Use correct span for structured suggestion

compiler/rustc_resolve/src/diagnostics.rs

@@ -398,20 +398,30 @@ impl<'a> Resolver<'a> {
                 err.help("use the `|| { ... }` closure form instead");
                 err
             }
-            ResolutionError::AttemptToUseNonConstantValueInConstant(ident, sugg) => {
+            ResolutionError::AttemptToUseNonConstantValueInConstant(ident, sugg, current) => {
                 let mut err = struct_span_err!(
                     self.session,
                     span,
                     E0435,
                     "attempt to use a non-constant value in a constant"
                 );
-                err.span_suggestion(
-                    ident.span,
-                    &sugg,
-                    "".to_string(),
-                    Applicability::MaybeIncorrect,
-                );
-                err.span_label(span, "non-constant value");
+                // let foo =...
+                //     ^^^ given this Span
+                // ------- get this Span to have an applicable suggestion
+                let sp =
+                    self.session.source_map().span_extend_to_prev_str(ident.span, current, true);
+                if sp.lo().0 == 0 {
+                    err.span_label(ident.span, &format!("this would need to be a `{}`", sugg));
+                } else {
+                    let sp = sp.with_lo(BytePos(sp.lo().0 - current.len() as u32));
+                    err.span_suggestion(
+                        sp,
+                        &format!("consider using `{}` instead of `{}`", sugg, current),
+                        format!("{} {}", sugg, ident),
+                        Applicability::MaybeIncorrect,
+                    );
+                    err.span_label(span, "non-constant value");
+                }
                 err
             }
             ResolutionError::BindingShadowsSomethingUnacceptable(what_binding, name, binding) => {

compiler/rustc_resolve/src/lib.rs

@@ -210,7 +210,11 @@ enum ResolutionError<'a> {
     /// Error E0434: can't capture dynamic environment in a fn item.
     CannotCaptureDynamicEnvironmentInFnItem,
     /// Error E0435: attempt to use a non-constant value in a constant.
-    AttemptToUseNonConstantValueInConstant(Ident, String),
+    AttemptToUseNonConstantValueInConstant(
+        Ident,
+        /* suggestion */ &'static str,
+        /* current */ &'static str,
+    ),
     /// Error E0530: `X` bindings cannot shadow `Y`s.
     BindingShadowsSomethingUnacceptable(&'static str, Symbol, &'a NameBinding<'a>),
     /// Error E0128: type parameters with a default cannot use forward-declared identifiers.
@@ -2614,18 +2618,19 @@ impl<'a> Resolver<'a> {
                                             ConstantItemKind::Const => "const",
                                             ConstantItemKind::Static => "static",
                                         };
-                                        let sugg = format!(
-                                            "consider using `let` instead of `{}`",
-                                            kind_str
-                                        );
-                                        (span, AttemptToUseNonConstantValueInConstant(ident, sugg))
+                                        (
+                                            span,
+                                            AttemptToUseNonConstantValueInConstant(
+                                                ident, "let", kind_str,
+                                            ),
+                                        )
                                     } else {
-                                        let sugg = "consider using `const` instead of `let`";
                                         (
                                             rib_ident.span,
                                             AttemptToUseNonConstantValueInConstant(
                                                 original_rib_ident_def,
-                                                sugg.to_string(),
+                                                "const",
+                                                "let",
                                             ),
                                         )
                                     };

compiler/rustc_span/src/source_map.rs

@@ -671,7 +671,9 @@ impl SourceMap {
             let pat = pat.to_owned() + ws;
             if let Ok(prev_source) = self.span_to_prev_source(sp) {
                 let prev_source = prev_source.rsplit(&pat).next().unwrap_or("").trim_start();
-                if !prev_source.is_empty() && (!prev_source.contains('\n') || accept_newlines) {
+                if prev_source.is_empty() && sp.lo().0 != 0 {
+                    return sp.with_lo(BytePos(sp.lo().0 - 1));
+                } else if !prev_source.contains('\n') || accept_newlines {
                     return sp.with_lo(BytePos(sp.lo().0 - prev_source.len() as u32));
                 }
             }

src/test/ui/error-codes/E0435.fixed

@@ -0,0 +1,6 @@
+// run-rustfix
+fn main () {
+    #[allow(non_upper_case_globals)]
+    const foo: usize = 42;
+    let _: [u8; foo]; //~ ERROR E0435
+}

src/test/ui/error-codes/E0435.rs

@@ -1,4 +1,6 @@
+// run-rustfix
 fn main () {
-    let foo = 42u32;
+    #[allow(non_upper_case_globals)]
+    let foo: usize = 42;
     let _: [u8; foo]; //~ ERROR E0435
 }

src/test/ui/error-codes/E0435.stderr

@@ -1,8 +1,8 @@
 error[E0435]: attempt to use a non-constant value in a constant
-  --> $DIR/E0435.rs:3:17
+  --> $DIR/E0435.rs:5:17
    |
-LL |     let foo = 42u32;
-   |         --- help: consider using `const` instead of `let`
+LL |     let foo: usize = 42;
+   |     ------- help: consider using `const` instead of `let`: `const foo`
 LL |     let _: [u8; foo];
    |                 ^^^ non-constant value
 

src/test/ui/impl-trait/bindings.stderr

@@ -2,33 +2,33 @@ error[E0435]: attempt to use a non-constant value in a constant
   --> $DIR/bindings.rs:5:29
    |
 LL |     const foo: impl Clone = x;
-   |           ---               ^ non-constant value
-   |           |
-   |           help: consider using `let` instead of `const`
+   |     ---------               ^ non-constant value
+   |     |
+   |     help: consider using `let` instead of `const`: `let foo`
 
 error[E0435]: attempt to use a non-constant value in a constant
   --> $DIR/bindings.rs:11:33
    |
 LL |         const foo: impl Clone = x;
-   |               ---               ^ non-constant value
-   |               |
-   |               help: consider using `let` instead of `const`
+   |         ---------               ^ non-constant value
+   |         |
+   |         help: consider using `let` instead of `const`: `let foo`
 
 error[E0435]: attempt to use a non-constant value in a constant
   --> $DIR/bindings.rs:18:33
    |
 LL |         const foo: impl Clone = x;
-   |               ---               ^ non-constant value
-   |               |
-   |               help: consider using `let` instead of `const`
+   |         ---------               ^ non-constant value
+   |         |
+   |         help: consider using `let` instead of `const`: `let foo`
 
 error[E0435]: attempt to use a non-constant value in a constant
   --> $DIR/bindings.rs:25:33
    |
 LL |         const foo: impl Clone = x;
-   |               ---               ^ non-constant value
-   |               |
-   |               help: consider using `let` instead of `const`
+   |         ---------               ^ non-constant value
+   |         |
+   |         help: consider using `let` instead of `const`: `let foo`
 
 warning: the feature `impl_trait_in_bindings` is incomplete and may not be safe to use and/or cause compiler crashes
   --> $DIR/bindings.rs:1:12

src/test/ui/issues/issue-27433.fixed

@@ -0,0 +1,7 @@
+// run-rustfix
+fn main() {
+    let foo = 42u32;
+    #[allow(unused_variables, non_snake_case)]
+    let FOO : u32 = foo;
+                   //~^ ERROR attempt to use a non-constant value in a constant
+}

src/test/ui/issues/issue-27433.rs

@@ -1,5 +1,7 @@
+// run-rustfix
 fn main() {
     let foo = 42u32;
+    #[allow(unused_variables, non_snake_case)]
     const FOO : u32 = foo;
                    //~^ ERROR attempt to use a non-constant value in a constant
 }

src/test/ui/issues/issue-27433.stderr

@@ -1,10 +1,10 @@
 error[E0435]: attempt to use a non-constant value in a constant
-  --> $DIR/issue-27433.rs:3:23
+  --> $DIR/issue-27433.rs:5:23
    |
 LL |     const FOO : u32 = foo;
-   |           ---         ^^^ non-constant value
-   |           |
-   |           help: consider using `let` instead of `const`
+   |     ---------         ^^^ non-constant value
+   |     |
+   |     help: consider using `let` instead of `const`: `let FOO`
 
 error: aborting due to previous error
 

src/test/ui/issues/issue-3521-2.fixed

@@ -0,0 +1,9 @@
+// run-rustfix
+fn main() {
+    let foo = 100;
+
+    let y: isize = foo + 1;
+    //~^ ERROR attempt to use a non-constant value in a constant
+
+    println!("{}", y);
+}

src/test/ui/issues/issue-3521-2.rs

@@ -1,3 +1,4 @@
+// run-rustfix
 fn main() {
     let foo = 100;
 

src/test/ui/issues/issue-3521-2.stderr

@@ -1,10 +1,10 @@
 error[E0435]: attempt to use a non-constant value in a constant
-  --> $DIR/issue-3521-2.rs:4:23
+  --> $DIR/issue-3521-2.rs:5:23
    |
 LL |     static y: isize = foo + 1;
-   |            -          ^^^ non-constant value
-   |            |
-   |            help: consider using `let` instead of `static`
+   |     --------          ^^^ non-constant value
+   |     |
+   |     help: consider using `let` instead of `static`: `let y`
 
 error: aborting due to previous error
 

src/test/ui/issues/issue-3521.fixed

@@ -0,0 +1,13 @@
+// run-rustfix
+fn main() {
+    #[allow(non_upper_case_globals)]
+    const foo: isize = 100;
+
+    #[derive(Debug)]
+    enum Stuff {
+        Bar = foo
+        //~^ ERROR attempt to use a non-constant value in a constant
+    }
+
+    println!("{:?}", Stuff::Bar);
+}

src/test/ui/issues/issue-3521.rs

@@ -1,5 +1,7 @@
+// run-rustfix
 fn main() {
-    let foo = 100;
+    #[allow(non_upper_case_globals)]
+    let foo: isize = 100;
 
     #[derive(Debug)]
     enum Stuff {

src/test/ui/issues/issue-3521.stderr

@@ -1,8 +1,8 @@
 error[E0435]: attempt to use a non-constant value in a constant
-  --> $DIR/issue-3521.rs:6:15
+  --> $DIR/issue-3521.rs:8:15
    |
-LL |     let foo = 100;
-   |         --- help: consider using `const` instead of `let`
+LL |     let foo: isize = 100;
+   |     ------- help: consider using `const` instead of `let`: `const foo`
 ...
 LL |         Bar = foo
    |               ^^^ non-constant value

src/test/ui/issues/issue-3668-2.fixed

@@ -0,0 +1,8 @@
+// run-rustfix
+#![allow(unused_variables, dead_code)]
+fn f(x:isize) {
+    let child: isize = x + 1;
+    //~^ ERROR attempt to use a non-constant value in a constant
+}
+
+fn main() {}

src/test/ui/issues/issue-3668-2.rs

@@ -1,3 +1,5 @@
+// run-rustfix
+#![allow(unused_variables, dead_code)]
 fn f(x:isize) {
     static child: isize = x + 1;
     //~^ ERROR attempt to use a non-constant value in a constant

src/test/ui/issues/issue-3668-2.stderr

@@ -1,10 +1,10 @@
 error[E0435]: attempt to use a non-constant value in a constant
-  --> $DIR/issue-3668-2.rs:2:27
+  --> $DIR/issue-3668-2.rs:4:27
    |
 LL |     static child: isize = x + 1;
-   |            -----          ^ non-constant value
-   |            |
-   |            help: consider using `let` instead of `static`
+   |     ------------          ^ non-constant value
+   |     |
+   |     help: consider using `let` instead of `static`: `let child`
 
 error: aborting due to previous error
 

src/test/ui/issues/issue-3668.stderr

@@ -2,9 +2,9 @@ error[E0435]: attempt to use a non-constant value in a constant
   --> $DIR/issue-3668.rs:8:34
    |
 LL |        static childVal: Box<P> = self.child.get();
-   |               --------           ^^^^ non-constant value
-   |               |
-   |               help: consider using `let` instead of `static`
+   |        ---------------           ^^^^ non-constant value
+   |        |
+   |        help: consider using `let` instead of `static`: `let childVal`
 
 error: aborting due to previous error
 

src/test/ui/issues/issue-42060.stderr

@@ -2,15 +2,15 @@ error[E0435]: attempt to use a non-constant value in a constant
   --> $DIR/issue-42060.rs:3:23
    |
 LL |     let thing = ();
-   |         ----- help: consider using `const` instead of `let`
+   |     --------- help: consider using `const` instead of `let`: `const thing`
 LL |     let other: typeof(thing) = thing;
    |                       ^^^^^ non-constant value
 
 error[E0435]: attempt to use a non-constant value in a constant
   --> $DIR/issue-42060.rs:9:13
    |
 LL |     let q = 1;
-   |         - help: consider using `const` instead of `let`
+   |     ----- help: consider using `const` instead of `let`: `const q`
 LL |     <typeof(q)>::N
    |             ^ non-constant value
 

src/test/ui/issues/issue-44239.fixed

@@ -0,0 +1,11 @@
+// run-rustfix
+#![allow(dead_code, non_upper_case_globals)]
+fn main() {
+    const n: usize = 0;
+
+    struct Foo;
+    impl Foo {
+        const N: usize = n;
+        //~^ ERROR attempt to use a non-constant value
+    }
+}

src/test/ui/issues/issue-44239.rs

@@ -1,5 +1,7 @@
+// run-rustfix
+#![allow(dead_code, non_upper_case_globals)]
 fn main() {
-    let n = 0;
+    let n: usize = 0;
 
     struct Foo;
     impl Foo {

src/test/ui/issues/issue-44239.stderr

@@ -1,8 +1,8 @@
 error[E0435]: attempt to use a non-constant value in a constant
-  --> $DIR/issue-44239.rs:6:26
+  --> $DIR/issue-44239.rs:8:26
    |
-LL |     let n = 0;
-   |         - help: consider using `const` instead of `let`
+LL |     let n: usize = 0;
+   |     ----- help: consider using `const` instead of `let`: `const n`
 ...
 LL |         const N: usize = n;
    |                          ^ non-constant value

src/test/ui/non-constant-expr-for-arr-len.stderr

@@ -2,9 +2,9 @@ error[E0435]: attempt to use a non-constant value in a constant
   --> $DIR/non-constant-expr-for-arr-len.rs:5:22
    |
 LL |     fn bar(n: usize) {
-   |            - help: consider using `const` instead of `let`
+   |            - this would need to be a `const`
 LL |         let _x = [0; n];
-   |                      ^ non-constant value
+   |                      ^
 
 error: aborting due to previous error
 

src/test/ui/repeat_count.stderr

@@ -2,7 +2,7 @@ error[E0435]: attempt to use a non-constant value in a constant
   --> $DIR/repeat_count.rs:5:17
    |
 LL |     let n = 1;
-   |         - help: consider using `const` instead of `let`
+   |     ----- help: consider using `const` instead of `let`: `const n`
 LL |     let a = [0; n];
    |                 ^ non-constant value
 

src/test/ui/type/type-dependent-def-issue-49241.stderr

@@ -2,9 +2,9 @@ error[E0435]: attempt to use a non-constant value in a constant
   --> $DIR/type-dependent-def-issue-49241.rs:3:22
    |
 LL |     const l: usize = v.count();
-   |           -          ^ non-constant value
-   |           |
-   |           help: consider using `let` instead of `const`
+   |     -------          ^ non-constant value
+   |     |
+   |     help: consider using `let` instead of `const`: `let l`
 
 error: aborting due to previous error