only be called at compile time")]
+pub(crate) struct ComptimeFnCalledOutsideConstContext {
+    #[primary_span]
+    pub span: Span,
+    #[subdiagnostic]
+    pub const_block: WrapConstBlock,
+}
+
+#[derive(Subdiagnostic)]
+#[multipart_suggestion(
+    "wrap the call in a const block to force compile time evaluation",
+    applicability = "maybe-incorrect"
+)]
+pub(crate) struct WrapConstBlock {
+    #[suggestion_part(code = "const {{ ")]
+    pub left: Span,
+    #[suggestion_part(code = " }}")]
+    pub right: Span,
+}

tests/ui/attributes/doc_examples/rustc_comptime.stderr

@@ -1,14 +1,24 @@
-error: comptime fns can only be called at compile time
+error: comptime functions can only be called at compile time
   --> $DIR/rustc_comptime.rs:10:5
    |
 10 |     comptime(); // ERROR
    |     ^^^^^^^^^^
+   |
+help: wrap the call in a const block to force compile time evaluation
+   |
+10 |     const { comptime() }; // ERROR
+   |     +++++++            +
 
-error: comptime fns can only be called at compile time
+error: comptime functions can only be called at compile time
   --> $DIR/rustc_comptime.rs:18:5
    |
 18 |     comptime();
    |     ^^^^^^^^^^
+   |
+help: wrap the call in a const block to force compile time evaluation
+   |
+18 |     const { comptime() };
+   |     +++++++            +
 
 error: aborting due to 2 previous errors
 

tests/ui/comptime/comptime_method.rs

@@ -13,5 +13,5 @@ const _: () = {
 
 fn main() {
     Bar.boo();
-    //~^ ERROR: comptime fns can only be called at compile time
+    //~^ ERROR: comptime functions can only be called at compile time
 }

tests/ui/comptime/comptime_method.stderr

@@ -1,8 +1,13 @@
-error: comptime fns can only be called at compile time
+error: comptime functions can only be called at compile time
   --> $DIR/comptime_method.rs:15:5
    |
 LL |     Bar.boo();
    |     ^^^^^^^^^
+   |
+help: wrap the call in a const block to force compile time evaluation
+   |
+LL |     const { Bar.boo() };
+   |     +++++++           +
 
 error: aborting due to 1 previous error
 

tests/ui/comptime/not_callable.rs

@@ -7,12 +7,12 @@ fn main() {
     // Ok
     const { foo() };
     // Not Ok
-    foo(); //~ ERROR: comptime fns can only be called at compile time
+    foo(); //~ ERROR: comptime functions can only be called at compile time
 }
 
 const fn bar() {
     // Not Ok
-    foo(); //~ ERROR: comptime fns can only be called at compile time
+    foo(); //~ ERROR: comptime functions can only be called at compile time
 }
 
 #[rustc_comptime]

tests/ui/comptime/not_callable.stderr

@@ -1,14 +1,24 @@
-error: comptime fns can only be called at compile time
+error: comptime functions can only be called at compile time
   --> $DIR/not_callable.rs:10:5
    |
 LL |     foo();
    |     ^^^^^
+   |
+help: wrap the call in a const block to force compile time evaluation
+   |
+LL |     const { foo() };
+   |     +++++++       +
 
-error: comptime fns can only be called at compile time
+error: comptime functions can only be called at compile time
   --> $DIR/not_callable.rs:15:5
    |
 LL |     foo();
    |     ^^^^^
+   |
+help: wrap the call in a const block to force compile time evaluation
+   |
+LL |     const { foo() };
+   |     +++++++       +
 
 error: aborting due to 2 previous errors
 

tests/ui/reflection/reflection_methods_in_runtime_code.rs

@@ -5,5 +5,5 @@ trait Trait {}
 fn main() {
     // Test the (lack of) usability of comptime fns in runtime code.
     std::any::TypeId::of::<[u8; usize::MAX]>().trait_info_of::<dyn Trait>();
-    //~^ ERROR: comptime fns can only be called at compile time
+    //~^ ERROR: comptime functions can only be called at compile time
 }

tests/ui/reflection/reflection_methods_in_runtime_code.stderr

@@ -1,8 +1,13 @@
-error: comptime fns can only be called at compile time
+error: comptime functions can only be called at compile time
   --> $DIR/reflection_methods_in_runtime_code.rs:7:5
    |
 LL |     std::any::TypeId::of::<[u8; usize::MAX]>().trait_info_of::<dyn Trait>();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+help: wrap the call in a const block to force compile time evaluation
+   |
+LL |     const { std::any::TypeId::of::<[u8; usize::MAX]>().trait_info_of::<dyn Trait>() };
+   |     +++++++                                                                         +
 
 error: aborting due to 1 previous error