Make #[diagnostic::on_unimplemented] format string parsing more robust

This commit fixes several issues with the format string parsing of the
`#[diagnostic::on_unimplemented]` attribute that were pointed out by
@ehuss.
In detail it fixes:

* Appearing format specifiers (display, etc). For these we generate a
warning that the specifier is unsupported. Otherwise we ignore them
* Positional arguments. For these we generate a warning that positional
arguments are unsupported in that location and replace them with the
format string equivalent (so `{}` or `{n}` where n is the index of the
positional argument)
* Broken format strings with enclosed }. For these we generate a warning
about the broken format string and set the emitted message literally to
the provided unformatted string
* Unknown format specifiers. For these we generate an additional warning
about the unknown specifier. Otherwise we emit the literal string as
message.

This essentially makes those strings behave like `format!` with the
minor difference that we do not generate hard errors but only warnings.
After that we continue trying to do something unsuprising (mostly either
ignoring the broken parts or falling back to just giving back the
literal string as provided).

Fix #122391

Author: weiznich

compiler/rustc_trait_selection/messages.ftl

@@ -19,6 +19,9 @@ trait_selection_closure_kind_mismatch = expected a closure that implements the `
 
 trait_selection_closure_kind_requirement = the requirement to implement `{$trait_prefix}{$expected}` derives from here
 
+trait_selection_disallowed_positional_argument = positional format arguments are not allowed here
+    .help = only named format arguments with the name of one of the generic types are allowed in this context
+
 trait_selection_dump_vtable_entries = vtable entries for `{$trait_ref}`: {$entries}
 
 trait_selection_empty_on_clause_in_rustc_on_unimplemented = empty `on`-clause in `#[rustc_on_unimplemented]`
@@ -30,6 +33,9 @@ trait_selection_ignored_diagnostic_option = `{$option_name}` is ignored due to p
 
 trait_selection_inherent_projection_normalization_overflow = overflow evaluating associated type `{$ty}`
 
+trait_selection_invalid_format_specifier = invalid format specifier
+    .help = no format specifier are supported in this position
+
 trait_selection_invalid_on_clause_in_rustc_on_unimplemented = invalid `on`-clause in `#[rustc_on_unimplemented]`
     .label = invalid on-clause here
 
@@ -60,3 +66,6 @@ trait_selection_unable_to_construct_constant_value = unable to construct a const
 
 trait_selection_unknown_format_parameter_for_on_unimplemented_attr = there is no parameter `{$argument_name}` on trait `{$trait_name}`
     .help = expect either a generic argument name or {"`{Self}`"} as format argument
+
+trait_selection_wrapped_parser_error = {$description}
+    .label = {$label}

compiler/rustc_trait_selection/src/traits/error_reporting/on_unimplemented.rs

@@ -367,6 +367,23 @@ pub struct UnknownFormatParameterForOnUnimplementedAttr {
     trait_name: Symbol,
 }
 
+#[derive(LintDiagnostic)]
+#[diag(trait_selection_disallowed_positional_argument)]
+#[help]
+pub struct DisallowedPositionalArgument;
+
+#[derive(LintDiagnostic)]
+#[diag(trait_selection_invalid_format_specifier)]
+#[help]
+pub struct InvalidFormatSpecifier;
+
+#[derive(LintDiagnostic)]
+#[diag(trait_selection_wrapped_parser_error)]
+pub struct WrappedParserError {
+    description: String,
+    label: String,
+}
+
 impl<'tcx> OnUnimplementedDirective {
     fn parse(
         tcx: TyCtxt<'tcx>,
@@ -758,64 +775,99 @@ impl<'tcx> OnUnimplementedFormatString {
         let trait_name = tcx.item_name(trait_def_id);
         let generics = tcx.generics_of(item_def_id);
         let s = self.symbol.as_str();
-        let parser = Parser::new(s, None, None, false, ParseMode::Format);
+        let mut parser = Parser::new(s, None, None, false, ParseMode::Format);
         let mut result = Ok(());
-        for token in parser {
+        for token in &mut parser {
             match token {
                 Piece::String(_) => (), // Normal string, no need to check it
-                Piece::NextArgument(a) => match a.position {
-                    Position::ArgumentNamed(s) => {
-                        match Symbol::intern(s) {
-                            // `{ThisTraitsName}` is allowed
-                            s if s == trait_name && !self.is_diagnostic_namespace_variant => (),
-                            s if ALLOWED_FORMAT_SYMBOLS.contains(&s)
-                                && !self.is_diagnostic_namespace_variant =>
-                            {
-                                ()
-                            }
-                            // So is `{A}` if A is a type parameter
-                            s if generics.params.iter().any(|param| param.name == s) => (),
-                            s => {
-                                if self.is_diagnostic_namespace_variant {
-                                    tcx.emit_node_span_lint(
-                                        UNKNOWN_OR_MALFORMED_DIAGNOSTIC_ATTRIBUTES,
-                                        tcx.local_def_id_to_hir_id(item_def_id.expect_local()),
-                                        self.span,
-                                        UnknownFormatParameterForOnUnimplementedAttr {
-                                            argument_name: s,
-                                            trait_name,
-                                        },
-                                    );
-                                } else {
-                                    result = Err(struct_span_code_err!(
-                                        tcx.dcx(),
-                                        self.span,
-                                        E0230,
-                                        "there is no parameter `{}` on {}",
-                                        s,
-                                        if trait_def_id == item_def_id {
-                                            format!("trait `{trait_name}`")
-                                        } else {
-                                            "impl".to_string()
-                                        }
-                                    )
-                                    .emit());
+                Piece::NextArgument(a) => {
+                    let format_spec = a.format;
+                    if self.is_diagnostic_namespace_variant
+                        && (format_spec.ty_span.is_some()
+                            || format_spec.width_span.is_some()
+                            || format_spec.precision_span.is_some()
+                            || format_spec.fill_span.is_some())
+                    {
+                        tcx.emit_node_span_lint(
+                            UNKNOWN_OR_MALFORMED_DIAGNOSTIC_ATTRIBUTES,
+                            tcx.local_def_id_to_hir_id(item_def_id.expect_local()),
+                            self.span,
+                            InvalidFormatSpecifier,
+                        );
+                    }
+                    match a.position {
+                        Position::ArgumentNamed(s) => {
+                            match Symbol::intern(s) {
+                                // `{ThisTraitsName}` is allowed
+                                s if s == trait_name && !self.is_diagnostic_namespace_variant => (),
+                                s if ALLOWED_FORMAT_SYMBOLS.contains(&s)
+                                    && !self.is_diagnostic_namespace_variant =>
+                                {
+                                    ()
+                                }
+                                // So is `{A}` if A is a type parameter
+                                s if generics.params.iter().any(|param| param.name == s) => (),
+                                s => {
+                                    if self.is_diagnostic_namespace_variant {
+                                        tcx.emit_node_span_lint(
+                                            UNKNOWN_OR_MALFORMED_DIAGNOSTIC_ATTRIBUTES,
+                                            tcx.local_def_id_to_hir_id(item_def_id.expect_local()),
+                                            self.span,
+                                            UnknownFormatParameterForOnUnimplementedAttr {
+                                                argument_name: s,
+                                                trait_name,
+                                            },
+                                        );
+                                    } else {
+                                        result = Err(struct_span_code_err!(
+                                            tcx.dcx(),
+                                            self.span,
+                                            E0230,
+                                            "there is no parameter `{}` on {}",
+                                            s,
+                                            if trait_def_id == item_def_id {
+                                                format!("trait `{trait_name}`")
+                                            } else {
+                                                "impl".to_string()
+                                            }
+                                        )
+                                        .emit());
+                                    }
                                 }
                             }
                         }
+                        // `{:1}` and `{}` are not to be used
+                        Position::ArgumentIs(..) | Position::ArgumentImplicitlyIs(_) => {
+                            if self.is_diagnostic_namespace_variant {
+                                tcx.emit_node_span_lint(
+                                    UNKNOWN_OR_MALFORMED_DIAGNOSTIC_ATTRIBUTES,
+                                    tcx.local_def_id_to_hir_id(item_def_id.expect_local()),
+                                    self.span,
+                                    DisallowedPositionalArgument,
+                                );
+                            } else {
+                                let reported = struct_span_code_err!(
+                                    tcx.dcx(),
+                                    self.span,
+                                    E0231,
+                                    "only named generic parameters are allowed"
+                                )
+                                .emit();
+                                result = Err(reported);
+                            }
+                        }
                     }
-                    // `{:1}` and `{}` are not to be used
-                    Position::ArgumentIs(..) | Position::ArgumentImplicitlyIs(_) => {
-                        let reported = struct_span_code_err!(
-                            tcx.dcx(),
-                            self.span,
-                            E0231,
-                            "only named generic parameters are allowed"
-                        )
-                        .emit();
-                        result = Err(reported);
-                    }
-                },
+                }
+            }
+        }
+        if self.is_diagnostic_namespace_variant {
+            for e in parser.errors {
+                tcx.emit_node_span_lint(
+                    UNKNOWN_OR_MALFORMED_DIAGNOSTIC_ATTRIBUTES,
+                    tcx.local_def_id_to_hir_id(item_def_id.expect_local()),
+                    self.span,
+                    WrappedParserError { description: e.description, label: e.label },
+                );
             }
         }
 
@@ -853,9 +905,9 @@ impl<'tcx> OnUnimplementedFormatString {
         let empty_string = String::new();
 
         let s = self.symbol.as_str();
-        let parser = Parser::new(s, None, None, false, ParseMode::Format);
+        let mut parser = Parser::new(s, None, None, false, ParseMode::Format);
         let item_context = (options.get(&sym::ItemContext)).unwrap_or(&empty_string);
-        parser
+        let constructed_message = (&mut parser)
             .map(|p| match p {
                 Piece::String(s) => s.to_owned(),
                 Piece::NextArgument(a) => match a.position {
@@ -895,9 +947,20 @@ impl<'tcx> OnUnimplementedFormatString {
                             }
                         }
                     }
+                    Position::ArgumentImplicitlyIs(_) if self.is_diagnostic_namespace_variant => {
+                        String::from("{}")
+                    }
+                    Position::ArgumentIs(idx) if self.is_diagnostic_namespace_variant => {
+                        format!("{{{idx}}}")
+                    }
                     _ => bug!("broken on_unimplemented {:?} - bad format arg", self.symbol),
                 },
             })
-            .collect()
+            .collect();
+        if self.is_diagnostic_namespace_variant && !parser.errors.is_empty() {
+            String::from(s)
+        } else {
+            constructed_message
+        }
     }
 }

tests/ui/diagnostic_namespace/on_unimplemented/broken_format.rs

@@ -0,0 +1,45 @@
+#[diagnostic::on_unimplemented(message = "{{Test } thing")]
+//~^WARN unmatched `}` found
+//~|WARN unmatched `}` found
+trait ImportantTrait1 {}
+
+#[diagnostic::on_unimplemented(message = "Test {}")]
+//~^WARN positional format arguments are not allowed here
+//~|WARN positional format arguments are not allowed here
+trait ImportantTrait2 {}
+
+#[diagnostic::on_unimplemented(message = "Test {1:}")]
+//~^WARN positional format arguments are not allowed here
+//~|WARN positional format arguments are not allowed here
+trait ImportantTrait3 {}
+
+#[diagnostic::on_unimplemented(message = "Test {Self:123}")]
+//~^WARN invalid format specifier
+//~|WARN invalid format specifier
+trait ImportantTrait4 {}
+
+#[diagnostic::on_unimplemented(message = "Test {Self:!}")]
+//~^WARN expected `'}'`, found `'!'`
+//~|WARN expected `'}'`, found `'!'`
+//~|WARN unmatched `}` found
+//~|WARN unmatched `}` found
+trait ImportantTrait5 {}
+
+fn check_1(_: impl ImportantTrait1) {}
+fn check_2(_: impl ImportantTrait2) {}
+fn check_3(_: impl ImportantTrait3) {}
+fn check_4(_: impl ImportantTrait4) {}
+fn check_5(_: impl ImportantTrait5) {}
+
+fn main() {
+    check_1(());
+    //~^ERROR {{Test } thing
+    check_2(());
+    //~^ERROR Test {}
+    check_3(());
+    //~^ERROR Test {1}
+    check_4(());
+    //~^ERROR Test ()
+    check_5(());
+    //~^ERROR Test {Self:!}
+}