Clarify message about unresolved use

Author: kornelski

compiler/rustc_error_codes/src/error_codes/E0433.md

@@ -1,17 +1,27 @@
-An undeclared type or module was used.
+An undeclared crate, module, or type was used.
 
 Erroneous code example:
 
 ```compile_fail,E0433
 let map = HashMap::new();
-// error: failed to resolve: use of undeclared type or module `HashMap`
+// error: failed to resolve: use of undeclared type `HashMap`
 ```
 
 Please verify you didn't misspell the type/module's name or that you didn't
 forget to import it:
 
-
 ```
 use std::collections::HashMap; // HashMap has been imported.
 let map: HashMap<u32, u32> = HashMap::new(); // So it can be used!
 ```
+
+If you've expected to use a crate name:
+
+```compile_fail
+use ferris_wheel::BigO;
+// error: failed to resolve: use of undeclared crate or module `ferris_wheel`
+```
+
+Make sure the crate has been added as a dependency in `Cargo.toml`.
+
+To use a module from your current crate, add the `crate::` prefix to the path.

compiler/rustc_resolve/src/lib.rs

@@ -2407,7 +2407,14 @@ impl<'a> Resolver<'a> {
                             (format!("maybe a missing crate `{}`?", ident), None)
                         }
                     } else if i == 0 {
-                        (format!("use of undeclared type or module `{}`", ident), None)
+                        if ident
+                            .name
+                            .with(|n| n.chars().next().map_or(false, |c| c.is_ascii_uppercase()))
+                        {
+                            (format!("use of undeclared type `{}`", ident), None)
+                        } else {
+                            (format!("use of undeclared crate or module `{}`", ident), None)
+                        }
                     } else {
                         let mut msg =
                             format!("could not find `{}` in `{}`", ident, path[i - 1].ident);

src/test/ui/attributes/register-attr-tool-prelude.rs

@@ -7,7 +7,7 @@
 #[no_implicit_prelude]
 mod m {
     #[attr] //~ ERROR cannot find attribute `attr` in this scope
-    #[tool::attr] //~ ERROR failed to resolve: use of undeclared type or module `tool`
+    #[tool::attr] //~ ERROR failed to resolve: use of undeclared crate or module `tool`
     fn check() {}
 }
 

src/test/ui/attributes/register-attr-tool-prelude.stderr

@@ -1,8 +1,8 @@
-error[E0433]: failed to resolve: use of undeclared type or module `tool`
+error[E0433]: failed to resolve: use of undeclared crate or module `tool`
   --> $DIR/register-attr-tool-prelude.rs:10:7
    |
 LL |     #[tool::attr]
-   |       ^^^^ use of undeclared type or module `tool`
+   |       ^^^^ use of undeclared crate or module `tool`
 
 error: cannot find attribute `attr` in this scope
   --> $DIR/register-attr-tool-prelude.rs:9:7

src/test/ui/bad/bad-module.rs

@@ -1,7 +1,7 @@
 fn main() {
     let foo = thing::len(Vec::new());
-    //~^ ERROR failed to resolve: use of undeclared type or module `thing`
+    //~^ ERROR failed to resolve: use of undeclared crate or module `thing`
 
     let foo = foo::bar::baz();
-    //~^ ERROR failed to resolve: use of undeclared type or module `foo`
+    //~^ ERROR failed to resolve: use of undeclared crate or module `foo`
 }

src/test/ui/bad/bad-module.stderr

@@ -1,14 +1,14 @@
-error[E0433]: failed to resolve: use of undeclared type or module `thing`
+error[E0433]: failed to resolve: use of undeclared crate or module `thing`
   --> $DIR/bad-module.rs:2:15
    |
 LL |     let foo = thing::len(Vec::new());
-   |               ^^^^^ use of undeclared type or module `thing`
+   |               ^^^^^ use of undeclared crate or module `thing`
 
-error[E0433]: failed to resolve: use of undeclared type or module `foo`
+error[E0433]: failed to resolve: use of undeclared crate or module `foo`
   --> $DIR/bad-module.rs:5:15
    |
 LL |     let foo = foo::bar::baz();
-   |               ^^^ use of undeclared type or module `foo`
+   |               ^^^ use of undeclared crate or module `foo`
 
 error: aborting due to 2 previous errors
 

src/test/ui/coherence/conflicting-impl-with-err.stderr

@@ -1,14 +1,14 @@
-error[E0433]: failed to resolve: use of undeclared type or module `nope`
+error[E0433]: failed to resolve: use of undeclared crate or module `nope`
   --> $DIR/conflicting-impl-with-err.rs:4:11
    |
 LL | impl From<nope::Thing> for Error {
-   |           ^^^^ use of undeclared type or module `nope`
+   |           ^^^^ use of undeclared crate or module `nope`
 
-error[E0433]: failed to resolve: use of undeclared type or module `nope`
+error[E0433]: failed to resolve: use of undeclared crate or module `nope`
   --> $DIR/conflicting-impl-with-err.rs:5:16
    |
 LL |     fn from(_: nope::Thing) -> Self {
-   |                ^^^^ use of undeclared type or module `nope`
+   |                ^^^^ use of undeclared crate or module `nope`
 
 error: aborting due to 2 previous errors
 

src/test/ui/derived-errors/issue-31997-1.stderr

@@ -1,4 +1,4 @@
-error[E0433]: failed to resolve: use of undeclared type or module `HashMap`
+error[E0433]: failed to resolve: use of undeclared type `HashMap`
   --> $DIR/issue-31997-1.rs:20:19
    |
 LL |     let mut map = HashMap::new();

src/test/ui/dyn-trait-compatibility.rs

@@ -1,14 +1,14 @@
 type A0 = dyn;
 //~^ ERROR cannot find type `dyn` in this scope
 type A1 = dyn::dyn;
-//~^ ERROR use of undeclared type or module `dyn`
+//~^ ERROR use of undeclared crate or module `dyn`
 type A2 = dyn<dyn, dyn>;
 //~^ ERROR cannot find type `dyn` in this scope
 //~| ERROR cannot find type `dyn` in this scope
 //~| ERROR cannot find type `dyn` in this scope
 type A3 = dyn<<dyn as dyn>::dyn>;
 //~^ ERROR cannot find type `dyn` in this scope
 //~| ERROR cannot find type `dyn` in this scope
-//~| ERROR use of undeclared type or module `dyn`
+//~| ERROR use of undeclared crate or module `dyn`
 
 fn main() {}

src/test/ui/dyn-trait-compatibility.stderr

@@ -1,14 +1,14 @@
-error[E0433]: failed to resolve: use of undeclared type or module `dyn`
+error[E0433]: failed to resolve: use of undeclared crate or module `dyn`
   --> $DIR/dyn-trait-compatibility.rs:3:11
    |
 LL | type A1 = dyn::dyn;
-   |           ^^^ use of undeclared type or module `dyn`
+   |           ^^^ use of undeclared crate or module `dyn`
 
-error[E0433]: failed to resolve: use of undeclared type or module `dyn`
+error[E0433]: failed to resolve: use of undeclared crate or module `dyn`
   --> $DIR/dyn-trait-compatibility.rs:9:23
    |
 LL | type A3 = dyn<<dyn as dyn>::dyn>;
-   |                       ^^^ use of undeclared type or module `dyn`
+   |                       ^^^ use of undeclared crate or module `dyn`
 
 error[E0412]: cannot find type `dyn` in this scope
   --> $DIR/dyn-trait-compatibility.rs:1:11