Auto merge of #30064 - fhartwig:macro-suggestions, r=sanxiyn

Fixes #13677
This does the same sort of suggestion for misspelt macros that we already do for misspelt identifiers.
Example. Compiling this program:

```rust
macro_rules! foo {
    ($e:expr) => ( $e )
}

fn main() {
    fob!("hello!");
}
```

gives the following error message:

```
/Users/mcp/temp/test.rs:7:5: 7:8 error: macro undefined: 'fob!'
/Users/mcp/temp/test.rs:7     fob!("hello!");
                              ^~~
/Users/mcp/temp/test.rs:7:5: 7:8 help: did you mean `foo`?
/Users/mcp/temp/test.rs:7     fob!("hello!");
```

I had to move the levenshtein distance function into libsyntax for this. Maybe this should live somewhere else (some utility crate?), but I couldn't find a crate to put it in that is imported by libsyntax and the other rustc crates.

2 files changed, 16 insertions, 0 deletions

diff --git a/src/libsyntax/ext/base.rs b/src/libsyntax/ext/base.rs
index 55f0fa5675a..cf6881ab650 100644
--- a/src/libsyntax/ext/base.rs
+++ b/src/libsyntax/ext/base.rs
@@ -24,6 +24,7 @@ use parse::token;
 use parse::token::{InternedString, intern, str_to_ident};
 use ptr::P;
 use util::small_vector::SmallVector;
+use util::lev_distance::{lev_distance, max_suggestion_distance};
 use ext::mtwt;
 use fold::Folder;
 
@@ -776,6 +777,20 @@ impl<'a> ExtCtxt<'a> {
     pub fn name_of(&self, st: &str) -> ast::Name {
         token::intern(st)
     }
+
+    pub fn suggest_macro_name(&mut self, name: &str, span: Span) {
+        let mut min: Option<(Name, usize)> = None;
+        let max_dist = max_suggestion_distance(name);
+        for macro_name in self.syntax_env.names.iter() {
+            let dist = lev_distance(name, &macro_name.as_str());
+            if dist <= max_dist && (min.is_none() || min.unwrap().1 > dist) {
+                min = Some((*macro_name, dist));
+            }
+        }
+        if let Some((suggestion, _)) = min {
+            self.fileline_help(span, &format!("did you mean `{}!`?", suggestion));
+        }
+    }
 }
 
 /// Extract a string literal from the macro expanded version of `expr`,
diff --git a/src/libsyntax/ext/expand.rs b/src/libsyntax/ext/expand.rs
index 9b1a7a50201..01e72cf2a8d 100644
--- a/src/libsyntax/ext/expand.rs
+++ b/src/libsyntax/ext/expand.rs
@@ -191,6 +191,7 @@ fn expand_mac_invoc<T, F, G>(mac: ast::Mac,
                 pth.span,
                 &format!("macro undefined: '{}!'",
                         &extname));
+            fld.cx.suggest_macro_name(&extname.as_str(), pth.span);
 
             // let compilation continue
             None