Add some UI tests and reword error-code docs

compiler/rustc_error_codes/src/error_codes/E0015.md

@@ -1,5 +1,4 @@
-A constant item was initialized with something that is not a constant
-expression.
+A non-`const` function was called in a `const` context.
 
 Erroneous code example:
 
@@ -8,26 +7,20 @@ fn create_some() -> Option<u8> {
     Some(1)
 }
 
-const FOO: Option<u8> = create_some(); // error!
+// error: cannot call non-const fn `create_some` in constants
+const FOO: Option<u8> = create_some();
 ```
 
-The only functions that can be called in static or constant expressions are
-`const` functions, and struct/enum constructors.
+All functions used in a `const` context (constant or static expression) must
+be marked `const`.
 
 To fix this error, you can declare `create_some` as a constant function:
 
 ```
-const fn create_some() -> Option<u8> { // declared as a const function
+// declared as a `const` function:
+const fn create_some() -> Option<u8> {
     Some(1)
 }
 
-const FOO: Option<u8> = create_some(); // ok!
-
-// These are also working:
-struct Bar {
-    x: u8,
-}
-
-const OTHER_FOO: Option<u8> = Some(1);
-const BAR: Bar = Bar {x: 1};
+const FOO: Option<u8> = create_some(); // no error!
 ```

src/test/ui/error-codes/E0013.rs

@@ -0,0 +1,4 @@
+static X: i32 = 42;
+const Y: i32 = X; //~ ERROR constants cannot refer to statics [E0013]
+
+fn main() {}

src/test/ui/error-codes/E0013.stderr

@@ -0,0 +1,11 @@
+error[E0013]: constants cannot refer to statics
+  --> $DIR/E0013.rs:2:16
+   |
+LL | const Y: i32 = X;
+   |                ^
+   |
+   = help: consider extracting the value of the `static` to a `const`, and referring to that
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0013`.

src/test/ui/error-codes/E0015.rs

@@ -0,0 +1,8 @@
+fn create_some() -> Option<u8> {
+    Some(1)
+}
+
+const FOO: Option<u8> = create_some();
+//~^ ERROR cannot call non-const fn `create_some` in constants [E0015]
+
+fn main() {}

src/test/ui/error-codes/E0015.stderr

@@ -0,0 +1,11 @@
+error[E0015]: cannot call non-const fn `create_some` in constants
+  --> $DIR/E0015.rs:5:25
+   |
+LL | const FOO: Option<u8> = create_some();
+   |                         ^^^^^^^^^^^^^
+   |
+   = note: calls in constants are limited to constant functions, tuple structs and tuple variants
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0015`.

src/tools/tidy/src/error_codes.rs

@@ -15,8 +15,6 @@
 //!
 //! 4. We check that the error code is actually emitted by the compiler.
 //!   - This is done by searching `compiler/` with a regex.
-//!
-//! This tidy check was merged and refactored from two others. See #PR_NUM for information about linting changes that occurred during this refactor.
 
 use std::{ffi::OsStr, fs, path::Path};
 
@@ -57,7 +55,7 @@ pub fn check(root_path: &Path, search_paths: &[&Path], verbose: bool, bad: &mut
     let no_longer_emitted = check_error_codes_docs(root_path, &error_codes, &mut errors, verbose);
 
     // Stage 3: check list has UI tests
-    check_error_codes_tests(root_path, &error_codes, &mut errors, verbose);
+    check_error_codes_tests(root_path, &error_codes, &mut errors, verbose, &no_longer_emitted);
 
     // Stage 4: check list is emitted by compiler
     check_error_codes_used(search_paths, &error_codes, &mut errors, &no_longer_emitted, verbose);
@@ -174,22 +172,21 @@ fn check_error_codes_docs(
             return;
         }
 
-        let (found_code_example, found_proper_doctest, emit_ignore_warning, emit_no_longer_warning) =
+        let (found_code_example, found_proper_doctest, emit_ignore_warning, no_longer_emitted) =
             check_explanation_has_doctest(&contents, &err_code);
+
         if emit_ignore_warning {
             verbose_print!(
                 verbose,
                 "warning: Error code `{err_code}` uses the ignore header. This should not be used, add the error code to the \
                 `IGNORE_DOCTEST_CHECK` constant instead."
             );
         }
-        if emit_no_longer_warning {
+
+        if no_longer_emitted {
             no_longer_emitted_codes.push(err_code.to_owned());
-            verbose_print!(
-                verbose,
-                "warning: Error code `{err_code}` is no longer emitted and should be removed entirely."
-            );
         }
+
         if !found_code_example {
             verbose_print!(
                 verbose,
@@ -226,7 +223,7 @@ fn check_explanation_has_doctest(explanation: &str, err_code: &str) -> (bool, bo
     let mut found_proper_doctest = false;
 
     let mut emit_ignore_warning = false;
-    let mut emit_no_longer_warning = false;
+    let mut no_longer_emitted = false;
 
     for line in explanation.lines() {
         let line = line.trim();
@@ -246,13 +243,13 @@ fn check_explanation_has_doctest(explanation: &str, err_code: &str) -> (bool, bo
         } else if line
             .starts_with("#### Note: this error code is no longer emitted by the compiler")
         {
-            emit_no_longer_warning = true;
+            no_longer_emitted = true;
             found_code_example = true;
             found_proper_doctest = true;
         }
     }
 
-    (found_code_example, found_proper_doctest, emit_ignore_warning, emit_no_longer_warning)
+    (found_code_example, found_proper_doctest, emit_ignore_warning, no_longer_emitted)
 }
 
 // Stage 3: Checks that each error code has a UI test in the correct directory
@@ -261,6 +258,7 @@ fn check_error_codes_tests(
     error_codes: &[String],
     errors: &mut Vec<String>,
     verbose: bool,
+    no_longer_emitted: &[String],
 ) {
     let tests_path = root_path.join(Path::new(ERROR_TESTS_PATH));
 
@@ -295,6 +293,11 @@ fn check_error_codes_tests(
             }
         };
 
+        if no_longer_emitted.contains(code) {
+            // UI tests *can't* contain error codes that are no longer emitted.
+            continue;
+        }
+
         let mut found_code = false;
 
         for line in file.lines() {