Disallow problematic identifiers

These newly disallowed identifiers were never actually supported in
practice and are problematic on multiple levels:

1. An identifier like `_` or `__` are bad names that don't help users
   understand an API.
2. Identifiers like `_` are problematic for languages like Rust that use
   this character for pattern matching.

docs/source/1.0/spec/core/idl.rst

@@ -216,6 +216,12 @@ The Smithy IDL is defined by the following ABNF:
     trait_structure_kvp :`node_object_key` `ws` ":" `ws` `node_value`
     apply_statement :"apply" `ws` `shape_id` `ws` `trait` `br`
 
+.. rubric:: Shape ID
+
+.. seealso::
+
+    Refer to :ref:`shape-id` for the ABNF grammar of shape IDs.
+
 
 .. _comments:
 

docs/source/1.0/spec/core/model.rst

@@ -445,7 +445,9 @@ Shape IDs are formally defined by the following ABNF:
     root_shape_id          :`absolute_root_shape_id` / `identifier`
     absolute_root_shape_id :`namespace` "#" `identifier`
     namespace              :`identifier` *("." `identifier`)
-    identifier             :(ALPHA / "_") *(ALPHA / DIGIT / "_")
+    identifier             :identifier_start *identifier_chars
+    identifier_start       :*"_" ALPHA
+    identifier_chars       :ALPHA / DIGIT / "_"
     shape_id_member        :"$" `identifier`
 
 .. rubric:: Best practices for defining shape names

smithy-model/src/main/java/software/amazon/smithy/model/loader/ParserUtils.java

@@ -147,22 +147,37 @@ public static void consumeNamespace(SimpleParser parser) {
      * Expects and skips over a Smithy identifier production.
      *
      * <pre>
-     *     identifier = (ALPHA / "_") *(ALPHA / DIGIT / "_")
+     *     identifier       = identifier_start *identifier_chars
+     *     identifier_start = *"_" ALPHA
+     *     identifier_chars = ALPHA / DIGIT / "_"
      * </pre>
      *
      * @param parser Parser to consume tokens from.
      */
     public static void consumeIdentifier(SimpleParser parser) {
-        // (ALPHA / "_")
-        if (!isIdentifierStart(parser.peek())) {
-            throw parser.syntax("Expected a valid identifier character, but found '"
-                                + parser.peekSingleCharForMessage() + '\'');
+        // Parse identifier_start
+        char c = parser.peek();
+        if (c == '_') {
+            parser.consumeUntilNoLongerMatches(next -> next == '_');
+            if (!ParserUtils.isValidIdentifierCharacter(parser.peek())) {
+                throw invalidIdentifier(parser);
+            }
+        } else if (!isAlphabetic(c)) {
+            throw invalidIdentifier(parser);
         }
 
-        // *(ALPHA / DIGIT / "_")
+        // Skip the first character since it's known to be valid.
+        parser.skip();
+
+        // Parse identifier_chars
         parser.consumeUntilNoLongerMatches(ParserUtils::isValidIdentifierCharacter);
     }
 
+    private static RuntimeException invalidIdentifier(SimpleParser parser) {
+        throw parser.syntax("Expected a valid identifier character, but found '"
+                            + parser.peekSingleCharForMessage() + '\'');
+    }
+
     /**
      * Returns true if the given character is allowed in an identifier.
      *
@@ -180,7 +195,7 @@ public static boolean isValidIdentifierCharacter(char c) {
      * @return Returns true if the character can start an identifier.
      */
     public static boolean isIdentifierStart(char c) {
-        return c == '_' || (c >= 'A' && c <= 'Z') || (c >= 'a' && c <= 'z');
+        return c == '_' ||  isAlphabetic(c);
     }
 
     /**
@@ -192,4 +207,15 @@ public static boolean isIdentifierStart(char c) {
     public static boolean isDigit(char c) {
         return c >= '0' && c <= '9';
     }
+
+    /**
+     * Returns true if the given character is an alphabetic character
+     * A-Z, a-z. This is a stricter version of {@link Character#isAlphabetic}.
+     *
+     * @param c Character to check.
+     * @return Returns true if the character is an alphabetic character.
+     */
+    public static boolean isAlphabetic(char c) {
+        return (c >= 'A' && c <= 'Z') || (c >= 'a' && c <= 'z');
+    }
 }

smithy-model/src/main/java/software/amazon/smithy/model/shapes/ShapeId.java

@@ -85,29 +85,29 @@ public static ShapeId from(String id) {
     public static boolean isValidNamespace(CharSequence namespace) {
         if (namespace == null) {
             return false;
+        } else if (namespace.equals(Prelude.NAMESPACE)) {
+            // Shortcut for prelude namespaces.
+            return true;
         }
 
-        // Shortcut for prelude namespaces.
-        if (namespace.equals(Prelude.NAMESPACE)) {
-            return true;
+        int length = namespace.length();
+        if (length == 0) {
+            return false;
         }
 
-        boolean start = true;
-        for (int position = 0; position < namespace.length(); position++) {
-            char c = namespace.charAt(position);
-            if (start) {
-                start = false;
-                if (!ParserUtils.isIdentifierStart(c)) {
-                    return false;
-                }
-            } else if (c == '.') {
-                start = true;
-            } else if (!ParserUtils.isValidIdentifierCharacter(c)) {
+        int position = 0;
+        while (true) {
+            position = parseIdentifier(namespace, position);
+            if (position == -1) { // Bad: did not parse a valid identifier.
                 return false;
-            }
+            } else if (position == length) { // Good: parsed and reached the end.
+                return true;
+            } else if (namespace.charAt(position) != '.') { // Bad: invalid character.
+                return false;
+            } else if (++position >= length) { // Bad: trailing '.'
+                return false;
+            } // continue parsing after '.', expecting an identifier.
         }
-
-        return !start;
     }
 
     /**
@@ -117,21 +117,40 @@ public static boolean isValidNamespace(CharSequence namespace) {
      * @return Returns true if this is a valid identifier.
      */
     public static boolean isValidIdentifier(CharSequence identifier) {
-        if (identifier == null || identifier.length() == 0) {
-            return false;
+        return parseIdentifier(identifier, 0) == identifier.length();
+    }
+
+    private static int parseIdentifier(CharSequence identifier, int offset) {
+        if (identifier == null || identifier.length() <= offset) {
+            return -1;
         }
 
-        if (!ParserUtils.isIdentifierStart(identifier.charAt(0))) {
-            return false;
+        // Parse the required identifier_start production.
+        char startingChar = identifier.charAt(offset);
+        if (startingChar == '_') {
+            while (identifier.charAt(offset) == '_') {
+                offset++;
+            }
+            if (!ParserUtils.isValidIdentifierCharacter(identifier.charAt(offset))) {
+                return -1;
+            }
+            offset++;
+        } else if (!ParserUtils.isAlphabetic(startingChar)) {
+            return -1;
         }
 
-        for (int i = 1; i < identifier.length(); i++) {
-            if (!ParserUtils.isValidIdentifierCharacter(identifier.charAt(i))) {
-                return false;
+        // Parse the optional identifier_chars production.
+        while (offset < identifier.length()) {
+            if (!ParserUtils.isValidIdentifierCharacter(identifier.charAt(offset))) {
+                // Return the position of the character that stops the identifier.
+                // This is either an invalid case (e.g., isValidIdentifier), or
+                // just the marker needed for isValidNamespace to find '.'.
+                return offset;
             }
+            offset++;
         }
 
-        return true;
+        return offset;
     }
 
     /**

smithy-model/src/test/java/software/amazon/smithy/model/selector/SelectorTest.java

@@ -941,7 +941,8 @@ public void parsesValidVariableAccess() {
                 "${\nfoo\n}\n",
                 "${a}",
                 "${a_b_c}",
-                "${_}");
+                "${_a}",
+                "${__a}");
 
         for (String expr : exprs) {
             Selector.parse(expr);
@@ -956,7 +957,9 @@ public void detectsInvalidVariableAccess() {
                 "${}",
                 "$}",
                 "${a",
-                "${*}");
+                "${*}",
+                "${_}",
+                "${__}");
 
         for (String expr : exprs) {
             Assertions.assertThrows(SelectorSyntaxException.class, () -> Selector.parse(expr));

smithy-model/src/test/java/software/amazon/smithy/model/shapes/ShapeIdTest.java

@@ -170,7 +170,6 @@ public static Collection<Object[]> shapeIdData() {
                 { "Mixed.case#name", false},
                 { "_foo.baz#name", false},
                 { "__foo.baz#name", false},
-                { "a._.b#name", false},
 
                 // invalid namespaces
                 { "#name", true},
@@ -179,6 +178,8 @@ public static Collection<Object[]> shapeIdData() {
                 { ".name.space#name", true},
                 { "name-space#name", true},
                 { "1namespace.foo#name", true},
+                { "a._.b#name", true},
+                { "a.____.b#name", true},
 
                 // valid shape names
                 { "ns.foo#shapename", false},

smithy-model/src/test/resources/software/amazon/smithy/model/loader/invalid/invalid-unquoted-shape-id.smithy

@@ -1,5 +1,5 @@
 // Parse error at line 3, column 19 near `#\n`: Expected a valid identifier character, but found '#'
 
-metadata abc = __$#
+metadata abc = aa$#
 
 namespace foo.baz

smithy-model/src/test/resources/software/amazon/smithy/model/loader/invalid/shape-name-syntax2.smithy

@@ -0,0 +1,4 @@
+// Parse error at line 4, column 11 near `%I`: Expected a valid identifier character, but found '%'
+namespace com.foo
+
+structure %Invalid {}

smithy-model/src/test/resources/software/amazon/smithy/model/loader/invalid/shape-name-syntax3.smithy

@@ -0,0 +1,4 @@
+// Parse error at line 4, column 12 near ` `: Expected a valid identifier character, but found ' '
+namespace com.foo
+
+structure _ {}

smithy-model/src/test/resources/software/amazon/smithy/model/loader/invalid/use/use-invalid-namespace.smithy

@@ -3,4 +3,4 @@ $version: "1.0"
 
 namespace smithy.example
 
-use _$#Bar
+use a$#Bar