Auto merge of rust-lang#96468 - davidtwco:diagnostic-translation-subdiagnostic, r=oli-obk

macros: subdiagnostic derive

Add a new macro, `#[derive(SessionSubdiagnostic)]`, which can be applied to structs that represent subdiagnostics, such as labels, notes, helps or suggestions.

`#[derive(SessionSubdiagnostic)]` can be used with the existing `#[derive(SessionDiagnostic)]`. All diagnostics implemented using either derive are translatable, and this new derive should make it easier to port existing diagnostics to using these derives.

For example, consider the following subdiagnostic types...

```rust
#[derive(SessionSubdiagnostic)]
pub enum ExpectedIdentifierLabel<'tcx> {
    #[label(slug = "parser-expected-identifier")]
    WithoutFound {
        #[primary_span]
        span: Span,
    }
    #[label(slug = "parser-expected-identifier-found")]
    WithFound {
        #[primary_span]
        span: Span,
        found: String,
    }
}

#[derive(SessionSubdiagnostic)]
#[suggestion_verbose(slug = "parser-raw-identifier")]
pub struct RawIdentifierSuggestion<'tcx> {
    #[primary_span]
    span: Span,
    #[applicability]
    applicability: Applicability,
    ident: Ident,
}
```

...and the corresponding Fluent messages:

```fluent
parser-expected-identifier = expected identifier

parser-expected-identifier-found = expected identifier, found {$found}

parser-raw-identifier = escape `{$ident}` to use it as an identifier
```

These can be emitted using the new `subdiagnostic` function on `Diagnostic`...

```rust
diag.subdiagnostic(ExpectedIdentifierLabel::WithoutFound { span });
diag.subdiagnostic(RawIdentifierSuggestion { span, applicability, ident });
```

...or as part of a larger `#[derive(SessionDiagnostic)]`:

```rust
#[derive(SessionDiagnostic)]
#[error(slug = "parser-expected-identifier")]
pub struct ExpectedIdentifier {
    #[primary_span]
    span: Span,
    token_descr: String,
    #[subdiagnostic]
    label: ExpectedIdentifierLabel,
    #[subdiagnostic]
    raw_identifier_suggestion: Option<RawIdentifierSuggestion>,
}
```

```rust
sess.emit_err(ExpectedIdentifier { ... });
```

r? `@oli-obk`
cc `@pvdrz`

Author: bors

compiler/rustc_borrowck/src/diagnostics/mutability_errors.rs

@@ -237,7 +237,7 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
                     err.span_suggestion_verbose(
                         span,
                         "consider changing this to be mutable",
-                        " mut ".into(),
+                        " mut ",
                         Applicability::MaybeIncorrect,
                     );
                 }

compiler/rustc_builtin_macros/src/deriving/default.rs

@@ -235,7 +235,7 @@ fn validate_default_attribute(
             .span_suggestion_hidden(
                 attr.span,
                 "try using `#[default]`",
-                "#[default]".into(),
+                "#[default]",
                 Applicability::MaybeIncorrect,
             )
             .emit();

compiler/rustc_error_messages/locales/en-US/typeck.ftl

@@ -82,3 +82,11 @@ typeck-value-of-associated-struct-already-specified =
 
 typeck-address-of-temporary-taken = cannot take address of a temporary
     .label = temporary value
+
+typeck-add-return-type-add = try adding a return type
+
+typeck-add-return-type-missing-here = a return type might be missing here
+
+typeck-expected-default-return-type = expected `()` because of default return type
+
+typeck-expected-return-type = expected `{$expected}` because of return type

compiler/rustc_errors/src/diagnostic.rs

@@ -78,6 +78,13 @@ impl<'source> Into<FluentValue<'source>> for DiagnosticArgValue<'source> {
     }
 }
 
+/// Trait implemented by error types. This should not be implemented manually. Instead, use
+/// `#[derive(SessionSubdiagnostic)]` -- see [rustc_macros::SessionSubdiagnostic].
+pub trait AddSubdiagnostic {
+    /// Add a subdiagnostic to an existing diagnostic.
+    fn add_to_diagnostic(self, diag: &mut Diagnostic);
+}
+
 #[must_use]
 #[derive(Clone, Debug, Encodable, Decodable)]
 pub struct Diagnostic {
@@ -605,7 +612,7 @@ impl Diagnostic {
         &mut self,
         sp: Span,
         msg: impl Into<DiagnosticMessage>,
-        suggestion: String,
+        suggestion: impl ToString,
         applicability: Applicability,
     ) -> &mut Self {
         self.span_suggestion_with_style(
@@ -623,13 +630,13 @@ impl Diagnostic {
         &mut self,
         sp: Span,
         msg: impl Into<DiagnosticMessage>,
-        suggestion: String,
+        suggestion: impl ToString,
         applicability: Applicability,
         style: SuggestionStyle,
     ) -> &mut Self {
         self.push_suggestion(CodeSuggestion {
             substitutions: vec![Substitution {
-                parts: vec![SubstitutionPart { snippet: suggestion, span: sp }],
+                parts: vec![SubstitutionPart { snippet: suggestion.to_string(), span: sp }],
             }],
             msg: msg.into(),
             style,
@@ -643,7 +650,7 @@ impl Diagnostic {
         &mut self,
         sp: Span,
         msg: impl Into<DiagnosticMessage>,
-        suggestion: String,
+        suggestion: impl ToString,
         applicability: Applicability,
     ) -> &mut Self {
         self.span_suggestion_with_style(
@@ -711,7 +718,7 @@ impl Diagnostic {
         &mut self,
         sp: Span,
         msg: impl Into<DiagnosticMessage>,
-        suggestion: String,
+        suggestion: impl ToString,
         applicability: Applicability,
     ) -> &mut Self {
         self.span_suggestion_with_style(
@@ -734,7 +741,7 @@ impl Diagnostic {
         &mut self,
         sp: Span,
         msg: impl Into<DiagnosticMessage>,
-        suggestion: String,
+        suggestion: impl ToString,
         applicability: Applicability,
     ) -> &mut Self {
         self.span_suggestion_with_style(
@@ -755,7 +762,7 @@ impl Diagnostic {
         &mut self,
         sp: Span,
         msg: impl Into<DiagnosticMessage>,
-        suggestion: String,
+        suggestion: impl ToString,
         applicability: Applicability,
     ) -> &mut Self {
         self.span_suggestion_with_style(
@@ -768,6 +775,13 @@ impl Diagnostic {
         self
     }
 
+    /// Add a subdiagnostic from a type that implements `SessionSubdiagnostic` - see
+    /// [rustc_macros::SessionSubdiagnostic].
+    pub fn subdiagnostic(&mut self, subdiagnostic: impl AddSubdiagnostic) -> &mut Self {
+        subdiagnostic.add_to_diagnostic(self);
+        self
+    }
+
     pub fn set_span<S: Into<MultiSpan>>(&mut self, sp: S) -> &mut Self {
         self.span = sp.into();
         if let Some(span) = self.span.primary_span() {

compiler/rustc_errors/src/diagnostic_builder.rs

@@ -477,7 +477,7 @@ impl<'a, G: EmissionGuarantee> DiagnosticBuilder<'a, G> {
         &mut self,
         sp: Span,
         msg: impl Into<DiagnosticMessage>,
-        suggestion: String,
+        suggestion: impl ToString,
         applicability: Applicability,
     ) -> &mut Self);
     forward!(pub fn span_suggestions(
@@ -497,28 +497,28 @@ impl<'a, G: EmissionGuarantee> DiagnosticBuilder<'a, G> {
         &mut self,
         sp: Span,
         msg: impl Into<DiagnosticMessage>,
-        suggestion: String,
+        suggestion: impl ToString,
         applicability: Applicability,
     ) -> &mut Self);
     forward!(pub fn span_suggestion_verbose(
         &mut self,
         sp: Span,
         msg: impl Into<DiagnosticMessage>,
-        suggestion: String,
+        suggestion: impl ToString,
         applicability: Applicability,
     ) -> &mut Self);
     forward!(pub fn span_suggestion_hidden(
         &mut self,
         sp: Span,
         msg: impl Into<DiagnosticMessage>,
-        suggestion: String,
+        suggestion: impl ToString,
         applicability: Applicability,
     ) -> &mut Self);
     forward!(pub fn tool_only_span_suggestion(
         &mut self,
         sp: Span,
         msg: impl Into<DiagnosticMessage>,
-        suggestion: String,
+        suggestion: impl ToString,
         applicability: Applicability,
     ) -> &mut Self);
 
@@ -530,6 +530,11 @@ impl<'a, G: EmissionGuarantee> DiagnosticBuilder<'a, G> {
         name: impl Into<Cow<'static, str>>,
         arg: DiagnosticArgValue<'static>,
     ) -> &mut Self);
+
+    forward!(pub fn subdiagnostic(
+        &mut self,
+        subdiagnostic: impl crate::AddSubdiagnostic
+    ) -> &mut Self);
 }
 
 impl<G: EmissionGuarantee> Debug for DiagnosticBuilder<'_, G> {

compiler/rustc_errors/src/lib.rs

@@ -370,8 +370,8 @@ impl fmt::Display for ExplicitBug {
 impl error::Error for ExplicitBug {}
 
 pub use diagnostic::{
-    Diagnostic, DiagnosticArg, DiagnosticArgValue, DiagnosticId, DiagnosticStyledString,
-    IntoDiagnosticArg, SubDiagnostic,
+    AddSubdiagnostic, Diagnostic, DiagnosticArg, DiagnosticArgValue, DiagnosticId,
+    DiagnosticStyledString, IntoDiagnosticArg, SubDiagnostic,
 };
 pub use diagnostic_builder::{DiagnosticBuilder, EmissionGuarantee};
 use std::backtrace::Backtrace;

compiler/rustc_expand/src/config.rs

@@ -511,7 +511,7 @@ pub fn parse_cfg<'a>(meta_item: &'a MetaItem, sess: &Session) -> Option<&'a Meta
             err.span_suggestion(
                 span,
                 "expected syntax is",
-                suggestion.into(),
+                suggestion,
                 Applicability::HasPlaceholders,
             );
         }

compiler/rustc_infer/src/infer/error_reporting/mod.rs

@@ -2198,7 +2198,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
             err.span_suggestion(
                 span.with_hi(before_close).shrink_to_hi(),
                 msg,
-                ",".into(),
+                ",",
                 Applicability::MachineApplicable,
             );
         } else {

compiler/rustc_lint/src/array_into_iter.rs

@@ -129,7 +129,7 @@ impl<'tcx> LateLintPass<'tcx> for ArrayIntoIter {
                 diag.span_suggestion(
                     call.ident.span,
                     "use `.iter()` instead of `.into_iter()` to avoid ambiguity",
-                    "iter".into(),
+                    "iter",
                     Applicability::MachineApplicable,
                 );
                 if self.for_expr_span == expr.span {

compiler/rustc_lint/src/context.rs

@@ -738,7 +738,7 @@ pub trait LintContext: Sized {
                     db.span_suggestion_verbose(
                         span.shrink_to_hi(),
                         "insert whitespace here to avoid this being parsed as a prefix in Rust 2021",
-                        " ".into(),
+                        " ",
                         Applicability::MachineApplicable,
                     );
                 }