Rollup merge of rust-lang#56572 - kevgrasso:let_self_err_dev, r=estebank

Contexually dependent error message for E0424 when value is assigned to "self"

This is an improvement for pull request rust-lang#54495 referencing issue rust-lang#54369. If the "self" keyword is assigned a value as though it were a valid identifier, it will now report:
```
let self = "self";
    ^^^^ `self` value is a keyword and may not be bound to variables or shadowed
```
instead of
```
let self = "self";
    ^^^^ `self` value is a keyword only available in methods with `self` parameter
```
If anyone has a better idea for what the error should be I'd be happy to modify it appropriately.

Author: pietroalbini

src/librustc_resolve/lib.rs

@@ -3010,6 +3010,7 @@ impl<'a, 'crateloader: 'a> Resolver<'a, 'crateloader> {
         // Visit all direct subpatterns of this pattern.
         let outer_pat_id = pat.id;
         pat.walk(&mut |pat| {
+            debug!("resolve_pattern pat={:?} node={:?}", pat, pat.node);
             match pat.node {
                 PatKind::Ident(bmode, ident, ref opt_pat) => {
                     // First try to resolve the identifier as some existing
@@ -3166,6 +3167,7 @@ impl<'a, 'crateloader: 'a> Resolver<'a, 'crateloader> {
                  format!("not found in {}", mod_str),
                  item_span)
             };
+
             let code = DiagnosticId::Error(code.into());
             let mut err = this.session.struct_span_err_with_code(base_span, &base_msg, code);
 
@@ -3189,11 +3191,22 @@ impl<'a, 'crateloader: 'a> Resolver<'a, 'crateloader> {
                 return (err, Vec::new());
             }
             if is_self_value(path, ns) {
+                debug!("smart_resolve_path_fragment E0424 source:{:?}", source);
+
                 __diagnostic_used!(E0424);
                 err.code(DiagnosticId::Error("E0424".into()));
-                err.span_label(span, format!("`self` value is a keyword \
-                                               only available in \
-                                               methods with `self` parameter"));
+                err.span_label(span, match source {
+                    PathSource::Pat => {
+                        format!("`self` value is a keyword \
+                                and may not be bound to \
+                                variables or shadowed")
+                    }
+                    _ => {
+                        format!("`self` value is a keyword \
+                                only available in methods \
+                                with `self` parameter")
+                    }
+                });
                 return (err, Vec::new());
             }
 

src/test/ui/error-codes/E0424.rs

@@ -19,4 +19,5 @@ impl Foo {
 }
 
 fn main () {
+    let self = "self"; //~ ERROR E0424
 }

src/test/ui/error-codes/E0424.stderr

@@ -4,6 +4,12 @@ error[E0424]: expected value, found module `self`
 LL |         self.bar(); //~ ERROR E0424
    |         ^^^^ `self` value is a keyword only available in methods with `self` parameter
 
-error: aborting due to previous error
+error[E0424]: expected unit struct/variant or constant, found module `self`
+  --> $DIR/E0424.rs:22:9
+   |
+LL |     let self = "self"; //~ ERROR E0424
+   |         ^^^^ `self` value is a keyword and may not be bound to variables or shadowed
+
+error: aborting due to 2 previous errors
 
 For more information about this error, try `rustc --explain E0424`.