Introduce #[diagnostic::on_type_error(message)]

compiler/rustc_attr_parsing/src/attributes/diagnostic/mod.rs

@@ -24,6 +24,7 @@ use crate::parser::{ArgParser, MetaItemListParser, MetaItemOrLitParser, MetaItem
 pub(crate) mod do_not_recommend;
 pub(crate) mod on_const;
 pub(crate) mod on_move;
+pub(crate) mod on_type_error;
 pub(crate) mod on_unimplemented;
 pub(crate) mod on_unknown;
 pub(crate) mod on_unmatch_args;
@@ -42,6 +43,8 @@ pub(crate) enum Mode {
     DiagnosticOnUnknown,
     /// `#[diagnostic::on_unmatch_args]`
     DiagnosticOnUnmatchArgs,
+    /// `#[diagnostic::on_type_error]`
+    DiagnosticOnTypeError,
 }
 
 impl Mode {
@@ -53,12 +56,15 @@ impl Mode {
             Self::DiagnosticOnMove => "diagnostic::on_move",
             Self::DiagnosticOnUnknown => "diagnostic::on_unknown",
             Self::DiagnosticOnUnmatchArgs => "diagnostic::on_unmatch_args",
+            Self::DiagnosticOnTypeError => "diagnostic::on_type_error",
         }
     }
 
     fn expected_options(&self) -> &'static str {
         const DEFAULT: &str =
             "at least one of the `message`, `note` and `label` options are expected";
+        const DIAGNOSTIC_ON_TYPE_ERROR_EXPECTED_OPTIONS: &str =
+            "at least a single `note` option is expected";
         match self {
             Self::RustcOnUnimplemented => {
                 "see <https://rustc-dev-guide.rust-lang.org/diagnostics.html#rustc_on_unimplemented>"
@@ -68,11 +74,14 @@ impl Mode {
             Self::DiagnosticOnMove => DEFAULT,
             Self::DiagnosticOnUnknown => DEFAULT,
             Self::DiagnosticOnUnmatchArgs => DEFAULT,
+            Self::DiagnosticOnTypeError => DIAGNOSTIC_ON_TYPE_ERROR_EXPECTED_OPTIONS,
         }
     }
 
     fn allowed_options(&self) -> &'static str {
         const DEFAULT: &str = "only `message`, `note` and `label` are allowed as options";
+        const DIAGNOSTIC_ON_TYPE_ERROR_ALLOWED_OPTIONS: &str =
+            "only `note` is allowed as option for `diagnostic::on_type_error`";
         match self {
             Self::RustcOnUnimplemented => {
                 "see <https://rustc-dev-guide.rust-lang.org/diagnostics.html#rustc_on_unimplemented>"
@@ -82,6 +91,7 @@ impl Mode {
             Self::DiagnosticOnMove => DEFAULT,
             Self::DiagnosticOnUnknown => DEFAULT,
             Self::DiagnosticOnUnmatchArgs => DEFAULT,
+            Self::DiagnosticOnTypeError => DIAGNOSTIC_ON_TYPE_ERROR_ALLOWED_OPTIONS,
         }
     }
 
@@ -105,6 +115,9 @@ impl Mode {
             Self::DiagnosticOnUnmatchArgs => {
                 "only `This` is allowed as a format argument, referring to the macro's name"
             }
+            Self::DiagnosticOnTypeError => {
+                "only `note` is allowed as option for `diagnostic::on_type_error`"
+            }
         }
     }
 }
@@ -294,15 +307,31 @@ fn parse_directive_items<'p>(
             }
         };
         match (mode, name) {
-            (_, sym::message) => {
+            (
+                Mode::RustcOnUnimplemented
+                | Mode::DiagnosticOnUnimplemented
+                | Mode::DiagnosticOnConst
+                | Mode::DiagnosticOnMove
+                | Mode::DiagnosticOnUnknown
+                | Mode::DiagnosticOnUnmatchArgs,
+                sym::message,
+            ) => {
                 let value = or_malformed!(value?);
                 if let Some(message) = &message {
                     duplicate!(name, message.0)
                 } else {
                     message = Some((item.span(), parse_format(value)));
                 }
             }
-            (_, sym::label) => {
+            (
+                Mode::RustcOnUnimplemented
+                | Mode::DiagnosticOnUnimplemented
+                | Mode::DiagnosticOnConst
+                | Mode::DiagnosticOnMove
+                | Mode::DiagnosticOnUnknown
+                | Mode::DiagnosticOnUnmatchArgs,
+                sym::label,
+            ) => {
                 let value = or_malformed!(value?);
                 if let Some(label) = &label {
                     duplicate!(name, label.0)
@@ -356,7 +385,6 @@ fn parse_directive_items<'p>(
                     malformed!();
                 }
             }
-
             _other => {
                 malformed!();
             }
@@ -430,13 +458,19 @@ fn parse_arg(
                 _ => FormatArg::This,
             },
 
+            (Mode::DiagnosticOnTypeError, sym::Found) => FormatArg::Found,
+            (Mode::DiagnosticOnTypeError, sym::Expected) => FormatArg::Expected,
+
             // Some diagnostic attributes can use `{This}` to refer to the annotated item.
             // For those that don't, we continue and maybe use it as a generic parameter.
             //
             // FIXME(mejrs) `DiagnosticOnUnimplemented` is intentionally not here;
             // that requires lang approval which is best kept for a standalone PR.
             (
-                Mode::DiagnosticOnUnknown | Mode::DiagnosticOnMove | Mode::DiagnosticOnUnmatchArgs,
+                Mode::DiagnosticOnUnknown
+                | Mode::DiagnosticOnMove
+                | Mode::DiagnosticOnUnmatchArgs
+                | Mode::DiagnosticOnTypeError,
                 sym::This,
             ) => FormatArg::This,
 
@@ -462,7 +496,8 @@ fn parse_arg(
                 Mode::RustcOnUnimplemented
                 | Mode::DiagnosticOnUnimplemented
                 | Mode::DiagnosticOnMove
-                | Mode::DiagnosticOnConst,
+                | Mode::DiagnosticOnConst
+                | Mode::DiagnosticOnTypeError,
                 generic_param,
             ) => FormatArg::GenericParam { generic_param, span },
 

compiler/rustc_attr_parsing/src/attributes/diagnostic/on_type_error.rs

@@ -0,0 +1,64 @@
+use rustc_feature::template;
+use rustc_hir::attrs::AttributeKind;
+use rustc_lint_defs::builtin::MISPLACED_DIAGNOSTIC_ATTRIBUTES;
+use rustc_span::sym;
+
+use crate::attributes::AttributeStability;
+use crate::attributes::diagnostic::*;
+use crate::attributes::prelude::*;
+use crate::context::AcceptContext;
+use crate::diagnostics::DiagnosticOnTypeErrorOnlyForAdt;
+use crate::parser::ArgParser;
+use crate::target_checking::{ALL_TARGETS, AllowedTargets};
+
+#[derive(Default)]
+pub(crate) struct OnTypeErrorParser {
+    span: Option<Span>,
+    directive: Option<(Span, Directive)>,
+}
+
+impl OnTypeErrorParser {
+    fn parse<'sess>(&mut self, cx: &mut AcceptContext<'_, 'sess>, args: &ArgParser, mode: Mode) {
+        if !cx.features().diagnostic_on_type_error() {
+            return;
+        }
+
+        let span = cx.attr_span;
+        self.span = Some(span);
+
+        if !matches!(cx.target, Target::Enum | Target::Struct | Target::Union) {
+            cx.emit_lint(MISPLACED_DIAGNOSTIC_ATTRIBUTES, DiagnosticOnTypeErrorOnlyForAdt, span);
+            return;
+        }
+
+        let Some(items) = parse_list(cx, args, mode) else { return };
+
+        if let Some(directive) = parse_directive_items(cx, mode, items.mixed(), true) {
+            merge_directives(cx, &mut self.directive, (span, directive));
+        }
+    }
+}
+
+impl AttributeParser for OnTypeErrorParser {
+    const ATTRIBUTES: AcceptMapping<Self> = &[(
+        &[sym::diagnostic, sym::on_type_error],
+        template!(List: &[r#"note = "...""#]),
+        AttributeStability::Stable,
+        |this, cx, args| {
+            this.parse(cx, args, Mode::DiagnosticOnTypeError);
+        },
+    )];
+
+    const ALLOWED_TARGETS: AllowedTargets = AllowedTargets::AllowList(ALL_TARGETS);
+
+    fn finalize(self, _cx: &FinalizeContext<'_, '_>) -> Option<AttributeKind> {
+        if let Some(span) = self.span {
+            Some(AttributeKind::OnTypeError {
+                span,
+                directive: self.directive.map(|d| Box::new(d.1)),
+            })
+        } else {
+            None
+        }
+    }
+}

compiler/rustc_attr_parsing/src/context.rs

@@ -32,6 +32,7 @@ use crate::attributes::deprecation::*;
 use crate::attributes::diagnostic::do_not_recommend::*;
 use crate::attributes::diagnostic::on_const::*;
 use crate::attributes::diagnostic::on_move::*;
+use crate::attributes::diagnostic::on_type_error::*;
 use crate::attributes::diagnostic::on_unimplemented::*;
 use crate::attributes::diagnostic::on_unknown::*;
 use crate::attributes::diagnostic::on_unmatch_args::*;
@@ -144,6 +145,7 @@ attribute_parsers!(
         NakedParser,
         OnConstParser,
         OnMoveParser,
+        OnTypeErrorParser,
         OnUnimplementedParser,
         OnUnknownParser,
         OnUnmatchArgsParser,

compiler/rustc_attr_parsing/src/diagnostics.rs

@@ -305,6 +305,10 @@ pub(crate) struct DiagnosticOnUnknownOnlyForImports {
 #[diag("`#[diagnostic::on_unmatch_args]` can only be applied to macro definitions")]
 pub(crate) struct DiagnosticOnUnmatchArgsOnlyForMacros;
 
+#[derive(Diagnostic)]
+#[diag("`#[diagnostic::on_type_error]` can only be applied to enums, structs or unions")]
+pub(crate) struct DiagnosticOnTypeErrorOnlyForAdt;
+
 #[derive(Diagnostic)]
 #[diag("`#[diagnostic::do_not_recommend]` can only be placed on trait implementations")]
 pub(crate) struct IncorrectDoNotRecommendLocation {

compiler/rustc_feature/src/unstable.rs

@@ -510,6 +510,8 @@ declare_features! (
     (unstable, diagnostic_on_const, "1.93.0", Some(143874)),
     /// Allows giving on-move borrowck custom diagnostic messages for a type
     (unstable, diagnostic_on_move, "1.96.0", Some(154181)),
+    /// Allows giving custom types diagnostic messages on type errors
+    (unstable, diagnostic_on_type_error, "CURRENT_RUSTC_VERSION", Some(155382)),
     /// Allows giving unresolved imports a custom diagnostic message
     (unstable, diagnostic_on_unknown, "1.96.0", Some(152900)),
     /// Allows macros to customize macro argument matcher diagnostics.

compiler/rustc_hir/src/attrs/data_structures.rs

@@ -1173,6 +1173,12 @@ pub enum AttributeKind {
         directive: Option<Box<Directive>>,
     },
 
+    /// Represents`#[diagnostic::on_type_error]`.
+    OnTypeError {
+        span: Span,
+        directive: Option<Box<Directive>>,
+    },
+
     /// Represents `#[rustc_on_unimplemented]` and `#[diagnostic::on_unimplemented]`.
     OnUnimplemented {
         /// None if the directive was malformed in some way.

compiler/rustc_hir/src/attrs/diagnostic.rs

@@ -149,6 +149,10 @@ impl FormatString {
                 }
                 Piece::Arg(FormatArg::This) => ret.push_str(&args.this),
 
+                // only for on_type_error
+                Piece::Arg(FormatArg::Found) => ret.push_str(&args.found),
+                Piece::Arg(FormatArg::Expected) => ret.push_str(&args.expected),
+
                 // It's only `rustc_onunimplemented` from here
                 Piece::Arg(FormatArg::ThisPath) => ret.push_str(&args.this_path),
                 Piece::Arg(FormatArg::ThisResolved) => {
@@ -209,6 +213,8 @@ pub struct FormatArgs {
     pub this: String,
     pub this_resolved: String = String::new(),
     pub this_path: String = String::new(),
+    pub found: String = String::new(),
+    pub expected: String = String::new(),
     pub item_context: &'static str = "",
     pub generic_args: Vec<(Symbol, String)> = Vec::new(),
 }
@@ -238,6 +244,10 @@ pub enum FormatArg {
     ItemContext,
     /// What the user typed, if it doesn't match anything we can use.
     AsIs(Symbol),
+    /// {Found} in diagnostic::on_type_error
+    Found,
+    /// {Expected} in diagnostic::on_type_error
+    Expected,
 }
 
 /// Represents the `on` filter in `#[rustc_on_unimplemented]`.

compiler/rustc_hir/src/attrs/encode_cross_crate.rs

@@ -78,6 +78,7 @@ impl AttributeKind {
             NonExhaustive(..) => Yes, // Needed for rustdoc
             OnConst { .. } => Yes,
             OnMove { .. } => Yes,
+            OnTypeError { .. } => Yes,
             OnUnimplemented { .. } => Yes,
             OnUnknown { .. } => Yes,
             OnUnmatchArgs { .. } => Yes,

compiler/rustc_lint_defs/src/builtin.rs

@@ -4523,6 +4523,7 @@ declare_lint! {
     Warn,
     "detects diagnostic attribute with malformed diagnostic format literals",
 }
+
 declare_lint! {
     /// The `ambiguous_glob_imports` lint detects glob imports that should report ambiguity
     /// errors, but previously didn't do that due to rustc bugs.

compiler/rustc_passes/src/check_attr.rs

@@ -39,8 +39,8 @@ use rustc_session::config::CrateType;
 use rustc_session::errors::feature_err;
 use rustc_session::lint;
 use rustc_session::lint::builtin::{
-    CONFLICTING_REPR_HINTS, INVALID_DOC_ATTRIBUTES, MALFORMED_DIAGNOSTIC_FORMAT_LITERALS,
-    MISPLACED_DIAGNOSTIC_ATTRIBUTES, UNUSED_ATTRIBUTES,
+    CONFLICTING_REPR_HINTS, INVALID_DOC_ATTRIBUTES, MALFORMED_DIAGNOSTIC_ATTRIBUTES,
+    MALFORMED_DIAGNOSTIC_FORMAT_LITERALS, MISPLACED_DIAGNOSTIC_ATTRIBUTES, UNUSED_ATTRIBUTES,
 };
 use rustc_span::edition::Edition;
 use rustc_span::{DUMMY_SP, Ident, Span, Symbol, sym};
@@ -249,6 +249,9 @@ impl<'tcx> CheckAttrVisitor<'tcx> {
             AttributeKind::OnMove { directive } => {
                 self.check_diagnostic_on_move(hir_id, directive.as_deref())
             }
+            AttributeKind::OnTypeError { directive, .. } => {
+                self.check_diagnostic_on_type_error(hir_id, directive.as_deref())
+            }
 
             // All of the following attributes have no specific checks.
             // tidy-alphabetical-start
@@ -599,6 +602,58 @@ impl<'tcx> CheckAttrVisitor<'tcx> {
         }
     }
 
+    fn check_diagnostic_on_type_error(&self, hir_id: HirId, directive: Option<&Directive>) {
+        if let Some(directive) = directive {
+            if let Node::Item(Item {
+                kind:
+                    ItemKind::Struct(_, generics, _)
+                    | ItemKind::Enum(_, generics, _)
+                    | ItemKind::Union(_, generics, _),
+                ..
+            }) = self.tcx.hir_node(hir_id)
+            {
+                let generic_count = generics
+                    .params
+                    .iter()
+                    .filter(|p| !matches!(p.kind, GenericParamKind::Lifetime { .. }))
+                    .count();
+
+                // Enforce: at most one generic
+                if generic_count != 1 {
+                    self.tcx.emit_node_span_lint(
+                        MALFORMED_DIAGNOSTIC_ATTRIBUTES,
+                        hir_id,
+                        generics.span,
+                        errors::OnTypeErrorNotExactlyOneGeneric { count: generic_count },
+                    );
+                }
+
+                directive.visit_params(&mut |argument_name, span| {
+                    let has_generic = generics.params.iter().any(|p| {
+                        if !matches!(p.kind, GenericParamKind::Lifetime { .. })
+                            && let ParamName::Plain(name) = p.name
+                            && name.name == argument_name
+                        {
+                            true
+                        } else {
+                            false
+                        }
+                    });
+
+                    let is_allowed = argument_name == sym::Expected || argument_name == sym::Found;
+                    if !(has_generic | is_allowed) {
+                        self.tcx.emit_node_span_lint(
+                            MALFORMED_DIAGNOSTIC_FORMAT_LITERALS,
+                            hir_id,
+                            span,
+                            errors::OnTypeErrorMalformedFormatLiterals { name: argument_name },
+                        )
+                    }
+                });
+            }
+        }
+    }
+
     /// Checks if an `#[inline]` is applied to a function or a closure.
     fn check_inline(&self, hir_id: HirId, attr_span: Span, kind: &InlineAttr, target: Target) {
         match target {

compiler/rustc_passes/src/errors.rs

@@ -1260,3 +1260,18 @@ pub(crate) struct UnknownFormatParameterForOnUnimplementedAttr {
 pub(crate) struct OnMoveMalformedFormatLiterals {
     pub name: Symbol,
 }
+
+#[derive(Diagnostic)]
+#[diag("unknown parameter `{$name}`")]
+#[help(r#"expect either a generic argument name, {"`{Self}`"}, {"`{Expected}`"} or {"`{Found}`"} as format argument"#)]
+pub(crate) struct OnTypeErrorMalformedFormatLiterals {
+    pub name: Symbol,
+}
+
+#[derive(Diagnostic)]
+#[diag(
+    "`#[diagnostic::on_type_error]` only supports exactly one ADT generic parameter, but found `{$count}`"
+)]
+pub(crate) struct OnTypeErrorNotExactlyOneGeneric {
+    pub count: usize,
+}