Rollup merge of #44125 - SergioBenitez:master, r=nrc

Initial diagnostic API for proc-macros.

This commit introduces the ability to create and emit `Diagnostic` structures from proc-macros, allowing for proc-macro authors to emit warning, error, note, and help messages just like the compiler does.

The API is somewhat based on the diagnostic API already present in `rustc` with several changes that improve usability. The entry point into the diagnostic API is a new `Diagnostic` type which is primarily created through new `error`, `warning`, `help`, and `note` methods on `Span`. The `Diagnostic` type records the diagnostic level, message, and optional `Span` for the top-level diagnostic and contains a `Vec` of all of the child diagnostics. Child diagnostics can be added through builder methods on `Diagnostic`.

A typical use of the API may look like:

```rust
let token = parse_token();
let val = parse_val();

val.span
    .error(format!("expected A but found {}", val))
    .span_note(token.span, "because of this token")
    .help("consider using a different token")
    .emit();
```

cc @jseyfried @nrc @dtolnay @alexcrichton

3 files changed, 157 insertions, 0 deletions

diff --git a/src/libproc_macro/Cargo.toml b/src/libproc_macro/Cargo.toml
index 1b5141773a9..cfd83e348a8 100644
--- a/src/libproc_macro/Cargo.toml
+++ b/src/libproc_macro/Cargo.toml
@@ -10,3 +10,4 @@ crate-type = ["dylib"]
 [dependencies]
 syntax = { path = "../libsyntax" }
 syntax_pos = { path = "../libsyntax_pos" }
+rustc_errors = { path = "../librustc_errors" }
diff --git a/src/libproc_macro/diagnostic.rs b/src/libproc_macro/diagnostic.rs
new file mode 100644
index 00000000000..c39aec896e6
--- /dev/null
+++ b/src/libproc_macro/diagnostic.rs
@@ -0,0 +1,134 @@
+// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+use Span;
+
+use rustc_errors as rustc;
+
+/// An enum representing a diagnostic level.
+#[unstable(feature = "proc_macro", issue = "38356")]
+#[derive(Copy, Clone, Debug)]
+pub enum Level {
+    /// An error.
+    Error,
+    /// A warning.
+    Warning,
+    /// A note.
+    Note,
+    /// A help message.
+    Help,
+    #[doc(hidden)]
+    __Nonexhaustive,
+}
+
+/// A structure representing a diagnostic message and associated children
+/// messages.
+#[unstable(feature = "proc_macro", issue = "38356")]
+#[derive(Clone, Debug)]
+pub struct Diagnostic {
+    level: Level,
+    message: String,
+    span: Option<Span>,
+    children: Vec<Diagnostic>
+}
+
+macro_rules! diagnostic_child_methods {
+    ($spanned:ident, $regular:ident, $level:expr) => (
+        /// Add a new child diagnostic message to `self` with the level
+        /// identified by this methods name with the given `span` and `message`.
+        #[unstable(feature = "proc_macro", issue = "38356")]
+        pub fn $spanned<T: Into<String>>(mut self, span: Span, message: T) -> Diagnostic {
+            self.children.push(Diagnostic::spanned(span, $level, message));
+            self
+        }
+
+        /// Add a new child diagnostic message to `self` with the level
+        /// identified by this method's name with the given `message`.
+        #[unstable(feature = "proc_macro", issue = "38356")]
+        pub fn $regular<T: Into<String>>(mut self, message: T) -> Diagnostic {
+            self.children.push(Diagnostic::new($level, message));
+            self
+        }
+    )
+}
+
+impl Diagnostic {
+    /// Create a new diagnostic with the given `level` and `message`.
+    #[unstable(feature = "proc_macro", issue = "38356")]
+    pub fn new<T: Into<String>>(level: Level, message: T) -> Diagnostic {
+        Diagnostic {
+            level: level,
+            message: message.into(),
+            span: None,
+            children: vec![]
+        }
+    }
+
+    /// Create a new diagnostic with the given `level` and `message` pointing to
+    /// the given `span`.
+    #[unstable(feature = "proc_macro", issue = "38356")]
+    pub fn spanned<T: Into<String>>(span: Span, level: Level, message: T) -> Diagnostic {
+        Diagnostic {
+            level: level,
+            message: message.into(),
+            span: Some(span),
+            children: vec![]
+        }
+    }
+
+    diagnostic_child_methods!(span_error, error, Level::Error);
+    diagnostic_child_methods!(span_warning, warning, Level::Warning);
+    diagnostic_child_methods!(span_note, note, Level::Note);
+    diagnostic_child_methods!(span_help, help, Level::Help);
+
+    /// Returns the diagnostic `level` for `self`.
+    #[unstable(feature = "proc_macro", issue = "38356")]
+    pub fn level(&self) -> Level {
+        self.level
+    }
+
+    /// Emit the diagnostic.
+    #[unstable(feature = "proc_macro", issue = "38356")]
+    pub fn emit(self) {
+        ::__internal::with_sess(move |(sess, _)| {
+            let handler = &sess.span_diagnostic;
+            let level = __internal::level_to_internal_level(self.level);
+            let mut diag = rustc::DiagnosticBuilder::new(handler, level, &*self.message);
+
+            if let Some(span) = self.span {
+                diag.set_span(span.0);
+            }
+
+            for child in self.children {
+                let span = child.span.map(|s| s.0);
+                let level = __internal::level_to_internal_level(child.level);
+                diag.sub(level, &*child.message, span);
+            }
+
+            diag.emit();
+        });
+    }
+}
+
+#[unstable(feature = "proc_macro_internals", issue = "27812")]
+#[doc(hidden)]
+pub mod __internal {
+    use super::{Level, rustc};
+
+    pub fn level_to_internal_level(level: Level) -> rustc::Level {
+        match level {
+            Level::Error => rustc::Level::Error,
+            Level::Warning => rustc::Level::Warning,
+            Level::Note => rustc::Level::Note,
+            Level::Help => rustc::Level::Help,
+            Level::__Nonexhaustive => unreachable!("Level::__Nonexhaustive")
+        }
+    }
+}
diff --git a/src/libproc_macro/lib.rs b/src/libproc_macro/lib.rs
index a86854cdbc7..e403e2a00c9 100644
--- a/src/libproc_macro/lib.rs
+++ b/src/libproc_macro/lib.rs
@@ -42,6 +42,12 @@
 #[macro_use]
 extern crate syntax;
 extern crate syntax_pos;
+extern crate rustc_errors;
+
+mod diagnostic;
+
+#[unstable(feature = "proc_macro", issue = "38356")]
+pub use diagnostic::{Diagnostic, Level};
 
 use std::{ascii, fmt, iter};
 use std::str::FromStr;
@@ -188,12 +194,28 @@ pub fn quote_span(span: Span) -> TokenStream {
     TokenStream(quote::Quote::quote(&span.0))
 }
 
+macro_rules! diagnostic_method {
+    ($name:ident, $level:expr) => (
+        /// Create a new `Diagnostic` with the given `message` at the span
+        /// `self`.
+        #[unstable(feature = "proc_macro", issue = "38356")]
+        pub fn $name<T: Into<String>>(self, message: T) -> Diagnostic {
+            Diagnostic::spanned(self, $level, message)
+        }
+    )
+}
+
 impl Span {
     /// The span of the invocation of the current procedural macro.
     #[unstable(feature = "proc_macro", issue = "38356")]
     pub fn call_site() -> Span {
         ::__internal::with_sess(|(_, mark)| Span(mark.expn_info().unwrap().call_site))
     }
+
+    diagnostic_method!(error, Level::Error);
+    diagnostic_method!(warning, Level::Warning);
+    diagnostic_method!(note, Level::Note);
+    diagnostic_method!(help, Level::Help);
 }
 
 /// A single token or a delimited sequence of token trees (e.g. `[1, (), ..]`).