1 files changed, 141 insertions, 0 deletions
diff --git a/compiler/rustc_macros/src/diagnostics/error.rs b/compiler/rustc_macros/src/diagnostics/error.rs
new file mode 100644
index 000000000..0b1ededa7
--- /dev/null
+++ b/compiler/rustc_macros/src/diagnostics/error.rs
@@ -0,0 +1,141 @@
+use proc_macro::{Diagnostic, Level, MultiSpan};
+use proc_macro2::TokenStream;
+use quote::quote;
+use syn::{spanned::Spanned, Attribute, Error as SynError, Meta, NestedMeta};
+
+#[derive(Debug)]
+pub(crate) enum DiagnosticDeriveError {
+    SynError(SynError),
+    ErrorHandled,
+}
+
+impl DiagnosticDeriveError {
+    pub(crate) fn to_compile_error(self) -> TokenStream {
+        match self {
+            DiagnosticDeriveError::SynError(e) => e.to_compile_error(),
+            DiagnosticDeriveError::ErrorHandled => {
+                // Return ! to avoid having to create a blank DiagnosticBuilder to return when an
+                // error has already been emitted to the compiler.
+                quote! {
+                    { unreachable!(); }
+                }
+            }
+        }
+    }
+}
+
+impl From<SynError> for DiagnosticDeriveError {
+    fn from(e: SynError) -> Self {
+        DiagnosticDeriveError::SynError(e)
+    }
+}
+
+/// Helper function for use with `throw_*` macros - constraints `$f` to an `impl FnOnce`.
+pub(crate) fn _throw_err(
+    diag: Diagnostic,
+    f: impl FnOnce(Diagnostic) -> Diagnostic,
+) -> DiagnosticDeriveError {
+    f(diag).emit();
+    DiagnosticDeriveError::ErrorHandled
+}
+
+/// Helper function for printing `syn::Path` - doesn't handle arguments in paths and these are
+/// unlikely to come up much in use of the macro.
+fn path_to_string(path: &syn::Path) -> String {
+    let mut out = String::new();
+    for (i, segment) in path.segments.iter().enumerate() {
+        if i > 0 || path.leading_colon.is_some() {
+            out.push_str("::");
+        }
+        out.push_str(&segment.ident.to_string());
+    }
+    out
+}
+
+/// Returns an error diagnostic on span `span` with msg `msg`.
+pub(crate) fn span_err(span: impl MultiSpan, msg: &str) -> Diagnostic {
+    Diagnostic::spanned(span, Level::Error, msg)
+}
+
+/// Emit a diagnostic on span `$span` with msg `$msg` (optionally performing additional decoration
+/// using the `FnOnce` passed in `diag`) and return `Err(ErrorHandled)`.
+///
+/// For methods that return a `Result<_, DiagnosticDeriveError>`:
+macro_rules! throw_span_err {
+    ($span:expr, $msg:expr) => {{ throw_span_err!($span, $msg, |diag| diag) }};
+    ($span:expr, $msg:expr, $f:expr) => {{
+        let diag = span_err($span, $msg);
+        return Err(crate::diagnostics::error::_throw_err(diag, $f));
+    }};
+}
+
+pub(crate) use throw_span_err;
+
+/// Returns an error diagnostic for an invalid attribute.
+pub(crate) fn invalid_attr(attr: &Attribute, meta: &Meta) -> Diagnostic {
+    let span = attr.span().unwrap();
+    let path = path_to_string(&attr.path);
+    match meta {
+        Meta::Path(_) => span_err(span, &format!("`#[{}]` is not a valid attribute", path)),
+        Meta::NameValue(_) => {
+            span_err(span, &format!("`#[{} = ...]` is not a valid attribute", path))
+        }
+        Meta::List(_) => span_err(span, &format!("`#[{}(...)]` is not a valid attribute", path)),
+    }
+}
+
+/// Emit a error diagnostic for an invalid attribute (optionally performing additional decoration
+/// using the `FnOnce` passed in `diag`) and return `Err(ErrorHandled)`.
+///
+/// For methods that return a `Result<_, DiagnosticDeriveError>`:
+macro_rules! throw_invalid_attr {
+    ($attr:expr, $meta:expr) => {{ throw_invalid_attr!($attr, $meta, |diag| diag) }};
+    ($attr:expr, $meta:expr, $f:expr) => {{
+        let diag = crate::diagnostics::error::invalid_attr($attr, $meta);
+        return Err(crate::diagnostics::error::_throw_err(diag, $f));
+    }};
+}
+
+pub(crate) use throw_invalid_attr;
+
+/// Returns an error diagnostic for an invalid nested attribute.
+pub(crate) fn invalid_nested_attr(attr: &Attribute, nested: &NestedMeta) -> Diagnostic {
+    let name = attr.path.segments.last().unwrap().ident.to_string();
+    let name = name.as_str();
+
+    let span = nested.span().unwrap();
+    let meta = match nested {
+        syn::NestedMeta::Meta(meta) => meta,
+        syn::NestedMeta::Lit(_) => {
+            return span_err(span, &format!("`#[{}(\"...\")]` is not a valid attribute", name));
+        }
+    };
+
+    let span = meta.span().unwrap();
+    let path = path_to_string(meta.path());
+    match meta {
+        Meta::NameValue(..) => {
+            span_err(span, &format!("`#[{}({} = ...)]` is not a valid attribute", name, path))
+        }
+        Meta::Path(..) => {
+            span_err(span, &format!("`#[{}({})]` is not a valid attribute", name, path))
+        }
+        Meta::List(..) => {
+            span_err(span, &format!("`#[{}({}(...))]` is not a valid attribute", name, path))
+        }
+    }
+}
+
+/// Emit a error diagnostic for an invalid nested attribute (optionally performing additional
+/// decoration using the `FnOnce` passed in `diag`) and return `Err(ErrorHandled)`.
+///
+/// For methods that return a `Result<_, DiagnosticDeriveError>`:
+macro_rules! throw_invalid_nested_attr {
+    ($attr:expr, $nested_attr:expr) => {{ throw_invalid_nested_attr!($attr, $nested_attr, |diag| diag) }};
+    ($attr:expr, $nested_attr:expr, $f:expr) => {{
+        let diag = crate::diagnostics::error::invalid_nested_attr($attr, $nested_attr);
+        return Err(crate::diagnostics::error::_throw_err(diag, $f));
+    }};
+}
+
+pub(crate) use throw_invalid_nested_attr;