Add `rustc_fluent_macro` to decouple fluent from `rustc_macros`

Fluent, with all the icu4x it brings in, takes quite some time to
compile. `fluent_messages!` is only needed in further downstream rustc
crates, but is blocking more upstream crates like `rustc_index`. By
splitting it out, we allow `rustc_macros` to be compiled earlier, which
speeds up `x check compiler` by about 5 seconds (and even more after the
needless dependency on `serde_json` is removed from
`rustc_data_structures`).

1 files changed, 64 insertions, 0 deletions

diff --git a/compiler/rustc_fluent_macro/src/lib.rs b/compiler/rustc_fluent_macro/src/lib.rs
new file mode 100644
index 00000000000..a01643cd67d
--- /dev/null
+++ b/compiler/rustc_fluent_macro/src/lib.rs
@@ -0,0 +1,64 @@
+#![doc(html_root_url = "https://doc.rust-lang.org/nightly/nightly-rustc/")]
+#![feature(proc_macro_diagnostic)]
+#![feature(proc_macro_span)]
+#![deny(rustc::untranslatable_diagnostic)]
+#![deny(rustc::diagnostic_outside_of_impl)]
+#![allow(rustc::default_hash_types)]
+
+use proc_macro::TokenStream;
+
+mod fluent;
+
+/// Implements the `fluent_messages` macro, which performs compile-time validation of the
+/// compiler's Fluent resources (i.e. that the resources parse and don't multiply define the same
+/// messages) and generates constants that make using those messages in diagnostics more ergonomic.
+///
+/// For example, given the following invocation of the macro..
+///
+/// ```ignore (rust)
+/// fluent_messages! { "./typeck.ftl" }
+/// ```
+/// ..where `typeck.ftl` has the following contents..
+///
+/// ```fluent
+/// typeck_field_multiply_specified_in_initializer =
+///     field `{$ident}` specified more than once
+///     .label = used more than once
+///     .label_previous_use = first use of `{$ident}`
+/// ```
+/// ...then the macro parse the Fluent resource, emitting a diagnostic if it fails to do so, and
+/// will generate the following code:
+///
+/// ```ignore (rust)
+/// pub static DEFAULT_LOCALE_RESOURCE: &'static [&'static str] = include_str!("./typeck.ftl");
+///
+/// mod fluent_generated {
+///     mod typeck {
+///         pub const field_multiply_specified_in_initializer: DiagnosticMessage =
+///             DiagnosticMessage::fluent("typeck_field_multiply_specified_in_initializer");
+///         pub const field_multiply_specified_in_initializer_label_previous_use: DiagnosticMessage =
+///             DiagnosticMessage::fluent_attr(
+///                 "typeck_field_multiply_specified_in_initializer",
+///                 "previous_use_label"
+///             );
+///     }
+/// }
+/// ```
+/// When emitting a diagnostic, the generated constants can be used as follows:
+///
+/// ```ignore (rust)
+/// let mut err = sess.struct_span_err(
+///     span,
+///     fluent::typeck::field_multiply_specified_in_initializer
+/// );
+/// err.span_default_label(span);
+/// err.span_label(
+///     previous_use_span,
+///     fluent::typeck::field_multiply_specified_in_initializer_label_previous_use
+/// );
+/// err.emit();
+/// ```
+#[proc_macro]
+pub fn fluent_messages(input: TokenStream) -> TokenStream {
+    fluent::fluent_messages(input)
+}