in which we check for confusable Unicodepoints in float literal exponent

The `FatalError.raise()` might seem unmotivated (in most places in
the compiler, `err.emit()` suffices), but it's actually used to
maintain behavior (viz., stop lexing, don't emit potentially spurious
errors looking for the next token after the bad Unicodepoint in the
exponent): the previous revision's `self.err_span_` ultimately calls
`Handler::emit`, which aborts if the `Handler`'s continue_after_error
flag is set, which seems to typically be true during lexing (see
`phase_1_parse_input` and and how `CompileController::basic` has
`continue_parse_after_error: false` in librustc_driver).

Also, let's avoid apostrophes in error messages (the present author
would argue that users expect a reassuringly detached, formal,
above-it-all tone from a Serious tool like a compiler), and use an
RLS-friendly structured suggestion.

Resolves #49746.

src/libsyntax/parse/lexer/mod.rs

@@ -1075,9 +1075,16 @@ fn scan_float_exponent(&mut self) {
                 self.bump();
             }
             if self.scan_digits(10, 10) == 0 {
-                self.err_span_(self.pos,
-                               self.next_pos,
-                               "expected at least one digit in exponent")
+                let mut err = self.struct_span_fatal(
+                    self.pos, self.next_pos,
+                    "expected at least one digit in exponent"
+                );
+                if let Some(ch) = self.ch {
+                    // check for e.g. Unicode minus '−' (Issue #49746)
+                    unicode_chars::check_for_substitution(self, ch, &mut err);
+                }
+                err.emit();
+                FatalError.raise();
             }
         }
     }

src/libsyntax/parse/lexer/unicode_chars.rs

@@ -344,9 +344,9 @@ pub fn check_for_substitution<'a>(reader: &StringReader<'a>,
         match ASCII_ARRAY.iter().find(|&&(c, _)| c == ascii_char) {
             Some(&(ascii_char, ascii_name)) => {
                 let msg =
-                    format!("unicode character '{}' ({}) looks like '{}' ({}), but it's not",
+                    format!("Unicode character '{}' ({}) looks like '{}' ({}), but it is not",
                             ch, u_name, ascii_char, ascii_name);
-                err.span_help(span, &msg);
+                err.span_suggestion(span, &msg, ascii_char.to_string());
             },
             None => {
                 let msg = format!("substitution character not found for '{}'", ch);

src/test/ui/did_you_mean/issue-49746-unicode-confusable-in-float-literal-expt.rs

@@ -0,0 +1,14 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+const UNIVERSAL_GRAVITATIONAL_CONSTANT = 6.674e−11; // m³⋅kg⁻¹⋅s⁻²
+//~^ ERROR expected at least one digit in exponent
+
+fn main() {}

src/test/ui/did_you_mean/issue-49746-unicode-confusable-in-float-literal-expt.stderr

@@ -0,0 +1,12 @@
+error: expected at least one digit in exponent
+  --> $DIR/issue-49746-unicode-confusable-in-float-literal-expt.rs:11:48
+   |
+LL | const UNIVERSAL_GRAVITATIONAL_CONSTANT = 6.674e−11; // m³⋅kg⁻¹⋅s⁻²
+   |                                                ^
+help: Unicode character '−' (Minus Sign) looks like '-' (Minus/Hyphen), but it is not
+   |
+LL | const UNIVERSAL_GRAVITATIONAL_CONSTANT = 6.674e-11; // m³⋅kg⁻¹⋅s⁻²
+   |                                                ^
+
+error: aborting due to previous error
+