lint: LintDiagnosticBuilder into rustc_errors

Signed-off-by: David Wood <david.wood@huawei.com>

compiler/rustc_errors/src/diagnostic_builder.rs

@@ -589,3 +589,26 @@ macro_rules! struct_span_err {
 macro_rules! error_code {
     ($code:ident) => {{ $crate::DiagnosticId::Error(stringify!($code).to_owned()) }};
 }
+
+/// Wrapper around a `DiagnosticBuilder` for creating lints.
+pub struct LintDiagnosticBuilder<'a, G: EmissionGuarantee>(DiagnosticBuilder<'a, G>);
+
+impl<'a, G: EmissionGuarantee> LintDiagnosticBuilder<'a, G> {
+    /// Return the inner `DiagnosticBuilder`, first setting the primary message to `msg`.
+    pub fn build(mut self, msg: impl Into<DiagnosticMessage>) -> DiagnosticBuilder<'a, G> {
+        self.0.set_primary_message(msg);
+        self.0.set_is_lint();
+        self.0
+    }
+
+    /// Create a `LintDiagnosticBuilder` from some existing `DiagnosticBuilder`.
+    pub fn new(err: DiagnosticBuilder<'a, G>) -> LintDiagnosticBuilder<'a, G> {
+        LintDiagnosticBuilder(err)
+    }
+}
+
+impl<'a> LintDiagnosticBuilder<'a, ErrorGuaranteed> {
+    pub fn forget_guarantee(self) -> LintDiagnosticBuilder<'a, ()> {
+        LintDiagnosticBuilder(self.0.forget_guarantee())
+    }
+}

compiler/rustc_errors/src/lib.rs

@@ -373,7 +373,7 @@ pub use diagnostic::{
     AddSubdiagnostic, Diagnostic, DiagnosticArg, DiagnosticArgValue, DiagnosticId,
     DiagnosticStyledString, IntoDiagnosticArg, SubDiagnostic,
 };
-pub use diagnostic_builder::{DiagnosticBuilder, EmissionGuarantee};
+pub use diagnostic_builder::{DiagnosticBuilder, EmissionGuarantee, LintDiagnosticBuilder};
 use std::backtrace::Backtrace;
 
 /// A handler deals with errors and other compiler output.