Try to improve format! error messages

Instead of just saying "unterminated format string" and friends, instead print
information about what was expected and what was found.

Closes #9931

Author: alexcrichton

src/libstd/fmt/parse.rs

@@ -170,9 +170,7 @@ impl<'self> Iterator<Piece<'self>> for Parser<'self> {
             Some((_, '{')) => {
                 self.cur.next();
                 let ret = Some(Argument(self.argument()));
-                if !self.consume('}') {
-                    self.err(~"unterminated format string");
-                }
+                self.must_consume('}');
                 ret
             }
             Some((pos, '\\')) => {
@@ -223,6 +221,25 @@ impl<'self> Parser<'self> {
         }
     }
 
+    /// Forces consumption of the specified character. If the character is not
+    /// found, an error is emitted.
+    fn must_consume(&mut self, c: char) {
+        self.ws();
+        match self.cur.clone().next() {
+            Some((_, maybe)) if c == maybe => {
+                self.cur.next();
+            }
+            Some((_, other)) => {
+                parse_error::cond.raise(
+                    format!("expected `{}` but found `{}`", c, other));
+            }
+            None => {
+                parse_error::cond.raise(
+                    format!("expected `{}` but string was terminated", c));
+            }
+        }
+    }
+
     /// Attempts to consume any amount of whitespace followed by a character
     fn wsconsume(&mut self, c: char) -> bool {
         self.ws(); self.consume(c)
@@ -386,15 +403,11 @@ impl<'self> Parser<'self> {
         self.ws();
         match self.word() {
             "select" => {
-                if !self.wsconsume(',') {
-                    self.err(~"`select` must be followed by `,`");
-                }
+                self.must_consume(',');
                 Some(self.select())
             }
             "plural" => {
-                if !self.wsconsume(',') {
-                    self.err(~"`plural` must be followed by `,`");
-                }
+                self.must_consume(',');
                 Some(self.plural())
             }
             "" => {
@@ -420,15 +433,11 @@ impl<'self> Parser<'self> {
                 self.err(~"cannot have an empty selector");
                 break
             }
-            if !self.wsconsume('{') {
-                self.err(~"selector must be followed by `{`");
-            }
+            self.must_consume('{');
             self.depth += 1;
             let pieces = self.collect();
             self.depth -= 1;
-            if !self.wsconsume('}') {
-                self.err(~"selector case must be terminated by `}`");
-            }
+            self.must_consume('}');
             if selector == "other" {
                 if !other.is_none() {
                     self.err(~"multiple `other` statements in `select");
@@ -475,9 +484,7 @@ impl<'self> Parser<'self> {
                             self.err(format!("expected `offset`, found `{}`",
                                              word));
                         } else {
-                            if !self.consume(':') {
-                                self.err(~"`offset` must be followed by `:`");
-                            }
+                            self.must_consume(':');
                             match self.integer() {
                                 Some(i) => { offset = Some(i); }
                                 None => {
@@ -524,15 +531,11 @@ impl<'self> Parser<'self> {
                     }
                 }
             };
-            if !self.wsconsume('{') {
-                self.err(~"selector must be followed by `{`");
-            }
+            self.must_consume('{');
             self.depth += 1;
             let pieces = self.collect();
             self.depth -= 1;
-            if !self.wsconsume('}') {
-                self.err(~"selector case must be terminated by `}`");
-            }
+            self.must_consume('}');
             if isother {
                 if !other.is_none() {
                     self.err(~"multiple `other` statements in `select");

src/test/compile-fail/ifmt-bad-arg.rs

@@ -36,20 +36,20 @@ fn main() {
 
     // bad syntax of the format string
 
-    format!("{"); //~ ERROR: unterminated format string
+    format!("{"); //~ ERROR: expected `}` but string was terminated
     format!("\\ "); //~ ERROR: invalid escape
     format!("\\"); //~ ERROR: expected an escape
 
     format!("{0, }", 1); //~ ERROR: expected method
     format!("{0, foo}", 1); //~ ERROR: unknown method
-    format!("{0, select}", "a"); //~ ERROR: must be followed by
-    format!("{0, plural}", 1); //~ ERROR: must be followed by
+    format!("{0, select}", "a"); //~ ERROR: expected `,` but found `}`
+    format!("{0, plural}", 1); //~ ERROR: expected `,` but found `}`
 
-    format!("{0, select, a{{}", 1); //~ ERROR: must be terminated
+    format!("{0, select, a{{}", 1); //~ ERROR: expected `}` but string was terminated
     format!("{0, select, {} other{}}", "a"); //~ ERROR: empty selector
     format!("{0, select, other{} other{}}", "a"); //~ ERROR: multiple `other`
     format!("{0, plural, offset: other{}}", "a"); //~ ERROR: must be an integer
-    format!("{0, plural, offset 1 other{}}", "a"); //~ ERROR: be followed by `:`
+    format!("{0, plural, offset 1 other{}}", "a"); //~ ERROR: expected `:` but found `1`
     format!("{0, plural, =a{} other{}}", "a"); //~ ERROR: followed by an integer
     format!("{0, plural, a{} other{}}", "a"); //~ ERROR: unexpected plural
     format!("{0, select, a{}}", "a"); //~ ERROR: must provide an `other`