Print a trace through types to show how to get to the problematic type

compiler/rustc_lint/src/builtin.rs

@@ -57,8 +57,6 @@
 
 use crate::nonstandard_style::{method_context, MethodLateContext};
 
-use std::fmt::Write;
-
 // hardwired lints from librustc_middle
 pub use rustc_session::lint::builtin::*;
 
@@ -2408,8 +2406,34 @@ enum InitKind {
         }
 
         /// Information about why a type cannot be initialized this way.
-        /// Contains an error message and optionally a span to point at.
+        struct InitError {
-        type InitError = (String, Option<Span>);
+            message: String,
+            /// Spans from struct fields and similar can be obtained from just the type.
+            span: Option<Span>,
+            /// Used to report a trace through adts.
+            nested: Option<Box<InitError>>,
+        }
+        impl InitError {
+            fn spanned(self, span: Span) -> InitError {
+                Self { span: Some(span), ..self }
+            }
+
+            fn nested(self, nested: InitError) -> InitError {
+                assert!(self.nested.is_none());
+                Self { nested: Some(Box::new(nested)), ..self }
+            }
+        }
+
+        impl<'a> From<&'a str> for InitError {
+            fn from(s: &'a str) -> Self {
+                s.to_owned().into()
+            }
+        }
+        impl From<String> for InitError {
+            fn from(message: String) -> Self {
+                Self { message, span: None, nested: None }
+            }
+        }
 
         /// Test if this constant is all-0.
         fn is_zero(expr: &hir::Expr<'_>) -> bool {
@@ -2471,17 +2495,10 @@ fn variant_find_init_error<'tcx>(
             init: InitKind,
         ) -> Option<InitError> {
             variant.fields.iter().find_map(|field| {
-                ty_find_init_error(cx, field.ty(cx.tcx, substs), init).map(|(mut msg, span)| {
+                ty_find_init_error(cx, field.ty(cx.tcx, substs), init).map(|err| {
-                    if span.is_none() {
+                    InitError::from(format!("in this {descr}"))
-                        // Point to this field, should be helpful for figuring
+                        .spanned(cx.tcx.def_span(field.did))
-                        // out where the source of the error is.
+                        .nested(err)
-                        let span = cx.tcx.def_span(field.did);
-                        write!(&mut msg, " (in this {descr})").unwrap();
-                        (msg, Some(span))
-                    } else {
-                        // Just forward.
-                        (msg, span)
-                    }
                 })
             })
         }
@@ -2496,30 +2513,30 @@ fn ty_find_init_error<'tcx>(
             use rustc_type_ir::sty::TyKind::*;
             match ty.kind() {
                 // Primitive types that don't like 0 as a value.
-                Ref(..) => Some(("references must be non-null".to_string(), None)),
+                Ref(..) => Some("references must be non-null".into()),
-                Adt(..) if ty.is_box() => Some(("`Box` must be non-null".to_string(), None)),
+                Adt(..) if ty.is_box() => Some("`Box` must be non-null".into()),
-                FnPtr(..) => Some(("function pointers must be non-null".to_string(), None)),
+                FnPtr(..) => Some("function pointers must be non-null".into()),
-                Never => Some(("the `!` type has no valid value".to_string(), None)),
+                Never => Some("the `!` type has no valid value".into()),
                 RawPtr(tm) if matches!(tm.ty.kind(), Dynamic(..)) =>
                 // raw ptr to dyn Trait
                 {
-                    Some(("the vtable of a wide raw pointer must be non-null".to_string(), None))
+                    Some("the vtable of a wide raw pointer must be non-null".into())
                 }
                 // Primitive types with other constraints.
                 Bool if init == InitKind::Uninit => {
-                    Some(("booleans must be either `true` or `false`".to_string(), None))
+                    Some("booleans must be either `true` or `false`".into())
                 }
                 Char if init == InitKind::Uninit => {
-                    Some(("characters must be a valid Unicode codepoint".to_string(), None))
+                    Some("characters must be a valid Unicode codepoint".into())
                 }
                 Int(_) | Uint(_) if init == InitKind::Uninit => {
-                    Some(("integers must not be uninitialized".to_string(), None))
+                    Some("integers must not be uninitialized".into())
                 }
                 Float(_) if init == InitKind::Uninit => {
-                    Some(("floats must not be uninitialized".to_string(), None))
+                    Some("floats must not be uninitialized".into())
                 }
                 RawPtr(_) if init == InitKind::Uninit => {
-                    Some(("raw pointers must not be uninitialized".to_string(), None))
+                    Some("raw pointers must not be uninitialized".into())
                 }
                 // Recurse and checks for some compound types. (but not unions)
                 Adt(adt_def, substs) if !adt_def.is_union() => {
@@ -2531,21 +2548,21 @@ fn ty_find_init_error<'tcx>(
                         // handle the attribute correctly.)
                         // We don't add a span since users cannot declare such types anyway.
                         (Bound::Included(lo), Bound::Included(hi)) if 0 < lo && lo < hi => {
-                            return Some((format!("`{}` must be non-null", ty), None));
+                            return Some(format!("`{}` must be non-null", ty).into());
                         }
                         (Bound::Included(lo), Bound::Unbounded) if 0 < lo => {
-                            return Some((format!("`{}` must be non-null", ty), None));
+                            return Some(format!("`{}` must be non-null", ty).into());
                         }
                         (Bound::Included(_), _) | (_, Bound::Included(_))
                             if init == InitKind::Uninit =>
                         {
-                            return Some((
+                            return Some(
                                 format!(
                                     "`{}` must be initialized inside its custom valid range",
                                     ty,
-                                ),
+                                )
-                                None,
+                                .into(),
-                            ));
+                            );
                         }
                         _ => {}
                     }
@@ -2576,7 +2593,7 @@ fn ty_find_init_error<'tcx>(
                         Some((variant, definitely_inhabited))
                     });
                     let Some(first_variant) = potential_variants.next() else {
-                        return Some(("enums with no inhabited variants have no valid value".to_string(), Some(span)));
+                        return Some(InitError::from("enums with no inhabited variants have no valid value").spanned(span));
                     };
                     // So we have at least one potentially inhabited variant. Might we have two?
                     let Some(second_variant) = potential_variants.next() else {
@@ -2600,10 +2617,9 @@ fn ty_find_init_error<'tcx>(
                                 .filter(|(_variant, definitely_inhabited)| *definitely_inhabited)
                                 .count();
                         if definitely_inhabited > 1 {
-                            return Some((
+                            return Some(InitError::from(
-                                "enums with multiple inhabited variants have to be initialized to a variant".to_string(),
+                                "enums with multiple inhabited variants have to be initialized to a variant",
-                                Some(span),
+                            ).spanned(span));
-                            ));
                         }
                     }
                     // We couldn't find anything wrong here.
@@ -2632,8 +2648,7 @@ fn ty_find_init_error<'tcx>(
             // using zeroed or uninitialized memory.
             // We are extremely conservative with what we warn about.
             let conjured_ty = cx.typeck_results().expr_ty(expr);
-            if let Some((msg, span)) =
+            if let Some(mut err) = with_no_trimmed_paths!(ty_find_init_error(cx, conjured_ty, init))
-                with_no_trimmed_paths!(ty_find_init_error(cx, conjured_ty, init))
             {
                 // FIXME(davidtwco): make translatable
                 cx.struct_span_lint(
@@ -2659,10 +2674,17 @@ fn ty_find_init_error<'tcx>(
                             "help: use `MaybeUninit<T>` instead, \
                             and only call `assume_init` after initialization is done",
                         );
-                        if let Some(span) = span {
+                        loop {
-                            lint.span_note(span, &msg);
+                            if let Some(span) = err.span {
+                                lint.span_note(span, &err.message);
                             } else {
-                            lint.note(&msg);
+                                lint.note(&err.message);
+                            }
+                            if let Some(e) = err.nested {
+                                err = *e;
+                            } else {
+                                break;
+                            }
                         }
                         lint
                     },

src/test/ui/consts/const-eval/validate_uninhabited_zsts.32bit.stderr

@@ -40,6 +40,11 @@ LL | const BAR: [empty::Empty; 3] = [unsafe { std::mem::transmute(()) }; 3];
    |                                          this code causes undefined behavior when executed
    |                                          help: use `MaybeUninit<T>` instead, and only call `assume_init` after initialization is done
    |
+note: in this struct field
+  --> $DIR/validate_uninhabited_zsts.rs:16:22
+   |
+LL |     pub struct Empty(Void);
+   |                      ^^^^
 note: enums with no inhabited variants have no valid value
   --> $DIR/validate_uninhabited_zsts.rs:13:5
    |

src/test/ui/consts/const-eval/validate_uninhabited_zsts.64bit.stderr

@@ -40,6 +40,11 @@ LL | const BAR: [empty::Empty; 3] = [unsafe { std::mem::transmute(()) }; 3];
    |                                          this code causes undefined behavior when executed
    |                                          help: use `MaybeUninit<T>` instead, and only call `assume_init` after initialization is done
    |
+note: in this struct field
+  --> $DIR/validate_uninhabited_zsts.rs:16:22
+   |
+LL |     pub struct Empty(Void);
+   |                      ^^^^
 note: enums with no inhabited variants have no valid value
   --> $DIR/validate_uninhabited_zsts.rs:13:5
    |

src/test/ui/lint/invalid_value.stderr

@@ -34,11 +34,12 @@ LL |         let _val: Wrap<&'static T> = mem::zeroed();
    |                                      this code causes undefined behavior when executed
    |                                      help: use `MaybeUninit<T>` instead, and only call `assume_init` after initialization is done
    |
-note: references must be non-null (in this struct field)
+note: in this struct field
   --> $DIR/invalid_value.rs:17:18
    |
 LL | struct Wrap<T> { wrapped: T }
    |                  ^^^^^^^^^^
+   = note: references must be non-null
 
 error: the type `Wrap<&T>` does not permit being left uninitialized
   --> $DIR/invalid_value.rs:58:38
@@ -49,11 +50,12 @@ LL |         let _val: Wrap<&'static T> = mem::uninitialized();
    |                                      this code causes undefined behavior when executed
    |                                      help: use `MaybeUninit<T>` instead, and only call `assume_init` after initialization is done
    |
-note: references must be non-null (in this struct field)
+note: in this struct field
   --> $DIR/invalid_value.rs:17:18
    |
 LL | struct Wrap<T> { wrapped: T }
    |                  ^^^^^^^^^^
+   = note: references must be non-null
 
 error: the type `!` does not permit zero-initialization
   --> $DIR/invalid_value.rs:65:23
@@ -160,11 +162,12 @@ LL |         let _val: Ref = mem::zeroed();
    |                         this code causes undefined behavior when executed
    |                         help: use `MaybeUninit<T>` instead, and only call `assume_init` after initialization is done
    |
-note: references must be non-null (in this struct field)
+note: in this struct field
   --> $DIR/invalid_value.rs:14:12
    |
 LL | struct Ref(&'static i32);
    |            ^^^^^^^^^^^^
+   = note: references must be non-null
 
 error: the type `Ref` does not permit being left uninitialized
   --> $DIR/invalid_value.rs:78:25
@@ -175,11 +178,12 @@ LL |         let _val: Ref = mem::uninitialized();
    |                         this code causes undefined behavior when executed
    |                         help: use `MaybeUninit<T>` instead, and only call `assume_init` after initialization is done
    |
-note: references must be non-null (in this struct field)
+note: in this struct field
   --> $DIR/invalid_value.rs:14:12
    |
 LL | struct Ref(&'static i32);
    |            ^^^^^^^^^^^^
+   = note: references must be non-null
 
 error: the type `fn()` does not permit zero-initialization
   --> $DIR/invalid_value.rs:80:26
@@ -212,11 +216,12 @@ LL |         let _val: Wrap<fn()> = mem::zeroed();
    |                                this code causes undefined behavior when executed
    |                                help: use `MaybeUninit<T>` instead, and only call `assume_init` after initialization is done
    |
-note: function pointers must be non-null (in this struct field)
+note: in this struct field
   --> $DIR/invalid_value.rs:17:18
    |
 LL | struct Wrap<T> { wrapped: T }
    |                  ^^^^^^^^^^
+   = note: function pointers must be non-null
 
 error: the type `Wrap<fn()>` does not permit being left uninitialized
   --> $DIR/invalid_value.rs:84:32
@@ -227,11 +232,12 @@ LL |         let _val: Wrap<fn()> = mem::uninitialized();
    |                                this code causes undefined behavior when executed
    |                                help: use `MaybeUninit<T>` instead, and only call `assume_init` after initialization is done
    |
-note: function pointers must be non-null (in this struct field)
+note: in this struct field
   --> $DIR/invalid_value.rs:17:18
    |
 LL | struct Wrap<T> { wrapped: T }
    |                  ^^^^^^^^^^
+   = note: function pointers must be non-null
 
 error: the type `WrapEnum<fn()>` does not permit zero-initialization
   --> $DIR/invalid_value.rs:86:36
@@ -242,11 +248,12 @@ LL |         let _val: WrapEnum<fn()> = mem::zeroed();
    |                                    this code causes undefined behavior when executed
    |                                    help: use `MaybeUninit<T>` instead, and only call `assume_init` after initialization is done
    |
-note: function pointers must be non-null (in this field of the only potentially inhabited enum variant)
+note: in this field of the only potentially inhabited enum variant
   --> $DIR/invalid_value.rs:18:28
    |
 LL | enum WrapEnum<T> { Wrapped(T) }
    |                            ^
+   = note: function pointers must be non-null
 
 error: the type `WrapEnum<fn()>` does not permit being left uninitialized
   --> $DIR/invalid_value.rs:87:36
@@ -257,11 +264,12 @@ LL |         let _val: WrapEnum<fn()> = mem::uninitialized();
    |                                    this code causes undefined behavior when executed
    |                                    help: use `MaybeUninit<T>` instead, and only call `assume_init` after initialization is done
    |
-note: function pointers must be non-null (in this field of the only potentially inhabited enum variant)
+note: in this field of the only potentially inhabited enum variant
   --> $DIR/invalid_value.rs:18:28
    |
 LL | enum WrapEnum<T> { Wrapped(T) }
    |                            ^
+   = note: function pointers must be non-null
 
 error: the type `Wrap<(RefPair, i32)>` does not permit zero-initialization
   --> $DIR/invalid_value.rs:89:42
@@ -272,11 +280,17 @@ LL |         let _val: Wrap<(RefPair, i32)> = mem::zeroed();
    |                                          this code causes undefined behavior when executed
    |                                          help: use `MaybeUninit<T>` instead, and only call `assume_init` after initialization is done
    |
-note: references must be non-null (in this struct field)
+note: in this struct field
+  --> $DIR/invalid_value.rs:17:18
+   |
+LL | struct Wrap<T> { wrapped: T }
+   |                  ^^^^^^^^^^
+note: in this struct field
   --> $DIR/invalid_value.rs:15:16
    |
 LL | struct RefPair((&'static i32, i32));
    |                ^^^^^^^^^^^^^^^^^^^
+   = note: references must be non-null
 
 error: the type `Wrap<(RefPair, i32)>` does not permit being left uninitialized
   --> $DIR/invalid_value.rs:90:42
@@ -287,11 +301,17 @@ LL |         let _val: Wrap<(RefPair, i32)> = mem::uninitialized();
    |                                          this code causes undefined behavior when executed
    |                                          help: use `MaybeUninit<T>` instead, and only call `assume_init` after initialization is done
    |
-note: references must be non-null (in this struct field)
+note: in this struct field
+  --> $DIR/invalid_value.rs:17:18
+   |
+LL | struct Wrap<T> { wrapped: T }
+   |                  ^^^^^^^^^^
+note: in this struct field
   --> $DIR/invalid_value.rs:15:16
    |
 LL | struct RefPair((&'static i32, i32));
    |                ^^^^^^^^^^^^^^^^^^^
+   = note: references must be non-null
 
 error: the type `NonNull<i32>` does not permit zero-initialization
   --> $DIR/invalid_value.rs:92:34
@@ -420,11 +440,12 @@ LL |         let _val: OneFruitNonZero = mem::zeroed();
    |                                     this code causes undefined behavior when executed
    |                                     help: use `MaybeUninit<T>` instead, and only call `assume_init` after initialization is done
    |
-note: `std::num::NonZeroU32` must be non-null (in this field of the only potentially inhabited enum variant)
+note: in this field of the only potentially inhabited enum variant
   --> $DIR/invalid_value.rs:39:12
    |
 LL |     Banana(NonZeroU32),
    |            ^^^^^^^^^^
+   = note: `std::num::NonZeroU32` must be non-null
 
 error: the type `OneFruitNonZero` does not permit being left uninitialized
   --> $DIR/invalid_value.rs:108:37
@@ -435,11 +456,12 @@ LL |         let _val: OneFruitNonZero = mem::uninitialized();
    |                                     this code causes undefined behavior when executed
    |                                     help: use `MaybeUninit<T>` instead, and only call `assume_init` after initialization is done
    |
-note: `std::num::NonZeroU32` must be non-null (in this field of the only potentially inhabited enum variant)
+note: in this field of the only potentially inhabited enum variant
   --> $DIR/invalid_value.rs:39:12
    |
 LL |     Banana(NonZeroU32),
    |            ^^^^^^^^^^
+   = note: `std::num::NonZeroU32` must be non-null
 
 error: the type `bool` does not permit being left uninitialized
   --> $DIR/invalid_value.rs:112:26
@@ -461,11 +483,12 @@ LL |         let _val: Wrap<char> = mem::uninitialized();
    |                                this code causes undefined behavior when executed
    |                                help: use `MaybeUninit<T>` instead, and only call `assume_init` after initialization is done
    |
-note: characters must be a valid Unicode codepoint (in this struct field)
+note: in this struct field
   --> $DIR/invalid_value.rs:17:18
    |
 LL | struct Wrap<T> { wrapped: T }
    |                  ^^^^^^^^^^
+   = note: characters must be a valid Unicode codepoint
 
 error: the type `NonBig` does not permit being left uninitialized
   --> $DIR/invalid_value.rs:118:28