span: move MultiSpan

`MultiSpan` contains labels, which are more complicated with the
introduction of diagnostic translation and will use types from
`rustc_errors` - however, `rustc_errors` depends on `rustc_span` so
`rustc_span` cannot use types like `DiagnosticMessage` without
dependency cycles. Introduce a new `rustc_error_messages` crate that can
contain `DiagnosticMessage` and `MultiSpan`.

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

compiler/rustc_error_messages/src/lib.rs

@@ -0,0 +1,171 @@
+use rustc_macros::{Decodable, Encodable};
+use rustc_span::Span;
+
+/// Abstraction over a message in a diagnostic to support both translatable and non-translatable
+/// diagnostic messages.
+#[derive(Clone, Debug, PartialEq, Eq, Hash, Encodable, Decodable)]
+pub enum DiagnosticMessage {
+    /// Non-translatable diagnostic message.
+    Str(String),
+    /// Identifier for a Fluent message corresponding to the diagnostic message.
+    FluentIdentifier(String),
+}
+
+impl DiagnosticMessage {
+    /// Convert `DiagnosticMessage` to a `&str`.
+    pub fn as_str(&self) -> &str {
+        match self {
+            DiagnosticMessage::Str(msg) => msg,
+            DiagnosticMessage::FluentIdentifier(..) => unimplemented!(),
+        }
+    }
+
+    /// Convert `DiagnosticMessage` to an owned `String`.
+    pub fn to_string(self) -> String {
+        match self {
+            DiagnosticMessage::Str(msg) => msg,
+            DiagnosticMessage::FluentIdentifier(..) => unimplemented!(),
+        }
+    }
+}
+
+/// A span together with some additional data.
+#[derive(Clone, Debug)]
+pub struct SpanLabel {
+    /// The span we are going to include in the final snippet.
+    pub span: Span,
+
+    /// Is this a primary span? This is the "locus" of the message,
+    /// and is indicated with a `^^^^` underline, versus `----`.
+    pub is_primary: bool,
+
+    /// What label should we attach to this span (if any)?
+    pub label: Option<DiagnosticMessage>,
+}
+
+/// A collection of `Span`s.
+///
+/// Spans have two orthogonal attributes:
+///
+/// - They can be *primary spans*. In this case they are the locus of
+///   the error, and would be rendered with `^^^`.
+/// - They can have a *label*. In this case, the label is written next
+///   to the mark in the snippet when we render.
+#[derive(Clone, Debug, Hash, PartialEq, Eq, Encodable, Decodable)]
+pub struct MultiSpan {
+    primary_spans: Vec<Span>,
+    span_labels: Vec<(Span, DiagnosticMessage)>,
+}
+
+impl MultiSpan {
+    #[inline]
+    pub fn new() -> MultiSpan {
+        MultiSpan { primary_spans: vec![], span_labels: vec![] }
+    }
+
+    pub fn from_span(primary_span: Span) -> MultiSpan {
+        MultiSpan { primary_spans: vec![primary_span], span_labels: vec![] }
+    }
+
+    pub fn from_spans(mut vec: Vec<Span>) -> MultiSpan {
+        vec.sort();
+        MultiSpan { primary_spans: vec, span_labels: vec![] }
+    }
+
+    pub fn push_span_label(&mut self, span: Span, label: String) {
+        self.span_labels.push((span, DiagnosticMessage::Str(label)));
+    }
+
+    pub fn push_span_message(&mut self, span: Span, message: DiagnosticMessage) {
+        self.span_labels.push((span, message));
+    }
+
+    /// Selects the first primary span (if any).
+    pub fn primary_span(&self) -> Option<Span> {
+        self.primary_spans.first().cloned()
+    }
+
+    /// Returns all primary spans.
+    pub fn primary_spans(&self) -> &[Span] {
+        &self.primary_spans
+    }
+
+    /// Returns `true` if any of the primary spans are displayable.
+    pub fn has_primary_spans(&self) -> bool {
+        self.primary_spans.iter().any(|sp| !sp.is_dummy())
+    }
+
+    /// Returns `true` if this contains only a dummy primary span with any hygienic context.
+    pub fn is_dummy(&self) -> bool {
+        let mut is_dummy = true;
+        for span in &self.primary_spans {
+            if !span.is_dummy() {
+                is_dummy = false;
+            }
+        }
+        is_dummy
+    }
+
+    /// Replaces all occurrences of one Span with another. Used to move `Span`s in areas that don't
+    /// display well (like std macros). Returns whether replacements occurred.
+    pub fn replace(&mut self, before: Span, after: Span) -> bool {
+        let mut replacements_occurred = false;
+        for primary_span in &mut self.primary_spans {
+            if *primary_span == before {
+                *primary_span = after;
+                replacements_occurred = true;
+            }
+        }
+        for span_label in &mut self.span_labels {
+            if span_label.0 == before {
+                span_label.0 = after;
+                replacements_occurred = true;
+            }
+        }
+        replacements_occurred
+    }
+
+    /// Returns the strings to highlight. We always ensure that there
+    /// is an entry for each of the primary spans -- for each primary
+    /// span `P`, if there is at least one label with span `P`, we return
+    /// those labels (marked as primary). But otherwise we return
+    /// `SpanLabel` instances with empty labels.
+    pub fn span_labels(&self) -> Vec<SpanLabel> {
+        let is_primary = |span| self.primary_spans.contains(&span);
+
+        let mut span_labels = self
+            .span_labels
+            .iter()
+            .map(|&(span, ref label)| SpanLabel {
+                span,
+                is_primary: is_primary(span),
+                label: Some(label.clone()),
+            })
+            .collect::<Vec<_>>();
+
+        for &span in &self.primary_spans {
+            if !span_labels.iter().any(|sl| sl.span == span) {
+                span_labels.push(SpanLabel { span, is_primary: true, label: None });
+            }
+        }
+
+        span_labels
+    }
+
+    /// Returns `true` if any of the span labels is displayable.
+    pub fn has_span_labels(&self) -> bool {
+        self.span_labels.iter().any(|(sp, _)| !sp.is_dummy())
+    }
+}
+
+impl From<Span> for MultiSpan {
+    fn from(span: Span) -> MultiSpan {
+        MultiSpan::from_span(span)
+    }
+}
+
+impl From<Vec<Span>> for MultiSpan {
+    fn from(spans: Vec<Span>) -> MultiSpan {
+        MultiSpan::from_spans(spans)
+    }
+}