Add beautiful error messages.

This is easy now that we have proper location encoding.

Example:

    test.sp(3) : error 017: undefined symbol "ham"
         3 |     int hello = ham;
    -------------------------^

    test.sp(4) : error 017: undefined symbol "hhello"
         4 |     return hhello
    --------------------^

compiler/errors.cpp

@@ -169,6 +169,7 @@ MessageBuilder::~MessageBuilder()
     auto& cc = CompileContext::get();
 
     ErrorReport report;
+    report.loc = where_;
     report.number = number_;
     if (where_.valid())
         report.fileno = cc.sources()->GetSourceFileIndex(where_);
@@ -180,8 +181,12 @@ MessageBuilder::~MessageBuilder()
     else
         report.file = cc.sources()->opened_files().at(0);
 
+    uint32_t actual_line = cc.sources()->GetLineAndCol(where_, &report.col);
+
+    // Rely on tokline when it's there, but... we should ditch it here, we
+    // have the technology.
     if (where_.valid() && !where_.line)
-        where_.line = cc.sources()->GetLineAndCol(where_, &report.col);
+        where_.line = actual_line;
     report.lineno = std::max(where_.line, 1);
 
     report.type = DeduceErrorType(number_);
@@ -264,9 +269,20 @@ ReportManager::ReportError(ErrorReport&& report)
         cc_.set_must_abort();
 }
 
-void
-ReportManager::DumpErrorReport(bool clear)
-{
+void DumpDiagnostic(FILE* fp, const ErrorReport& report) {
+    auto line = report.file->GetLine(report.lineno);
+    while (!line.empty() && (line.back() == '\r' || line.back() == '\n'))
+        line.pop_back();
+
+    fprintf(fp, "%6u | %s\n", report.lineno, line.c_str());
+
+    uint32_t num_dashes = 9 + report.col - 1;
+    for (uint32_t i = 0; i < num_dashes; i++)
+        fprintf(fp, "-");
+    fprintf(fp, "^\n");
+}
+
+void ReportManager::DumpErrorReport(bool clear) {
     FILE* stdfp = cc_.options()->use_stderr ? stderr : stdout;
 
     FILE* fp = nullptr;
@@ -282,8 +298,13 @@ ReportManager::DumpErrorReport(bool clear)
         return a.fileno > b.fileno;
     });
 
-    for (const auto& report : error_list_)
+    for (const auto& report : error_list_) {
         fprintf(fp, "%s", report.message.c_str());
+        if (report.loc.valid())
+            DumpDiagnostic(fp, report);
+        if (&report != &error_list_.back())
+            fprintf(fp, "\n");
+    }
     fflush(fp);
 
     if (fp != stdfp)

compiler/source-file.cpp

@@ -117,7 +117,8 @@ void SourceFile::ComputeLineExtents() {
     if (!line_extents_.empty())
         return;
 
-    line_extents_.emplace_back(0);
+    tr::vector<uint32_t> extents;
+    extents.emplace_back(0);
     for (uint32_t i = 0; i < data_.size(); i++) {
         if (data_[i] != '\r' && data_[i] != '\n')
             continue;
@@ -126,10 +127,13 @@ void SourceFile::ComputeLineExtents() {
             if (i + 1 < data_.size() && data_[i + 1] == '\n')
                 i++;
         }
-        line_extents_.emplace_back(i + 1);
+        extents.emplace_back(i + 1);
     }
 
-    line_extents_.shrink_to_fit();
+
+    line_extents_.resize(extents.size());
+    for (size_t i = 0; i < extents.size(); i++)
+        line_extents_[i] = extents[i];
 }
 
 bool SourceFile::OffsetToLineAndCol(uint32_t offset, uint32_t* line, uint32_t* col) {
@@ -178,4 +182,32 @@ bool SourceFile::OffsetToLineAndCol(uint32_t offset, uint32_t* line, uint32_t* c
     return false;
 }
 
+bool SourceFile::OffsetOfLine(uint32_t line, uint32_t* offset) {
+    ComputeLineExtents();
+
+    uint32_t line_index = line - 1;
+    if (line_index > line_extents_.size())
+        return false;
+
+    if (line_index == line_extents_.size())
+        *offset = data_.size();
+    else
+        *offset = line_extents_[line_index];
+    return true;
+}
+
+tr::string SourceFile::GetLine(uint32_t line) {
+    ComputeLineExtents();
+
+    uint32_t offset;
+    if (!OffsetOfLine(line, &offset))
+        return {};
+
+    uint32_t end;
+    if (!OffsetOfLine(line + 1, &end))
+        end = data_.size();
+
+    return data_.substr(offset, end - offset);
+}
+
 } // namespace sp

compiler/source-file.h

@@ -61,6 +61,8 @@ class SourceFile : public std::enable_shared_from_this<SourceFile>
     std::shared_ptr<SourceFile> to_shared() { return shared_from_this(); }
 
     bool OffsetToLineAndCol(uint32_t offset, uint32_t* line, uint32_t* col = nullptr);
+    bool OffsetOfLine(uint32_t line, uint32_t* offset);
+    tr::string GetLine(uint32_t line);
 
   private:
     bool Open(const std::string& file_name);