Don't emit general/vague errors if we already diagnosed something more specific

Makes error messages less noisy to read

Author: hsutter

regression-tests/test-results/pure2-bounds-safety-pointer-arithmetic-error.cpp2.output

@@ -1,8 +1,3 @@
 pure2-bounds-safety-pointer-arithmetic-error.cpp2...
-pure2-bounds-safety-pointer-arithmetic-error.cpp2(15,13): error: 'delete' and owning raw pointers are not supported in Cpp2
-pure2-bounds-safety-pointer-arithmetic-error.cpp2(15,13): error:   - use unique.new<T>, shared.new<T>, or gc.new<T> instead (in that order)
-pure2-bounds-safety-pointer-arithmetic-error.cpp2(9,25): error: invalid while loop body (at '{')
-pure2-bounds-safety-pointer-arithmetic-error.cpp2(3,3): error: ill-formed initializer (at '{')
-pure2-bounds-safety-pointer-arithmetic-error.cpp2(2,1): error: unexpected text at end of Cpp2 code section (at 'main')
-pure2-bounds-safety-pointer-arithmetic-error.cpp2(1,0): error: parse failed for section starting here
+pure2-bounds-safety-pointer-arithmetic-error.cpp2(15,13): error: 'delete' and owning raw pointers are not supported in Cpp2 - use unique.new<T>, shared.new<T>, or gc.new<T> instead (in that order)
 

source/common.h

@@ -236,9 +236,18 @@ struct error
     source_position where;
     std::string     msg;
     bool            internal = false;
+    bool            fallback = false;   // only emit this message if there was nothing better
 
-    error( source_position w, std::string const& m, bool i = false )
-        : where{w}, msg{m}, internal{i}
+    error(
+        source_position    w,
+        std::string const& m,
+        bool               i = false,
+        bool               f = false
+    )
+        : where{w}
+        , msg{m}
+        , internal{i}
+        , fallback{f}
     { }
 
     auto operator==(error const& that)

source/cppfront.cpp

@@ -952,10 +952,10 @@ class cppfront
     //
     cppfront(std::string const& filename)
         : sourcefile{ filename }
-        , source{ errors }
-        , tokens{ errors }
-        , parser{ errors }
-        , sema{ errors }
+        , source    { errors }
+        , tokens    { errors }
+        , parser    { errors }
+        , sema      { errors }
     {
         //  "Constraints enable creativity in the right directions"
         //  sort of applies here
@@ -998,7 +998,9 @@ class cppfront
                     if (!parser.parse(entry, tokens.get_generated())) {
                         errors.emplace_back(
                             source_position(line, 0),
-                            "parse failed for section starting here"
+                            "parse failed for section starting here",
+                            false,
+                            true    // a noisy fallback error message
                         );
                     }
                 }
@@ -4454,7 +4456,7 @@ class cppfront
                     break;case passing_style::copy:
                           case passing_style::forward:
                           default:
-                        errors.emplace_back( n.position(), "ICE: invalid parameter passing style, should have been rejected");
+                        errors.emplace_back( n.position(), "ICE: invalid parameter passing style, should have been rejected", true);
                     }
 
                     switch (this_->mod) {
@@ -4844,8 +4846,20 @@ class cppfront
             printer.abandon();
         }
 
-        error const* prev = nullptr;
-        for (auto&& error : errors) {
+        error const* prev                  = {};
+        bool         print_fallback_errors = true; // true until we find a non-fallback message
+
+        for (auto&& error : errors)
+        {
+            //  Only print fallback error messages if we
+            //  haven't found a better (non-fallback) one yet
+            if (!error.fallback) {
+                print_fallback_errors = false;
+            }
+            if (error.fallback && !print_fallback_errors) {
+                continue;
+            }
+
             //  Suppress adjacent duplicates (e.g., can arise when we
             //  reenter operator= to emit it as an assignment operator)
             if (
@@ -4857,6 +4871,7 @@ class cppfront
             }
             prev = &error;
         }
+
         if (violates_lifetime_safety) {
             std::cerr << "  ==> program violates lifetime safety guarantee - see previous errors\n";
         }

source/lex.h

@@ -349,7 +349,9 @@ auto expand_string_literal(
     if (text.back() != '"') {
         errors.emplace_back(
             source_position( src_pos ),
-            "not a legal string literal"
+            "not a legal string literal",
+            false,
+            true    // a noisy fallback error message
         );
         return {};
     }
@@ -1551,7 +1553,9 @@ auto lex_line(
                             if (std::ssize(s) <= j + 1) {
                                 errors.emplace_back(
                                     source_position( lineno, i ),
-                                    "not a legal string literal"
+                                    "not a legal string literal",
+                                    false,
+                                    true    // a noisy fallback error message
                                 );
                                 return {};
                             }
@@ -1632,17 +1636,13 @@ auto lex_line(
                         if (tokens.back() == "union") {
                             errors.emplace_back(
                                 source_position(lineno, i),
-                                "unsafe 'union's are not supported in Cpp2 - use std::variant instead"
+                                "unsafe 'union's are not supported in Cpp2 - use std::variant instead (or, in the future, the Cpp2 'union' metaclass function, but that is not yet implemented)"
                             );
                         }
                         if (tokens.back() == "delete") {
                             errors.emplace_back(
                                 source_position(lineno, i),
-                                "'delete' and owning raw pointers are not supported in Cpp2"
-                            );
-                            errors.emplace_back(
-                                source_position(lineno, i),
-                                "  - use unique.new<T>, shared.new<T>, or gc.new<T> instead (in that order)"
+                                "'delete' and owning raw pointers are not supported in Cpp2 - use unique.new<T>, shared.new<T>, or gc.new<T> instead (in that order)"
                             );
                         }
                     }
@@ -1653,7 +1653,9 @@ auto lex_line(
                 else if (!isspace(line[i])) {
                     errors.emplace_back(
                         source_position(lineno, i),
-                        std::string("unexpected text '") + line[i] + "'"
+                        std::string("unexpected text '") + line[i] + "'",
+                        false,
+                        true    // a noisy fallback error message
                     );
                 }
             }

source/load.h

@@ -957,7 +957,9 @@ class source
         {
             errors.emplace_back(
                 source_position(lineno_t(std::ssize(lines)), 0),
-                std::string("unexpected error reading source lines - did not reach EOF")
+                std::string("unexpected error reading source lines - did not reach EOF"),
+                false,
+                true    // a noisy fallback error
             );
             return false;
         }