feat: Add parsing for expressions.

This commit adds support for parsing expression using the pratt parsing
approach.

src/diagnostic.rs

@@ -0,0 +1,196 @@
+use std::{fmt::Display, path::Path, process::exit};
+
+use yansi::Paint;
+
+use crate::token::Span;
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord)]
+pub enum Severity {
+    Note,
+    Warning,
+    Error,
+    Critical,
+}
+
+impl Display for Severity {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        match self {
+            Severity::Note => write!(f, "{}", "Note".bold().cyan()),
+            Severity::Warning => write!(f, "{}", "Warning".bold().yellow()),
+            Severity::Error => write!(f, "{}", "Error".bold().red()),
+            Severity::Critical => write!(f, "{}", "Critical".bold().magenta()),
+        }
+    }
+}
+
+pub struct Diagnostic {
+    pub severity: Severity,
+    pub span: Option<Span>,
+    pub message: String,
+    pub labels: Vec<(Span, String)>,
+}
+
+impl Diagnostic {
+    pub fn new(severity: Severity, message: impl ToString) -> Self {
+        Self {
+            severity,
+            span: None,
+            message: message.to_string(),
+            labels: Vec::new(),
+        }
+    }
+
+    pub fn with_span(mut self, span: Span) -> Self {
+        self.span = Some(span);
+        self
+    }
+
+    pub fn add_label(mut self, span: Span, message: impl ToString) -> Self {
+        self.labels.push((span, message.to_string()));
+        self
+    }
+
+    pub fn report(self, file_name: &Path, source: &str) {
+        eprintln!("{}: {}", self.severity, self.message.bold());
+
+        let Some(primary_span) = self.span else {
+            eprintln!("  {} {}", "-->".bright_black(), file_name.display());
+            if self.severity == Severity::Critical {
+                exit(-1);
+            }
+            return;
+        };
+
+        // Guard: no source context available (e.g. critical error before any
+        // file is read).
+        if source.is_empty() || primary_span.start as usize >= source.len() {
+            eprintln!("  {} {}", "-->".bright_black(), file_name.display());
+            if self.severity == Severity::Critical {
+                exit(-1);
+            }
+            return;
+        }
+
+        let (primary_line, primary_col) = get_line_col(source, primary_span.start);
+
+        // Partition labels: those on the *exact same span* as the primary are
+        // merged into the primary underline as inline text.  All others are
+        // rendered as separate snippets below the primary.
+        let (same_span, other_span): (Vec<_>, Vec<_>) = self
+            .labels
+            .into_iter()
+            .partition(|(s, _)| *s == primary_span);
+
+        let primary_label: Option<String> = same_span.into_iter().next().map(|(_, m)| m);
+
+        // Gutter must be wide enough for the highest line number we'll print.
+        let max_line = other_span
+            .iter()
+            .filter(|(s, _)| (s.start as usize) < source.len())
+            .map(|(s, _)| get_line_col(source, s.start).0)
+            .fold(primary_line, usize::max);
+        let gutter_w = count_digits(max_line);
+        let pad = " ".repeat(gutter_w);
+
+        // " --> file:line:col"
+        eprintln!(
+            "{} {}:{}:{}",
+            format!("{pad} -->").bright_black(),
+            file_name.display(),
+            primary_line,
+            primary_col,
+        );
+        eprintln!("{}", format!("{pad} |").bright_black());
+
+        // Primary snippet.
+        render_snippet(
+            source,
+            primary_span,
+            primary_label.as_deref(),
+            gutter_w,
+            self.severity,
+        );
+
+        // Additional-context labels (different locations).
+        for (span, msg) in &other_span {
+            if (span.start as usize) < source.len() {
+                render_snippet(source, *span, Some(msg.as_str()), gutter_w, Severity::Note);
+            }
+        }
+
+        eprintln!("{}", format!("{pad} |").bright_black());
+
+        if self.severity == Severity::Critical {
+            exit(-1);
+        }
+    }
+}
+
+/// Render a single source-line snippet: the numbered source line followed by
+/// a `^^^` underline.  When `label` is `Some`, the text is appended after the
+/// carets on the same line.
+fn render_snippet(
+    source: &str,
+    span: Span,
+    label: Option<&str>,
+    gutter_w: usize,
+    severity: Severity,
+) {
+    let (line_num, _) = get_line_col(source, span.start);
+    let (line_start, line_content) = get_line_content(source, span.start);
+
+    let pad = " ".repeat(gutter_w);
+    let bar = format!("{}", "|".bright_black());
+    let line_num_str = format!("{:>width$}", line_num, width = gutter_w);
+
+    // "N | source text"
+    eprintln!("{} {bar} {line_content}", line_num_str.bright_black());
+
+    // Caret underline, clamped to the current line.
+    let col_offset = span.start as usize - line_start;
+    let line_end_byte = line_start + line_content.len();
+    let underline_len = (span.end as usize)
+        .min(line_end_byte)
+        .saturating_sub(span.start as usize)
+        .max(1);
+
+    let spaces = " ".repeat(col_offset);
+    let carets = "^".repeat(underline_len);
+    let colored_carets = paint_severity(&carets, severity);
+    let label_text = label
+        .map(|l| format!(" {}", paint_severity(l, severity)))
+        .unwrap_or_default();
+
+    // "  | ^^^label"
+    eprintln!("{pad} {bar} {spaces}{colored_carets}{label_text}");
+}
+
+fn paint_severity(s: &str, severity: Severity) -> String {
+    match severity {
+        Severity::Note => format!("{}", s.bold().bright_cyan()),
+        Severity::Warning => format!("{}", s.bold().bright_yellow()),
+        Severity::Error | Severity::Critical => format!("{}", s.bold().bright_red()),
+    }
+}
+
+fn count_digits(n: usize) -> usize {
+    format!("{n}").len()
+}
+
+/// Returns `(line_start_byte, line_content)` for the line that contains
+/// `position`.  The returned content does *not* include the trailing newline.
+fn get_line_content(source: &str, position: u32) -> (usize, &str) {
+    let pos = position as usize;
+    let line_start = source[..pos].rfind('\n').map(|i| i + 1).unwrap_or(0);
+    let rest = &source[line_start..];
+    let line_len = rest.find('\n').unwrap_or(rest.len());
+    (line_start, &rest[..line_len])
+}
+
+fn get_line_col(source: &str, position: u32) -> (usize, usize) {
+    let prefix = &source[..position as usize];
+    let line = prefix.bytes().filter(|&b| b == b'\n').count() + 1;
+    let line_start_byte = prefix.rfind('\n').map(|i| i + 1).unwrap_or(0);
+    let col = prefix[line_start_byte..].chars().count() + 1;
+    (line, col)
+}