Properly calculate best failure in macro matching

Previously, we used spans. This was not good. Sometimes, the span of the
token that failed to match may come from a position later in the file
which has been transcribed into a token stream way earlier in the file.
If precisely this token fails to match, we think that it was the best
match because its span is so high, even though other arms might have
gotten further in the token stream.

We now try to properly use the location in the token stream.

2 files changed, 32 insertions, 0 deletions

diff --git a/src/test/ui/macros/best-failure.rs b/src/test/ui/macros/best-failure.rs
new file mode 100644
index 00000000000..bbdd465d5ec
--- /dev/null
+++ b/src/test/ui/macros/best-failure.rs
@@ -0,0 +1,11 @@
+macro_rules! number {
+    (neg false, $self:ident) => { $self };
+    ($signed:tt => $ty:ty;) => {
+        number!(neg $signed, $self);
+        //~^ ERROR no rules expected the token `$`
+    };
+}
+
+number! { false => u8; }
+
+fn main() {}
diff --git a/src/test/ui/macros/best-failure.stderr b/src/test/ui/macros/best-failure.stderr
new file mode 100644
index 00000000000..a52fc5e3da6
--- /dev/null
+++ b/src/test/ui/macros/best-failure.stderr
@@ -0,0 +1,21 @@
+error: no rules expected the token `$`
+  --> $DIR/best-failure.rs:4:30
+   |
+LL | macro_rules! number {
+   | ------------------- when calling this macro
+...
+LL |         number!(neg $signed, $self);
+   |                              ^^^^^ no rules expected this token in macro call
+...
+LL | number! { false => u8; }
+   | ------------------------ in this macro invocation
+   |
+note: while trying to match meta-variable `$self:ident`
+  --> $DIR/best-failure.rs:2:17
+   |
+LL |     (neg false, $self:ident) => { $self };
+   |                 ^^^^^^^^^^^
+   = note: this error originates in the macro `number` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: aborting due to previous error
+