Add note about why no_mangle and export_name are unsafe

compiler/rustc_lint/src/builtin.rs

@@ -236,6 +236,18 @@ impl UnsafeCode {
 
         cx.struct_span_lint(UNSAFE_CODE, span, decorate);
     }
+
+    fn report_overriden_symbol_name(&self, cx: &EarlyContext<'_>, span: Span, msg: &str) {
+        self.report_unsafe(cx, span, |lint| {
+            lint.build(msg)
+                .note(
+                    "the linker's behavior with multiple libraries exporting duplicate symbol \
+                    names is undefined and Rust cannot provide guarantees when you manually \
+                    override them",
+                )
+                .emit();
+        })
+    }
 }
 
 impl EarlyLintPass for UnsafeCode {
@@ -279,27 +291,35 @@ impl EarlyLintPass for UnsafeCode {
 
             ast::ItemKind::Fn(..) => {
                 if attr::contains_name(&it.attrs, sym::no_mangle) {
-                    self.report_unsafe(cx, it.ident.span, |lint| {
-                        lint.build("declaration of a `no_mangle` function").emit();
-                    })
+                    self.report_overriden_symbol_name(
+                        cx,
+                        it.ident.span,
+                        "declaration of a `no_mangle` function",
+                    );
                 }
                 if attr::contains_name(&it.attrs, sym::export_name) {
-                    self.report_unsafe(cx, it.ident.span, |lint| {
-                        lint.build("declaration of a function with `export_name`").emit();
-                    })
+                    self.report_overriden_symbol_name(
+                        cx,
+                        it.ident.span,
+                        "declaration of a function with `export_name`",
+                    );
                 }
             }
 
             ast::ItemKind::Static(..) => {
                 if attr::contains_name(&it.attrs, sym::no_mangle) {
-                    self.report_unsafe(cx, it.ident.span, |lint| {
-                        lint.build("declaration of a `no_mangle` static").emit();
-                    })
+                    self.report_overriden_symbol_name(
+                        cx,
+                        it.ident.span,
+                        "declaration of a `no_mangle` static",
+                    );
                 }
                 if attr::contains_name(&it.attrs, sym::export_name) {
-                    self.report_unsafe(cx, it.ident.span, |lint| {
-                        lint.build("declaration of a static with `export_name`").emit();
-                    })
+                    self.report_overriden_symbol_name(
+                        cx,
+                        it.ident.span,
+                        "declaration of a static with `export_name`",
+                    );
                 }
             }
 

src/test/ui/lint/lint-unsafe-code.stderr

@@ -9,24 +9,31 @@ note: the lint level is defined here
    |
 LL | #![deny(unsafe_code)]
    |         ^^^^^^^^^^^
+   = note: the linker's behavior with multiple libraries exporting duplicate symbol names is undefined and Rust cannot provide guarantees when you manually override them
 
 error: declaration of a `no_mangle` static
   --> $DIR/lint-unsafe-code.rs:32:21
    |
 LL | #[no_mangle] static FOO: u32 = 5;
    |                     ^^^
+   |
+   = note: the linker's behavior with multiple libraries exporting duplicate symbol names is undefined and Rust cannot provide guarantees when you manually override them
 
 error: declaration of a function with `export_name`
   --> $DIR/lint-unsafe-code.rs:34:27
    |
 LL | #[export_name = "bar"] fn bar() {}
    |                           ^^^
+   |
+   = note: the linker's behavior with multiple libraries exporting duplicate symbol names is undefined and Rust cannot provide guarantees when you manually override them
 
 error: declaration of a static with `export_name`
   --> $DIR/lint-unsafe-code.rs:35:31
    |
 LL | #[export_name = "BAR"] static BAR: u32 = 5;
    |                               ^^^
+   |
+   = note: the linker's behavior with multiple libraries exporting duplicate symbol names is undefined and Rust cannot provide guarantees when you manually override them
 
 error: declaration of an `unsafe` function
   --> $DIR/lint-unsafe-code.rs:37:1
@@ -115,6 +122,7 @@ LL |         #[no_mangle] fn foo() {}
 LL |     unsafe_in_macro!()
    |     ------------------ in this macro invocation
    |
+   = note: the linker's behavior with multiple libraries exporting duplicate symbol names is undefined and Rust cannot provide guarantees when you manually override them
    = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error: declaration of a `no_mangle` static
@@ -126,6 +134,7 @@ LL |         #[no_mangle] static FOO: u32 = 5;
 LL |     unsafe_in_macro!()
    |     ------------------ in this macro invocation
    |
+   = note: the linker's behavior with multiple libraries exporting duplicate symbol names is undefined and Rust cannot provide guarantees when you manually override them
    = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error: declaration of a function with `export_name`
@@ -137,6 +146,7 @@ LL |         #[export_name = "bar"] fn bar() {}
 LL |     unsafe_in_macro!()
    |     ------------------ in this macro invocation
    |
+   = note: the linker's behavior with multiple libraries exporting duplicate symbol names is undefined and Rust cannot provide guarantees when you manually override them
    = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error: declaration of a static with `export_name`
@@ -148,6 +158,7 @@ LL |         #[export_name = "BAR"] static BAR: u32 = 5;
 LL |     unsafe_in_macro!()
    |     ------------------ in this macro invocation
    |
+   = note: the linker's behavior with multiple libraries exporting duplicate symbol names is undefined and Rust cannot provide guarantees when you manually override them
    = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error: usage of an `unsafe` block