RFC: proc macro include!

text/0000-proc-macro-include.md

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+- Feature Name: `proc_macro_include`
+- Start Date: 2021-11-24
+- RFC PR: [rust-lang/rfcs#0000](https://github.com/rust-lang/rfcs/pull/0000)
+- Rust Issue: [rust-lang/rust#0000](https://github.com/rust-lang/rust/issues/0000)
+
+# Summary
+[summary]: #summary
+
+Proc macros can now effectively `include!` other files and process their contents.
+This both allows proc macros to communicate that they read external files,
+and to maintain spans into the external file for more useful error messages.
+
+# Motivation
+[motivation]: #motivation
+
+- `include!` and `include_str!` are no longer required to be compiler built-ins,
+  and could be implemented as proc macros.
+- Help incremental builds and build determinism, by proc macros telling rustc which files they read.
+- Improve proc macro sandboxability and cacheability, by offering a way to implement this class of
+  file-reading macros without using OS APIs directly.
+
+# Guide-level explanation
+[guide-level-explanation]: #guide-level-explanation
+
+## For users of proc macros
+
+Nothing changes! You'll just see nicer errors and fewer rebuilds
+from procedural macros which read external files.
+
+## For writers of proc macros
+
+Three new functions are provided in the `proc_macro` interface crate:
+
+```rust
+/// Read the contents of a file as a `TokenStream` and add it to build dependency info.
+///
+/// The build system executing the compiler will know that the file was accessed during compilation,
+/// and will be able to rerun the build when the contents of the file changes.
+///
+/// May fail for a number of reasons, for example, if the string contains unbalanced delimiters
+/// or characters not existing in the language.
+///
+/// If the file fails to be read, this is not automatically a fatal error. The proc macro may
+/// gracefully handle the missing file, or emit a compile error noting the missing dependency.
+///
+/// Source spans are constructed for the read file. If you use the spans of this token stream,
+/// any resulting errors will correctly point at the tokens in the read file.
+///
+/// NOTE: some errors may cause panics instead of returning `io::Error`.
+/// We reserve the right to change these errors into `io::Error`s later.
+fn include<P: AsRef<str>>(path: P) -> Result<TokenStream, std::io::Error>;
+
+/// Read the contents of a file as a string and add it to build dependency info.
+///
+/// The build system executing the compiler will know that the file was accessed during compilation,
+/// and will be able to rerun the build when the contents of the file changes.
+///
+/// If the file fails to be read, this is not automatically a fatal error. The proc macro may
+/// gracefully handle the missing file, or emit a compile error noting the missing dependency.
+///
+/// NOTE: some errors may cause panics instead of returning `io::Error`.
+/// We reserve the right to change these errors into `io::Error`s later.
+fn include_str<P: AsRef<str>>(path: P) -> Result<String, std::io::Error>;
+
+/// Read the contents of a file as raw bytes and add it to build dependency info.
+///
+/// The build system executing the compiler will know that the file was accessed during compilation,
+/// and will be able to rerun the build when the contents of the file changes.
+///
+/// If the file fails to be read, this is not automatically a fatal error. The proc macro may
+/// gracefully handle the missing file, or emit a compile error noting the missing dependency.
+///
+/// NOTE: some errors may cause panics instead of returning `io::Error`.
+/// We reserve the right to change these errors into `io::Error`s later.
+fn include_bytes<P: AsRef<str>>(path: P) -> Result<Vec<u8>, std::io::Error>;
+```
+
+As an example, consider a potential implementation of [`core::include`](https://doc.rust-lang.org/stable/core/macro.include.html):
+
+```rust
+#[proc_macro]
+pub fn include(input: TokenStream) -> TokenStream {
+    let mut iter = input.into_iter();
+
+    let result = 'main: if let Some(tt) = iter.next() {
+        let TokenTree::Literal(lit) = tt &&
+        let LiteralValue::Str(path) = lit.value() else {
+            Diagnostic::spanned(tt.span(), Level::Error, "argument must be a string literal").emit();
+            break 'main TokenStream::new();
+        }
+
+        match proc_macro::include(&path) {
+            Ok(token_stream) => token_stream,
+            Err(err) => {
+                Diagnostic::spanned(Span::call_site(), Level::Error, format_args!("couldn't read {path}: {err}")).emit();
+                TokenStream::new()
+            }
+        }
+    } else {
+        Diagnostic::spanned(Span::call_site(), Level::Error, "include! takes 1 argument").emit();
+        TokenStream::new()
+    }
+
+    if let Some(_) = iter.next() {
+        Diagnostic::spanned(Span::call_site(), Level::Error, "include! takes 1 argument").emit();
+    }
+
+    result
+}
+```
+
+(RFC note: this example uses unstable and even unimplemented features for clarity.
+However, this RFC in no way requires these features to be useful on its own.)
+
+# Reference-level explanation
+[reference-level-explanation]: #reference-level-explanation
+
+If a file read is unsuccessful, an encoding of the responsible `io::Error` is passed over the RPC bridge.
+If a file is successfully read but fails to lex, `ErrorKind::Other` is returned.
+
+None of these three APIs should ever cause compilation to fail.
+It is the responsibility of the proc macro to fail compilation if a failed file read is fatal.
+
+The author is unsure of the technical details required to implement this in the compiler.
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+This is more API surface for the `proc_macro` crate, and the `proc_macro` bridge is already complicated.
+Additionally, this is likely to lead to more proc macros which read external files.
+Moving the handling of `include!`-like macros later in the compiler pipeline
+(read: dependent on name resolution)
+likely is also significantly more complicated than the current `include!` implementation.
+
+# Alternatives
+[rationale-and-alternatives]: #rationale-and-alternatives
+
+- [`proc_macro::tracked_path`](https://doc.rust-lang.org/stable/proc_macro/tracked_path/fn.path.html) (unstable)
+
+This just tells the proc_macro driver that the proc macro has a dependency on the given path.
+This is sufficient for tracking the file, as the proc macro can just also read the file itself,
+but lacks the ability to require the proc macro go through this API, or to provide spans for errors.
+
+Meaningfully, it'd be nice to be able to sandbox proc macros in wasm à la [watt](https://crates.io/crates/watt)
+while still having proc macros capable of reading the filesystem (in a proc_macro driver controlled manner).
+
+- Status quo
+
+Proc macros can continue to read files and use `include_str!` to indicate a build dependency.
+This is error prone, easy to forget to do, and all around not a great experience.
+
+# Prior art
+[prior-art]: #prior-art
+
+No known prior art.
+
+# Unresolved questions
+[unresolved-questions]: #unresolved-questions
+
+- It would be nice for `include` to allow emitting a useful lexer error directly.
+  This is not currently provided for by the proposed API.
+- Unknown unknowns.
+
+# Future possibilities
+[future-possibilities]: #future-possibilities
+
+Future expansion of the proc macro APIs are almost entirely orthogonal from this feature.
+As such, here is a small list of potential uses for this API:
+
+- Processing a Rust-lexer-compatible DSL
+  - Multi-file parser specifications for toolchains like LALRPOP or pest
+  - Larger scale Rust syntax experimentations
+- Pre-processing `include!`ed assets
+  - Embedding compiled-at-rustc-time shaders
+  - Escaping text at compile time for embedding in a document format