Fix logical error with what text is considered whitespace.

There is a logical issue around what counts as leading white-space.
There is code which does a subtraction assuming that no errors will be reported
inside the leading whitespace. However we compute the length of
that whitespace with std::char::is_whitespace and not
rustc_lexer::is_whitespace. The former will include a no-break space while
later will excluded it. We can only safely make the assumption that no errors
will be reported  in whitespace if it is all "Rust Standard" whitespace.
Indeed an error does occur in unicode whitespace if it contains a no-break
space.

2 files changed, 24 insertions, 0 deletions

diff --git a/tests/ui/errors/emitter-overflow-bad-whitespace.rs b/tests/ui/errors/emitter-overflow-bad-whitespace.rs
new file mode 100644
index 00000000000..d024d1f3921
--- /dev/null
+++ b/tests/ui/errors/emitter-overflow-bad-whitespace.rs
@@ -0,0 +1,11 @@
+// Invalid whitespace (not listed here: https://doc.rust-lang.org/reference/whitespace.html
+// e.g. \u{a0}) before any other syntax on the line should not cause any integer overflow
+// in the emitter, even when the terminal width causes the line to be truncated.
+//
+// issue #132918
+
+//@ check-fail
+//@ needs-rustc-debug-assertions
+//@ compile-flags: --diagnostic-width=1
+                                         fn main() {              return;              }
+//~^ ERROR unknown start of token: \u{a0}
diff --git a/tests/ui/errors/emitter-overflow-bad-whitespace.stderr b/tests/ui/errors/emitter-overflow-bad-whitespace.stderr
new file mode 100644
index 00000000000..8c0c097bcb9
--- /dev/null
+++ b/tests/ui/errors/emitter-overflow-bad-whitespace.stderr
@@ -0,0 +1,13 @@
+error: unknown start of token: \u{a0}
+  --> $DIR/emitter-overflow-bad-whitespace.rs:10:1
+   |
+LL |     ...
+   | ^
+   |
+help: Unicode character ' ' (No-Break Space) looks like ' ' (Space), but it is not
+   |
+LL |                                          fn main() {              return;              }
+   | +
+
+error: aborting due to 1 previous error
+