From afbbd383ccee8b91b1d1bbdcfb0f7a906bba71f7 Mon Sep 17 00:00:00 2001 From: Ayaz Hafiz Date: Sat, 13 Jun 2020 16:22:24 -0700 Subject: [PATCH] Note numeric literals that can never fit in an expected type re https://github.com/rust-lang/rust/pull/72380#discussion_r438289385 Given the toy code ```rust fn is_positive(n: usize) { n > -1_isize; } ``` We currently get a type mismatch error like the following: ``` error[E0308]: mismatched types --> src/main.rs:2:9 | 2 | n > -1_isize; | ^^^^^^^^ expected `usize`, found `isize` | help: you can convert an `isize` to `usize` and panic if the converted value wouldn't fit | 2 | n > (-1_isize).try_into().unwrap(); | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ ``` But clearly, `-1` can never fit into a `usize`, so the suggestion will always panic. A more useful message would tell the user that the value can never fit in the expected type: ``` error[E0308]: mismatched types --> test.rs:2:9 | 2 | n > -1_isize; | ^^^^^^^^ expected `usize`, found `isize` | note: `-1_isize` can never fit into `usize` --> test.rs:2:9 | 2 | n > -1_isize; | ^^^^^^^^ ``` Which is what this commit implements. I only added this check for negative literals because - Currently we can only perform such a check for literals (constant value propagation is outside the scope of the typechecker at this point) - A lint error for out-of-range numeric literals is already emitted IMO it makes more sense to put this check in librustc_lint, but as far as I can tell the typecheck pass happens before the lint pass, so I've added it here. r? @estebank --- src/librustc_typeck/check/demand.rs | 14 +++- src/test/ui/numeric/numeric-cast-no-fix.rs | 22 ++++++ .../ui/numeric/numeric-cast-no-fix.stderr | 74 +++++++++++++++++++ src/test/ui/repeat_count.stderr | 12 +++ 4 files changed, 121 insertions(+), 1 deletion(-) create mode 100644 src/test/ui/numeric/numeric-cast-no-fix.rs create mode 100644 src/test/ui/numeric/numeric-cast-no-fix.stderr diff --git a/src/librustc_typeck/check/demand.rs b/src/librustc_typeck/check/demand.rs index 019b4ca66060c..7fc880b6c7c6c 100644 --- a/src/librustc_typeck/check/demand.rs +++ b/src/librustc_typeck/check/demand.rs @@ -783,6 +783,9 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> { let literal_is_ty_suffixed = |expr: &hir::Expr<'_>| { if let hir::ExprKind::Lit(lit) = &expr.kind { lit.node.is_suffixed() } else { false } }; + let is_negative_int = + |expr: &hir::Expr<'_>| matches!(expr.kind, hir::ExprKind::Unary(hir::UnOp::UnNeg, ..)); + let is_uint = |ty: Ty<'_>| matches!(ty.kind, ty::Uint(..)); let in_const_context = self.tcx.hir().is_inside_const_context(expr.hir_id); @@ -807,7 +810,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> { "you can convert `{}` from `{}` to `{}`, matching the type of `{}`", lhs_src, expected_ty, checked_ty, src ); - let suggestion = format!("{}::from({})", checked_ty, lhs_src,); + let suggestion = format!("{}::from({})", checked_ty, lhs_src); (lhs_expr.span, msg, suggestion) } else { let msg = format!("{} and panic if the converted value wouldn't fit", msg); @@ -822,8 +825,17 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> { |err: &mut DiagnosticBuilder<'_>, found_to_exp_is_fallible: bool, exp_to_found_is_fallible: bool| { + let always_fallible = found_to_exp_is_fallible + && (exp_to_found_is_fallible || expected_ty_expr.is_none()); let msg = if literal_is_ty_suffixed(expr) { &lit_msg + } else if always_fallible && (is_negative_int(expr) && is_uint(expected_ty)) { + // We now know that converting either the lhs or rhs is fallible. Before we + // suggest a fallible conversion, check if the value can never fit in the + // expected type. + let msg = format!("`{}` can never fit into `{}`", src, expected_ty); + err.span_note(expr.span, &msg); + return; } else if in_const_context { // Do not recommend `into` or `try_into` in const contexts. return; diff --git a/src/test/ui/numeric/numeric-cast-no-fix.rs b/src/test/ui/numeric/numeric-cast-no-fix.rs new file mode 100644 index 0000000000000..8bfd833354164 --- /dev/null +++ b/src/test/ui/numeric/numeric-cast-no-fix.rs @@ -0,0 +1,22 @@ +#[allow(unused_must_use)] +fn main() { + let x_usize: usize = 1; + let x_u128: u128 = 2; + let x_u64: u64 = 3; + let x_u32: u32 = 4; + let x_u16: u16 = 5; + let x_u8: u8 = 6; + + x_usize > -1_isize; + //~^ ERROR mismatched types + x_u128 > -1_isize; + //~^ ERROR mismatched types + x_u64 > -1_isize; + //~^ ERROR mismatched types + x_u32 > -1_isize; + //~^ ERROR mismatched types + x_u16 > -1_isize; + //~^ ERROR mismatched types + x_u8 > -1_isize; + //~^ ERROR mismatched types +} diff --git a/src/test/ui/numeric/numeric-cast-no-fix.stderr b/src/test/ui/numeric/numeric-cast-no-fix.stderr new file mode 100644 index 0000000000000..51e263d636f98 --- /dev/null +++ b/src/test/ui/numeric/numeric-cast-no-fix.stderr @@ -0,0 +1,74 @@ +error[E0308]: mismatched types + --> $DIR/numeric-cast-no-fix.rs:10:15 + | +LL | x_usize > -1_isize; + | ^^^^^^^^ expected `usize`, found `isize` + | +note: `-1_isize` can never fit into `usize` + --> $DIR/numeric-cast-no-fix.rs:10:15 + | +LL | x_usize > -1_isize; + | ^^^^^^^^ + +error[E0308]: mismatched types + --> $DIR/numeric-cast-no-fix.rs:12:14 + | +LL | x_u128 > -1_isize; + | ^^^^^^^^ expected `u128`, found `isize` + | +note: `-1_isize` can never fit into `u128` + --> $DIR/numeric-cast-no-fix.rs:12:14 + | +LL | x_u128 > -1_isize; + | ^^^^^^^^ + +error[E0308]: mismatched types + --> $DIR/numeric-cast-no-fix.rs:14:13 + | +LL | x_u64 > -1_isize; + | ^^^^^^^^ expected `u64`, found `isize` + | +note: `-1_isize` can never fit into `u64` + --> $DIR/numeric-cast-no-fix.rs:14:13 + | +LL | x_u64 > -1_isize; + | ^^^^^^^^ + +error[E0308]: mismatched types + --> $DIR/numeric-cast-no-fix.rs:16:13 + | +LL | x_u32 > -1_isize; + | ^^^^^^^^ expected `u32`, found `isize` + | +note: `-1_isize` can never fit into `u32` + --> $DIR/numeric-cast-no-fix.rs:16:13 + | +LL | x_u32 > -1_isize; + | ^^^^^^^^ + +error[E0308]: mismatched types + --> $DIR/numeric-cast-no-fix.rs:18:13 + | +LL | x_u16 > -1_isize; + | ^^^^^^^^ expected `u16`, found `isize` + | +note: `-1_isize` can never fit into `u16` + --> $DIR/numeric-cast-no-fix.rs:18:13 + | +LL | x_u16 > -1_isize; + | ^^^^^^^^ + +error[E0308]: mismatched types + --> $DIR/numeric-cast-no-fix.rs:20:12 + | +LL | x_u8 > -1_isize; + | ^^^^^^^^ expected `u8`, found `isize` + | +help: you can convert `x_u8` from `u8` to `isize`, matching the type of `-1_isize` + | +LL | isize::from(x_u8) > -1_isize; + | ^^^^^^^^^^^^^^^^^ + +error: aborting due to 6 previous errors + +For more information about this error, try `rustc --explain E0308`. diff --git a/src/test/ui/repeat_count.stderr b/src/test/ui/repeat_count.stderr index 6a081e23d9d37..a1d5b36931dac 100644 --- a/src/test/ui/repeat_count.stderr +++ b/src/test/ui/repeat_count.stderr @@ -39,12 +39,24 @@ error[E0308]: mismatched types | LL | let f = [0; -4_isize]; | ^^^^^^^^ expected `usize`, found `isize` + | +note: `-4_isize` can never fit into `usize` + --> $DIR/repeat_count.rs:19:17 + | +LL | let f = [0; -4_isize]; + | ^^^^^^^^ error[E0308]: mismatched types --> $DIR/repeat_count.rs:22:23 | LL | let f = [0_usize; -1_isize]; | ^^^^^^^^ expected `usize`, found `isize` + | +note: `-1_isize` can never fit into `usize` + --> $DIR/repeat_count.rs:22:23 + | +LL | let f = [0_usize; -1_isize]; + | ^^^^^^^^ error[E0308]: mismatched types --> $DIR/repeat_count.rs:25:17