Unconditionally error at definition if default field value has const errors

Emit E0080 always on struct definition with default fields that have unconditional const errors and remove `default_field_always_invalid_const` lint.

Author: estebank

compiler/rustc_hir_analysis/src/check/wfcheck.rs

@@ -1104,6 +1104,25 @@ fn check_type_defn<'tcx>(
         for variant in variants.iter() {
             // All field types must be well-formed.
             for field in &variant.fields {
+                if let Some(def_id) = field.value
+                    && let Some(_ty) = tcx.type_of(def_id).no_bound_vars()
+                {
+                    // FIXME(generic_const_exprs, default_field_values): this is a hack and needs to
+                    // be refactored to check the instantiate-ability of the code better.
+                    if let Some(def_id) = def_id.as_local()
+                        && let hir::Node::AnonConst(anon) = tcx.hir_node_by_def_id(def_id)
+                        && let expr = &tcx.hir().body(anon.body).value
+                        && let hir::ExprKind::Path(hir::QPath::Resolved(None, path)) = expr.kind
+                        && let Res::Def(DefKind::ConstParam, _def_id) = path.res
+                    {
+                        // Do not evaluate bare `const` params, as those would ICE and are only
+                        // usable if `#![feature(generic_const_exprs)]` is enabled.
+                    } else {
+                        // Evaluate the constant proactively, to emit an error if the constant has
+                        // an unconditional error. We only do so if the const has no type params.
+                        let _ = tcx.const_eval_poly(def_id.into());
+                    }
+                }
                 let field_id = field.did.expect_local();
                 let hir::FieldDef { ty: hir_ty, .. } =
                     tcx.hir_node_by_def_id(field_id).expect_field();

compiler/rustc_lint/messages.ftl

@@ -211,10 +211,6 @@ lint_dangling_pointers_from_temporaries = a dangling pointer will be produced be
     .note = pointers do not have a lifetime; when calling `{$callee}` the `{$ty}` will be deallocated at the end of the statement because nothing is referencing it as far as the type system is concerned
     .help = for more information, see <https://doc.rust-lang.org/reference/destructors.html>
 
-lint_default_field_always_invalid_const = default field fails const-evaluation
-    .label = this field's constant fails const-evaluation, as seen in the previous error
-    .help = you can skip const-evaluation of default fields by enabling this lint
-
 lint_default_hash_types = prefer `{$preferred}` over `{$used}`, it has better performance
     .note = a `use rustc_data_structures::fx::{$preferred}` may be necessary
 

compiler/rustc_lint/src/default_field_always_invalid.rs

@@ -41,7 +41,6 @@ mod async_fn_in_trait;
 pub mod builtin;
 mod context;
 mod dangling;
-mod default_field_always_invalid;
 mod deref_into_dyn_supertrait;
 mod drop_forget_useless;
 mod early;
@@ -86,7 +85,6 @@ use async_closures::AsyncClosureUsage;
 use async_fn_in_trait::AsyncFnInTrait;
 use builtin::*;
 use dangling::*;
-use default_field_always_invalid::*;
 use deref_into_dyn_supertrait::*;
 use drop_forget_useless::*;
 use enum_intrinsics_non_enums::EnumIntrinsicsNonEnums;
@@ -195,7 +193,6 @@ late_lint_methods!(
             DropForgetUseless: DropForgetUseless,
             ImproperCTypesDeclarations: ImproperCTypesDeclarations,
             ImproperCTypesDefinitions: ImproperCTypesDefinitions,
-            DefaultFieldAlwaysInvalid: DefaultFieldAlwaysInvalid,
             InvalidFromUtf8: InvalidFromUtf8,
             VariantSizeDifferences: VariantSizeDifferences,
             PathStatements: PathStatements,

compiler/rustc_lint/src/lib.rs

@@ -730,15 +730,6 @@ pub(crate) struct UndroppedManuallyDropsSuggestion {
     pub end_span: Span,
 }
 
-#[derive(LintDiagnostic)]
-#[diag(lint_default_field_always_invalid_const)]
-pub(crate) struct DefaultFieldAlwaysInvalidConst {
-    #[label]
-    pub span: Span,
-    #[help]
-    pub help: (),
-}
-
 // invalid_from_utf8.rs
 #[derive(LintDiagnostic)]
 pub(crate) enum InvalidFromUtf8Diag {

compiler/rustc_lint/src/lints.rs

@@ -21,6 +21,12 @@ error: default fields are not supported in tuple structs
 LL | pub struct Rak(i32 = 42);
    |                      ^^ default fields are only supported on structs
 
+error: generic `Self` types are currently not permitted in anonymous constants
+  --> $DIR/default-field-values-failures.rs:20:14
+   |
+LL |     bar: S = Self::S,
+   |              ^^^^
+
 error[E0277]: the trait bound `S: Default` is not satisfied
   --> $DIR/default-field-values-failures.rs:14:5
    |
@@ -106,12 +112,6 @@ LL -     let _ = Rak(.., 0);
 LL +     let _ = Rak(0);
    |
 
-error: generic `Self` types are currently not permitted in anonymous constants
-  --> $DIR/default-field-values-failures.rs:20:14
-   |
-LL |     bar: S = Self::S,
-   |              ^^^^
-
 error: aborting due to 9 previous errors
 
 Some errors have detailed explanations: E0061, E0277, E0308.

tests/ui/structs/default-field-values-failures.stderr

@@ -1,18 +1,15 @@
 #![feature(default_field_values, generic_const_exprs)]
 #![allow(incomplete_features)]
 
-#[warn(default_field_always_invalid_const)] //~ WARN lint `default_field_always_invalid_const` can't be warned on
 pub struct Bat {
     pub bax: u8 = panic!("asdf"),
     //~^ ERROR evaluation of constant value failed
-    //~| WARN default field fails const-evaluation
 }
 
 pub struct Baz<const C: u8> {
     pub bax: u8 = 130 + C, // ok
     pub bat: u8 = 130 + 130,
     //~^ ERROR evaluation of `Baz::<C>::bat::{constant#0}` failed
-    //~| ERROR default field fails const-evaluation
     pub bay: u8 = 1, // ok
 }
 

tests/ui/structs/default-field-values-invalid-const.rs

@@ -1,45 +1,17 @@
-warning: lint `default_field_always_invalid_const` can't be warned on
-  --> $DIR/default-field-values-invalid-const.rs:4:8
-   |
-LL | #[warn(default_field_always_invalid_const)]
-   |        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ either `deny` or `allow`, no other lint level is supported for this lint
-
 error[E0080]: evaluation of constant value failed
-  --> $DIR/default-field-values-invalid-const.rs:6:19
+  --> $DIR/default-field-values-invalid-const.rs:5:19
    |
 LL |     pub bax: u8 = panic!("asdf"),
-   |                   ^^^^^^^^^^^^^^ the evaluated program panicked at 'asdf', $DIR/default-field-values-invalid-const.rs:6:19
+   |                   ^^^^^^^^^^^^^^ the evaluated program panicked at 'asdf', $DIR/default-field-values-invalid-const.rs:5:19
    |
    = note: this error originates in the macro `$crate::panic::panic_2015` which comes from the expansion of the macro `panic` (in Nightly builds, run with -Z macro-backtrace for more info)
 
-warning: default field fails const-evaluation
-  --> $DIR/default-field-values-invalid-const.rs:6:5
-   |
-LL |     pub bax: u8 = panic!("asdf"),
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ this field's constant fails const-evaluation, as seen in the previous error
-   |
-   = help: you can skip const-evaluation of default fields by enabling this lint
-note: the lint level is defined here
-  --> $DIR/default-field-values-invalid-const.rs:4:8
-   |
-LL | #[warn(default_field_always_invalid_const)]
-   |        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-
 error[E0080]: evaluation of `Baz::<C>::bat::{constant#0}` failed
-  --> $DIR/default-field-values-invalid-const.rs:13:19
+  --> $DIR/default-field-values-invalid-const.rs:11:19
    |
 LL |     pub bat: u8 = 130 + 130,
    |                   ^^^^^^^^^ attempt to compute `130_u8 + 130_u8`, which would overflow
 
-error: default field fails const-evaluation
-  --> $DIR/default-field-values-invalid-const.rs:13:5
-   |
-LL |     pub bat: u8 = 130 + 130,
-   |     ^^^^^^^^^^^^^^^^^^^^^^^ this field's constant fails const-evaluation, as seen in the previous error
-   |
-   = help: you can skip const-evaluation of default fields by enabling this lint
-   = note: `#[deny(default_field_always_invalid_const)]` on by default
-
-error: aborting due to 3 previous errors; 2 warnings emitted
+error: aborting due to 2 previous errors
 
 For more information about this error, try `rustc --explain E0080`.